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Title: Metabolic Adaptation to Climate and Distribution of the Raccoon Procyon Lotor and Other Procyonidae
Author: Mugaas, John N., Seidensticker, John, Mahlke-Johnson, Kathleen P.
Language: English
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*** Start of this LibraryBlog Digital Book "Metabolic Adaptation to Climate and Distribution of the Raccoon Procyon Lotor and Other Procyonidae" ***
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY · NUMBER 542
Metabolic Adaptation to Climate
and Distribution of the Raccoon
_Procyon lotor_ and Other Procyonidae
_John N. Mugaas, John Seidensticker,
and Kathleen P. Mahlke-Johnson_
[Smithsonian Institution logo]
SMITHSONIAN INSTITUTION PRESS
Washington, D.C.
1993
ABSTRACT
Mugaas, J. N., J. Seidensticker, and K. Mahlke-Johnson. Metabolic
Adaptation to Climate and Distribution of the Raccoon _Procyon lotor_
and Other Procyonidae. _Smithsonian Contributions to Zoology_, number
542, 34 pages, 8 figures, 12 tables, 1993.--Although the family
Procyonidae is largely a Neotropical group, the North American
raccoon, _Procyon lotor_, is more versatile in its use of climate, and
it is found in nearly every habitat from Panama to 60°N in Canada. We
hypothesized that most contemporary procyonids have remained in tropic
and subtropic climates because they have retained the metabolic
characteristics of their warm-adapted ancestors, whereas _Procyon
lotor_ evolved a different set of adaptations that have enabled it to
generalize its use of habitats and climates. To test this hypothesis
we compared _Procyon lotor_ with several other procyonids
(_Bassariscus astutus_, _Nasua nasua_, _Nasua narica_, _Procyon
cancrivorus_, and _Potos flavus_) with respect to (1) basal metabolic
rate ([.H]_{b}), (2) minimum wet thermal conductance (C_{mw}), (3)
diversity of diet (D_{d}), (4) intrinsic rate of natural increase
(r_{max}), and, where possible, (5) capacity for evaporative cooling
(E_{c}). We measured basal and thermoregulatory metabolism,
evaporative water loss, and body temperature of both sexes of _Procyon
lotor_ from north central Virginia, in summer and winter. Metabolic
data for other procyonids were from literature, as were dietary and
reproductive data for all species.
Procyon lotor differed from other procyonids in all five
variables. (1) _Procyon lotor_'s mass specific [.H]_{b} (0.46
mL O_{2}·g^{-1}·h^{-1}) was 1.45 to 1.86 times greater than values for
other procyonids. (2) Because of its annual molt, _Procyon lotor_'s
C_{mw} was about 49% higher in summer than winter, 0.0256 and 0.0172
mL O_{2}·g^{-1}·h^{-1}·°C^{-1}, respectively. The ratio of measured to
predicted C_{mw} for _Procyon lotor_ in winter (1.15) was similar to
values calculated for _Potos flavus_ (1.02) and _Procyon cancrivorus_
(1.25). Values for other procyonids were higher than this, but less
than the value for _Procyon lotor_ (1.76) in summer. On a mass
specific basis, _Bassariscus astutus_ had the lowest C_{mw} with a
ratio of 0.85. (3) _Procyon lotor_ utilized three times as many food
categories as _Procyon cancrivorus_, _Nasua nasua_, and _Bassariscus
astutus_; about two times as many as _Nasua narica_; and nine times as
many as _Potos flavus_. (4) Intrinsic rate of natural increase
correlated positively with [.H]_{b}. _Procyon lotor_ had the highest
r_{max} (2.52 of expected) and _Potos flavus_ the lowest (0.48 of
expected). The other procyonids examined also had low [.H]_{b}, but
their r_{max}'s were higher than predicted (1.11-1.32 of expected).
Early age of first female reproduction, fairly large litter size, long
life span, high-quality diet, and, in one case, female social
organization all compensated for low [.H]_{b} and elevated r_{max}.
(5) Although data on the capacity for evaporative cooling were
incomplete, this variable appeared to be best developed in _Procyon
lotor_ and _Bassariscus astutus_, the two species that have been most
successful at including temperate climates in their distributions.
These five variables are functionally interrelated, and have
co-evolved in each species to form a unique adaptive unit that
regulates body temperature and energy balance throughout each annual
cycle. The first four variables were converted into normalized
dimensionless numbers, which were used to derive a composite score
that represented each species' adaptive unit. _Procyon lotor_ had the
highest composite score (1.47) and _Potos flavus_ the lowest (0.39).
Scores for the other procyonids were intermediate to these extremes
(0.64-0.79). There was a positive correlation between the number of
climates a species occupies and the magnitude of its composite score.
Linear regression of this relationship indicated that 89% of the
variance in climatic distribution was attributed to the composite
scores. Differences in metabolic adaptation, therefore, have played a
role in delimiting climatic distribution of these species.
It was clear that _Procyon lotor_ differed from the other procyonids
with respect to thermoregulatory ability, diet, and reproductive
potential. These differences have enabled it to become a highly
successful climate generalist, and its evolution of an [.H]_{b} that
is higher than the procyonid norm appears to be the cornerstone of its
success.
OFFICIAL PUBLICATION DATE is handstamped in a limited number of
initial copies and is recorded in the Institution's annual report,
_Smithsonian Year_. SERIES COVER DESIGN: The coral _Montastrea
cavernosa_ (Linnaeus).
Library of Congress Cataloging-in-Publication Data
Mugaas, John N.
Metabolic adaptation to climate and distribution of the raccoon
Procyon lotor and other Procyonidae / John N. Mugaas, John
Seidensticker, and Kathleen P. Mahlke-Johnson.
p. cm.--(Smithsonian contributions to zoology; no. 542)
Includes bibliographical references (p. )
1. Raccoons-Metabolism-Climatic factors.
2. Procyonidae-Metabolism-Climatic factors.
3. Raccoons-Geographical distribution.
4. Procyonidae-Geographical distribution.
I. Seidensticker, John.
II. Mahlke-Johnson, Kathleen.
III. Title.
IV. Series.
QL1.S54 no. 542 [QL737.C26] 591 s-dc20 [599.74´443´04542] 93-3119
[permanent paper symbol] The paper used in this publication meets the
minimum requirements of the American National Standard for Permanence
of Paper for Printed Library Materials z39.48--1984.
Contents
_Page_
Introduction 1
Defining the Problem 1
Procyonid Origins 1
Typical Procyonids 2
The Atypical Procyonid 3
The Hypothesis 4
Hypothesis Testing 4
Adaptive Significance of the Variables 4
Basal Metabolic Rate and Intrinsic Rate of Natural Increase 4
Minimum Thermal Conductance 4
Capacity for Evaporative Cooling 5
Diet 5
Experimental Design and Summary 5
Acknowledgments 5
Materials and Methods 6
Live-trapping 6
Metabolic Studies 6
Basal and Thermoregulatory Metabolism 6
Evaporative Water Loss 7
Body Temperature 7
Calibrations 7
Calorimeter 7
Body Temperature Transmitters 8
Statistical Methods 8
Estimating Intrinsic Rate of Natural Increase 8
Comparison of Adaptive Units 8
Results 8
Body Mass 8
Basal Metabolic Rate 9
Minimum Thermal Conductance 9
Evaporative Water Loss 11
Thermoregulation at Low Temperatures 12
Body Temperature 12
Summer 14
Winter 14
Thermoregulation at High Temperatures 16
Body Temperature 16
Summer 16
Winter 16
Daily Cycle of Body Temperature 16
Discussion 16
Basal Metabolic Rate 16
Background 16
Captive versus Wild Raccoons 17
Seasonal Metabolism of Raccoons 17
Comparison of _Procyon lotor_ with Other Procyonids 17
Influence of Diet on Basal Metabolism 18
Background 18
Food Habits of Procyonids 18
Food Habits and Basal Metabolism 19
Summary 19
Basal Metabolism and Intrinsic Rate of Natural Increase 19
Background 19
_Procyon lotor_ 19
_Bassariscus astutus_ 19
_Nasua narica_ 19
_Nasua nasua_ 20
_Procyon cancrivorus_ 20
_Potos flavus_ 20
Summary 20
Basal Metabolism and Climatic Distribution 21
_Procyon lotor_ 21
Other Procyonids 21
Minimum Thermal Conductance 21
Background 21
Effect of Molt on Thermal Conductance 21
Comparison of Thermal Conductances 22
_Procyon lotor_ versus Tropical Procyonids 22
_Bassariscus astutus_ 22
Thermoregulation and Use of Stored Fat at Low Temperatures 22
Background 22
Thermoregulation 22
Stored Fat 23
Thermal Model of the Raccoon and Its Den 23
Metabolic Advantage of the Den 23
Thermoregulation at High Temperatures 24
Background 24
Comparison of Procyonid Responses to Heat Stress 24
_Potos flavus_ 24
_Nasua nasua and Nasua narica_ 24
_Bassariscus astutus_ 24
_Procyon lotor_ 24
_Procyon cancrivorus_ 24
Summary 24
Composite Scores of Adaptive Units and Geographic Distribution 25
Evolution of Metabolic Adaptations 26
Evolution of Low Basal Metabolic Rate 26
Evolution of High Basal Metabolic Rate 27
Summary 28
Appendix: List of Symbols 29
Literature Cited 30
Metabolic Adaptation to Climate
and Distribution of the Raccoon
_Procyon lotor_ and Other Procyonidae
_John N. Mugaas, John Seidensticker,
and Kathleen P. Mahlke-Johnson_
_John N. Mugaas, Department of Physiology, Division of Functional
Biology, West Virginia School of Osteopathic Medicine, Lewisburg,
West Virginia 24901. John Seidensticker and Kathleen P.
Mahlke-Johnson, National Zoological Park, Smithsonian Institution,
Washington, D.C. 20008._
$Introduction$
DEFINING THE PROBLEM
_Procyonid Origins_
The major carnivore radiations took place about 40 million years
before present (MYBP) in the late Eocene and early Oligocene (Ewer,
1973:363; Wayne et al., 1989). Between 30 and 40 MYBP, a progenitor
split into the ursid and procyonid lineages, which evolved into
present-day bears, pandas, and raccoons (Wayne et al., 1989). The
taxonomic relatedness of pandas to bears and raccoons has been tested
extensively and a number of authors have summarized current thinking
on the problem (Martin, 1989; Wayne et al., 1989; Wozencraft, 1989a,
1989b; Decker and Wozencraft, 1991). Davis (1964:322-327) and others
(Leone and Wiens, 1956; Todd and Pressman, 1968; Sarich, 1976; O'Brien
et al., 1985) place the giant panda, _Ailuropoda melanoleuca_, with
the ursids. The taxonomic status of the red panda, _Ailurus fulgens_,
appears to be less certain. Some current investigations align the red
panda with bears (Segall, 1943; Todd and Pressman, 1968; Hunt, 1974;
Ginsburg, 1982; Wozencraft, 1984:56-110; 1989a), whereas others place
them intermediate to procyonids and bears (Wurster and Benirschke,
1968; Sarich, 1976; O'Brien et al., 1985), or in close relationship to
the giant panda (Tagle et al., 1986).
The procyonid radiation took place in North America and produced forms
that were mostly arboreal and omnivorous (Eisenberg, 1981:122; Martin,
1989). The center of this diversification occurred in Middle America
(Baskin, 1982; Webb, 1985b) during the Miocene (Darlington, 1963:367;
Webb, 1985b). Fossil procyonids from the late Miocene are represented
in Florida, California, Texas, Nebraska, Kansas, and South Dakota
(Baskin, 1982; Martin, 1989) and include such genera as _Bassariscus_,
_Arctonasua_, _Cyonasua_, _Paranasua_, _Nasua_, and _Procyon_ (Baskin,
1982; Webb, 1985b). During the Miocene procyonids underwent a modest
radiation within tropical and subtropical climates of North America's
central and middle latitudes. _Cyonasua_, which has close affinities
to _Arctonasua_ (Baskin, 1982), appears in tropical South America in
the late Miocene and immigrated there either by rafting across the
Bolivar Trough or by island-hopping through the Antilles archipelagoes
(Marshall et al., 1982; Marshall, 1988). Thus, procyonids were found
on both continents prior to formation of the Panamanian land bridge
(Darlington, 1963:367, 395; Marshall et al., 1982; Marshall, 1988).
Origins of _Bassaricyon_ and _Potos_ are obscure but probably occurred
in tropical rainforests of Middle America (Baskin, 1982; Webb, 1985b).
A subsequent Pleistocene dispersal carried several modern genera
(Table 1) across the Panamanian land bridge into South America (Webb,
1985b). _Bassariscus_ and _Bassaricyon_ represent the most primitive
genera in Procyoninae and Potosinae subfamilies, respectively (Table 1;
Wozencraft, 1989a; Decker and Wozencraft, 1991).
In the early Tertiary, mid-latitudes of North America were much warmer
than they are now, but not fully tropical, and temperate deciduous
forests, associated with strongly seasonal climates, occurred only in
the far north (Barghoorn, 1953; Colbert, 1953; Darlington, 1963:589,
590). Major climatic deteriorations, with their attendant cooling of
northern continents, occurred during the Eo-Oligocene transition, in
the middle Miocene, at the end of the Miocene, and at about 3 MYBP
(late Pliocene). This last deterioration corresponds with closure of
the Panamanian isthmus (Berggren, 1982; Webb, 1985a). Climatic
deterioration went on at an accelerating rate during the late
Tertiary, with glacial conditions developing at the poles by the
mid-Pliocene (Barghoorn, 1953). Therefore, throughout the Tertiary, as
continents cooled, northern climate zones moved toward the tropics
(Barghoorn, 1953; Colbert, 1953; Darlington, 1963:589, 590, 594, 595;
Webb, 1985a).
TABLE 1.--Classification of recent Procyonidae after Wozencraft
(1989a) and Decker and Wozencraft (1991). Information in
parenthesis indicates general geographic distribution (modified from
Kortlucke and Ramirez-Pulido (1982) and Poglayen-Neuwall (1975)):
S.A. = South America; C.A. = Central America; M. = Mexico;
U.S. = United States; C. = Canada. Lower case letters preceding
geographic areas signify north (n), south (s), and west (w).
Order CARNIVORA Bowdich, 1821
Suborder CANIFORMIA Kretzoi, 1945
Family PROCYONIDAE Gray, 1825
Subfamily POTOSINAE Trouessart, 1904
Genus _Potos_ E. Geoffroy and G. Cuvier, 1795
_P. flavus_ (S.A., C.A., M.)
Genus _Bassaricyon_ Allen, 1876
_B. alleni_[a] (S.A.)
_B. beddardi_[a] (S.A.)
_B. gabbii_[a] (nS.A., C.A.)
_B. lasius_[a] (C.A.)
_B. pauli_[a] (C.A.)
Subfamily PROCYONINAE Gray, 1825
Genus _Bassariscus_ Coues, 1887
_B. astutus_ (M., wU.S.)
_B. sumichrasti_ (C.A., M.)
Genus _Nasua_ Storr, 1780
_N. narica_[b] (nS.A., C.A., M., swU.S.)
_N. nasua_[b] (S.A., sC.A.)
Genus _Nasuella_ Hollister, 1915
_N. olivacea_ (S.A.)
Genus _Procyon_ Storr, 1780
_P. cancrivorus_ (S.A., sC.A.)
_P. gloveralleni_[c] (Barbados)
_P. insularis_[c] (Maria Madre Is., Maria Magdalene Is.)
_P. lotor_[c] (C.A., M., U.S., sC.)
_P. maynardi_[c] (Bahamas, New Providence Is.)
_P. minor_[c] (Guadeloupe Is.)
_P. pygmaeus_[c] (M., Quintana Roo, Cozumel Is.)
[a] The several named forms of _Bassaricyon_ are a single
species, _Bassaricyon gabbii_ (Wozencraft, 1989a).
[b] These are considered conspecific in some current
taxonomies (Kortlucke and Ramirez-Pulido, 1982); however,
the scheme followed here maintains them as separate species
(Decker, 1991).
[c] Several named forms of _Procyon_ are a single species,
_Procyon lotor_ (Wozencraft, 1989a).
During the late Miocene, late Pliocene, and Pleistocene, the Bering
land bridge between North America and Asia formed periodically,
offering an avenue for dispersal between northern continents
(Darlington, 1963:366; Webb, 1985a). However, by the late Tertiary,
northern continents had cooled to the extent that climate, with its
attendant sharply defined vegetative zones, became the major factor
limiting dispersal by this route (Darlington, 1963:366; Webb, 1985a).
Those Holarctic mammals that did cross the Bering land bridge in the
late Tertiary were "cold-adapted" species associated with relatively
cool, but not alpine, climates (Darlington, 1963:366; Ewer, 1973:369).
Among carnivores this included some canids, ursids, mustelids, and
felids (Darlington, 1963:393-395, 397; Webb, 1985a). Procyonids,
however, did not cross the Bering land bridge into Asia, and Ewer
(1973:369) ascribes this to their being an "essentially tropical
group." Miocene radiation of procyonids occurred at a time when two of
the four major climatic deteriorations (middle and late Miocene) were
taking place (Webb, 1985a, 1985b). These deteriorations had the effect
of cooling the middle latitudes to the extent that temperate forest
forms began to appear in mid-latitude floras, along with a rapid
influx of herbaceous plants (Barghoorn, 1953). The procyonid radiation
did not penetrate beyond these climatically changing middle latitudes,
which implies that these animals were "warm-adapted," and were,
therefore, physiologically excluded from reaching the Bering land
bridge. Today, three of the six genera and over half of the 18 species
that comprise Procyonidae (Table 1; Wozencraft, 1989b) remain confined
to tropical regions of North and South America (Hall and Kelson,
1959:878-897; Poglayen-Neuwall, 1975; Kortlucke and Ramirez-Pulido,
1982; Nowak and Paradiso, 1983:977-985).
_Typical Procyonids_
McNab (1988a) contends that basal metabolism is a highly plastic
character in evolution, and he has amply shown that ecologically
uniform species are more apt to share common metabolic rates than
taxonomically allied species from drastically different environments
(McNab, 1984a, 1986a, 1986b, 1988a). Procyonids represent a
taxonomically allied group that shared a common ecological situation
for millions of years; consequently, members of this family might be
expected to show some uniformity in their [.H]_{b}. Basal and
thermoregulatory metabolism of several procyonids have been measured:
kinkajou, _Potos flavus_ (Müller and Kulzer, 1977; McNab, 1978a;
Müller and Rost, 1983), coatis, _Nasua nasua_ (Chevillard-Hugot et
al., 1980; Mugaas et al., in prep.), and _Nasua narica_ (Scholander et
al., 1950c; Mugaas et al., in prep.), ringtail, _Bassariscus astutus_
(Chevalier, 1985), and crab-eating raccoon, _Procyon cancrivorus_
(Scholander et al., 1950c). In general, these species have [.H]_{b}'s
that are 40%-80% of the values predicted for them by the Kleiber
(1961:206) equation. Lower than predicted [.H]_{b} is viewed as an
energy-saving adaptation for procyonids living in relatively stable
tropical climates (Müller and Kulzer, 1977; Chevillard-Hugot et al.,
1980; Müller and Rost, 1983). This implies that lower than predicted
[.H]_{b} is a general procyonid condition and that it represents a
characteristic that evolved in response to the family's long
association with tropical and subtropical forest environments.
_The Atypical Procyonid_
Although most procyonids are found in only tropical to subtropical
climates, the North American raccoon, _Procyon lotor_, (Figure 1) has
a much broader distribution that extends from tropical Panama (8°N) to
southern Canada. In Alberta, Canada, its range reaches the edge of the
Hudsonian Life Zone at 60°N (for distribution maps see Hall and
Kelson, 1959:878-897, and Poglayen-Neuwall, 1975). Range extensions
and an increase in numbers have been noted in Canada and in parts of
the United States since the 19th century (Lotze and Anderson, 1979;
Kaufmann, 1982; Nowak and Paradiso, 1983:977-985). Thus, _Procyon
lotor_ is more complex ecologically than other procyonids,
particularly when one takes into account its highly generalized food
habits (Hamilton, 1936; Stuewer, 1943; Stains, 1956:39-51; Greenwood,
1981) and the wide range of habitat types (forest, prairie, desert,
mountain, coastal marsh, freshwater marsh) and climates (tropical to
north temperate) in which it is successful (Whitney and Underwood,
1952:1; Hall and Kelson, 1959:885; Lotze and Anderson, 1979;
Kaufmann, 1982). On this basis it is clear that _Procyon lotor_ has
deviated from the typical procyonid portrait and has become the
consummate generalist of the Procyonidae.
[Illustration: FIGURE 1.--North American raccoon, _Procyon lotor_.]
_The Hypothesis_
Our general hypothesis was that whereas most contemporary procyonids
have retained the metabolic characteristics of their warm-adapted
ancestors, _Procyon lotor_ possesses a different set of adaptations,
which either evolved as characteristics unique to this species or were
acquired from its ancestral stock. In either case, its unique
adaptations have given _Procyon lotor_ the physiological flexibility
to generalize its use of habitats and climates and expand its
geographic distribution to a much greater extent than other
procyonids.
_Hypothesis Testing_
We tested our hypothesis by comparing _Procyon lotor_ with several
other procyonids (_Bassariscus astutus_, _Nasua nasua_, _Nasua
narica_, _Procyon cancrivorus_, and _Potos flavus_) on the basis of
their (1) basal metabolic rate ([.H]_{b}), (2) minimum wet thermal
conductance (C_{mw}), (3) diversity of diet (D_{d}), (4) intrinsic
rate of natural increase (r_{max}), and, when data were available, (5)
capacity for evaporative cooling (E_{c}). In a genetic sense each one
of these variables is a complex adaptive characteristic, expression of
which is determined by the interaction of several genes (Prosser,
1986:110-165). Experience has shown that a given species will express
each one of these variables in a specific manner that is relevant to
its mass, physiology, behavior, and environmental circumstance. Thus,
different expressions of these variables may represent specific
climatic adaptations (Prosser, 1986:16) that have been selected-for by
evolutionary process. Because these variables are interrelated with
respect to regulation of body temperature and energy balance, they
have co-evolved in each species to form an adaptive unit. For each
species, measured and calculated values for the first four variables
were converted into dimensionless numbers and used to derive a
composite score that represented its adaptive unit. Climatic
distributions of these species were then compared relative to their
composite scores.
ADAPTIVE SIGNIFICANCE OF THE VARIABLES
_Basal Metabolic Rate and Intrinsic Rate of Natural Increase_
Basal metabolic rate represents the minimum energy required by an
animal to maintain basic homeostasis (Lusk, 1917:141; Kleiber, 1932,
1961:251; Benedict, 1938; Brody, 1945:59; Robbins, 1983:105-111). For
mammals, [.H]_b appears to be determined by complex interactions
between their body size (Kleiber, 1932, 1961:206; Benedict, 1938;
Brody, 1945:368-374; Hemmingsen, 1960:15-36; McNab, 1983b; Calder,
1987), the climate in which they live (Scholander et al., 1950c; McNab
and Morrison, 1963; Hulbert and Dawson, 1974; Shkolnik and
Schmidt-Nielsen, 1976; McNab, 1979a; Vogel, 1980), their food habits
(McNab, 1978a, 1978b, 1980a, 1983a, 1984a, 1986a, 1986b, 1988a, 1989),
and their circadian period (Aschoff and Pohl, 1970; Prothero, 1984).
Some species have higher mass-specific [.H]_{b} than others, and this
variation appears to be tied to ecological circumstances rather than
taxonomic affinities (McNab, 1988a, 1989). Basal metabolic rate is
important ecologically because it serves as a measure of a species'
minimum "obligatory" energy requirement, and under many circumstances,
it represents the largest energy demand associated with a daily energy
budget (King, 1974:38-55; McNab, 1980a; Mugaas and King, 1981:37-40).
Recently it also has been implicated as a permissive factor with
respect to r_{max} of mammals (Hennemann, 1983; Lillegraven et al.,
1987; Nicoll and Thompson, 1987; Thompson, 1987) via its direct effect
on their rates of development and fecundity (McNab, 1980a, 1983a,
1986b; Hennemann, 1983; Schmitz and Lavigne, 1984; Glazier, 1985a,
1985b). The implication of this latter point is that those species
with higher [.H]_{b}'s also have faster rates of development and
greater fecundity and hence enjoy the competitive advantage of a
higher r_{max}. Basal metabolism is, therefore, "a highly plastic
character in the course of evolution" (McNab, 1988a:25) that has a
profound influence on each species' life history.
_Minimum Thermal Conductance_
Whole-body resistance to passive heat transfer is equal to tissue
resistance plus coat resistance. Within limits, these resistances can
be altered; tissue resistance can be varied by changes in blood flow,
whereas coat resistance can be changed by piloerection, molt, and
behavior. When whole-body resistance is maximized (maximum tissue and
coat resistances), passive heat transfer is minimized. The inverse of
resistance is conductance; therefore, maximum whole-body resistance is
the inverse of minimum thermal conductance (C_{m}). Minimum thermal
conductance is readily derived from metabolic chamber data, and it is
commonly used to describe an animal's capacity to minimize passive
heat transfer. Minimum thermal conductance interacts with [.H]_{b} and
body mass to set the maximum temperature differential a mammal can
maintain without increasing its basal level of heat production. The
low temperature in this differential is the lower critical temperature
(T_{lc}).
Mass-specific C_{m} for mammals is negatively correlated with body
mass (McNab and Morrison, 1963; Herreid and Kessel, 1967; McNab, 1970,
1979b; Bradley and Deavers, 1980; Aschoff, 1981), and for any given
mass its magnitude is 52% higher during the active, rather than the
inactive, phase of the daily cycle (Aschoff, 1981). However, some
mammals have C_{m}'s that are higher or lower than would be predicted
for them on the basis of body mass and circadian phase. Seasonal
variation in C_{m} (higher values during summer than winter) has been
reported for many northern mammals that experience large annual
variations in air temperature (Scholander et al., 1950a; Irving et
al., 1955; Hart, 1956, 1957; Irving, 1972:165). Some tropical mammals
with very thin fur coats, and others with nearly hairless bodies, have
high C_{m}'s (McNab, 1984a), as do burrowing mammals (McNab, 1966,
1979b, 1984a) and the kit fox, _Vulpes macrotis_ (Golightly and
Ohmart, 1983). Some small mammals with low basal metabolic rates tend
to have lower than predicted C_{m}'s: small marsupials (McNab, 1978a),
heteromyid rodents (McNab, 1979a), several ant eaters (McNab, 1984a),
the arctic hare, _Lepus arcticus_ (Wang et al., 1973), the ringtail,
_Bassariscus astutus_ (Chevalier, 1985), and the fennec, _Fennecus
zerda_ (Noll-Banholzer, 1979). Thus, in spite of its mass dependence,
C_{m} also has been modified during the course of evolution by
selective factors in the environment and by the animal's own metabolic
characteristics.
_Capacity for Evaporative Cooling_
Latent heat loss occurs as a result of evaporation from the
respiratory tract and through the skin, and except under conditions of
heat stress, it "is a liability in thermal and osmotic homeostasis"
(Calder and King, 1974:302). E_{c}, defined as the ratio of
evaporative heat lost to metabolic heat produced, can be used to
quantify thermoregulatory effectiveness of evaporative cooling and to
make comparisons of heat tolerance between species. Thermoregulatory
effectiveness of latent heat loss is not just a function of the rate
of evaporative water loss but also of the rate of metabolic heat
production (Lasiewski and Seymour, 1972). For example, a low metabolic
rate minimizes endogenous heat load and thus conserves water, whereas
the opposite is true of high metabolic rates (Lasiewski and Seymour,
1972). Some mammals that live in arid regions have evolved low
metabolic rates and thus capitalize on this relationship to reduce
their thermoregulatory water requirement (McNab and Morrison, 1963;
McNab, 1966; MacMillen and Lee, 1970; Noll-Banholzer, 1979). What is
evident, therefore, is that an animal's capacity for increasing latent
heat loss must evolve together with its [.H]_{b} and C_{m} in response
to specific environmental demands.
_Diet_
McNab (1986a, 1988a, 1989) demonstrated that, for mammals, departures
of [.H]_{b} from the Kleiber (1961:206) "norm" are highly correlated
with diet and independent of phylogenetic relationships. McNab's
analysis indicates that for mammals that feed on invertebrates, those
species with body mass less than 100 g have [.H]_{b}'s that are equal
to or greater than values predicted by the Kleiber equation, whereas
those with body mass greater than 100 g have metabolic rates that are
lower than predicted. Grazers, vertebrate eaters, nut eaters, and
terrestrial frugivores also have [.H]_{b}'s that are equal to or
greater than predicted, whereas insectivorous bats, arboreal
folivores, arboreal frugivores, and terrestrial folivores all have
rates that are lower than predicted. McNab (1986a) found animals with
mixed diets harder to categorize, but in general he predicted that
their [.H]_{b}'s would be related to (1) a food item that is
constantly available throughout the year, (2) a food item that is most
available during the worst conditions of the year, or (3) a mix of
foods available during the worst time of the year. Although these
correlations do not establish cause and effect between food habits and
[.H]_{b}, McNab's analysis does make it clear that the relationship
between these variables has very real consequences for an animal's
physiology, ecology, and evolution.
EXPERIMENTAL DESIGN AND SUMMARY
In this investigation we measured basal and thermoregulatory
metabolism, evaporative water loss, and body temperature of raccoons
from north central Virginia. Measurements were conducted on both sexes
in summer and winter to determine how season and sex influenced these
variables. We then compared the data for this widely distributed
generalist with data from literature for its ecologically more
restricted relatives. Dietary data for all species were taken from
literature, as were reproductive data for calculation of r_{max}.
Our analysis demonstrated clear differences between _Procyon lotor_
and other procyonids with respect to [.H]_{b}, C_{mw}, D_{d}, and
r_{max}. The composite score calculated from these variables for
_Procyon lotor_ was much higher than those derived for other species,
and there was a positive correlation between the number of climates a
species occupies and the magnitude of its composite score. Data on
evaporative water loss, although not complete for all species,
suggested that tropical and subtropical procyonids have less capacity
for evaporative cooling than _Procyon lotor_ or _Bassariscus astutus_.
It was clear, therefore, that with respect to its thermal physiology,
_Procyon lotor_ differed markedly from other procyonids, and we
contend that these differences have allowed this species to become a
highly successful climate generalist and to expand its distribution
into many different habitats and climates. Our analysis also suggested
that the cornerstone of _Procyon lotor_'s success as a climate
generalist is its [.H]_{b}, which is higher than the procyonid norm.
ACKNOWLEDGMENTS
The authors would like to thank John Eisenberg and Devra Kleiman for
their support and encouragement throughout the study. This
investigation was supported by research grants from the West Virginia
School of Osteopathic Medicine (WVSOM), and Friends of the National
Zoo (FONZ). Logistic support was provided by the National Zoological
Park's Conservation and Research Center (CRC), and the departments of
Mammalogy and Zoological Research. Our ability to conduct
physiological research at CRC was made possible by the thoughtful
support and encouragement provided by Chris Wemmer. His excellent
staff at CRC, especially Jack Williams, Junior Allison, and Red
McDaniel, were very helpful in providing hospitality and logistical
support to the senior author and his family during their various
visits to the Center. The assistance of several people at the National
Zoo also is gratefully acknowledged: Mitch Bush and Lyndsay Phillips
not only provided veterinary support throughout the investigation,
but also performed surgical procedures required to implant
temperature-sensitive radio transmitters in several raccoons; Olav
Oftedal made his laboratory available to us at various times and
loaned us equipment to use at CRC; Miles Roberts and his staff
provided care for our captive raccoons in the Department of Zoological
Research during various parts of the investigation. Greg Sanders and
Ken Halama, supported by FONZ assistantships, cared for our captive
raccoons at CRC, provided assistance in the laboratory whenever
needed, and were an invaluable source of aid. Their friendship and
help is gratefully acknowledged. Ellen Broudy and Andy Meyer,
supported by WVSOM and a student work study grant, respectively,
provided assistance in the laboratory. David Brown, John Eisenberg,
Mary Etta Hight, Brian McNab, Steve Thompson, and W. Chris Wozencraft
critically reviewed various phases of the manuscript and provided many
helpful suggestions. We deeply appreciate the work of Jean B.
McConville, whose beneficial editorial suggestions helped us improve
several early versions of the manuscript. We also gratefully
acknowledge Diane M. Tyler, our editor at the Smithsonian Institution
Press, whose expertise helped us mold the manuscript into its final
form. Jill Mellon and Sriyanie Miththalapa, supported by FONZ
traineeships, assisted in measuring the daily cycle of body
temperature in raccoons. The Virginia Commission of Game and Inland
Fisheries gave us permission to use wild-caught raccoons in this
project.
$Materials and Methods$
LIVE-TRAPPING
Raccoons were caught from May 1980 through December 1984 on a trapping
grid of 30 to 35 stations (one or two "live traps" per station) that
covered about one-third of the National Zoological Park's Conservation
and Research Center (CRC) near Front Royal, Virginia (Seidensticker et
al., 1988; Hallett et al., 1991). Animals were trapped during 10
consecutive days each month, and in this five-year interval 407
raccoons were captured and marked with tattoos and ear tags. All
captured animals were individualized with respect to age, reproductive
status, physical condition, parasite load, and mass and body
dimensions. These data characterized the structure and dynamics of the
raccoon population at CRC and provided information on the annual
cycle of fattening for raccoons in north central Virginia.
Animals used for metabolic measurements were captured at CRC about
1.5 km south of the trapping grid and thus were genetically
representative of the area. Six males were captured and measured during
the summer of 1983. These animals were kept isolated for a week before
being measured and were released later that summer at the site of their
capture. The other seven animals used in our study were from the
collection of the National Zoological Park and all of them had their
origins at CRC.
METABOLIC STUDIES
_Basal and Thermoregulatory Metabolism_
Metabolic measurements, conducted at CRC, were carried out on eight
males during July and August 1983, on four females and three males
from November 1983 through March 1984, and on four females during June
and July 1984.
Raccoons were housed throughout the study such that they were
constantly exposed to a natural cycle of temperature and photoperiod.
Weather records for the Front Royal area indicate that average
temperatures are around -0.5°C in January and 23.3°C in July
(Crockett, 1972). Light:dark (L:D) periods for the latitude of CRC
(48°55'N; United States Department of the Interior Geological Survey,
1972), calculated from duration of daylight tables (List, 1971:506-512),
were 14.9:9.1 and 9.4:14.6 hours L:D for summer and winter solstices,
respectively, and 12.2:11.8 hours L:D for vernal and autumnal equinoxes.
Our animals were fed a measured amount of food daily, and they usually
ate most of what was provided. Occasionally these animals would eat
very little or none of their ration, and on some days they would eat
all that was given to them. We fed them either feline diet (ground
horse meat) or canned mackerel (Star-kist(R)[1]) along with
high-protein dog chow (Purina(R)). When available, fresh fruit also
was added to their diet. Water was always provided ad libitum.
[1] _The use of product brand names in this publication is
not intended as an endorsement of the products by the
Smithsonian Institution._
Measurements were conducted during the raccoons' daily inactive period
(sunrise to sunset) in both summer and winter. Oxygen consumption was
measured in a flow-through metabolism chamber at 5°C intervals from
-10°C to 35°C. Animals were held at each temperature until the lowest
rate of oxygen consumption had been obtained and maintained for at
least 15 minutes. During each determination, oxygen consumption was
monitored for 30 minutes to one hour beyond a suspected minimum value
to see if an even lower reading could be obtained. Raccoons attained
minimum levels of oxygen consumption more quickly at warm (>10°C) than
at cold temperatures. Depending on the temperature, therefore, each
measurement took from two to five hours to complete. On days when two
measurements could be completed, the second trial was always at a
temperature 10°C warmer than the first.
The metabolism chamber was constructed from galvanized sheet metal
(77.5 × 45.5 × 51.0 cm = 180 liters) and was painted black inside.
Within the chamber, the animal was held in a cage (71 × 39 × 33 cm)
constructed from turkey wire that also was painted black. This cage
prevented the raccoons from coming into contact with the walls of the
chamber, yet it was large enough to allow them to stand and freely
move about. The bottom of the cage was 11 cm above the chamber floor,
which was covered to a depth of one cm with mineral oil to trap urine
and feces.
During measurements, the metabolism chamber was placed in a
controlled-temperature cabinet (modified Montgomery Ward model 8969
freezer). Air temperature (T_{a}) in the metabolism chamber was
regulated with a Yellow Springs Instrument model 74 temperature
controller. T_{a} was controlled to ±1.0°C at temperatures below
freezing, and to ±0.5°C at temperatures above freezing. The chamber
air and wall temperatures were recorded continuously (Linseis model
LS-64 recorder) during each experiment, and, except during temperature
changes, they were always within 0.5°C of each other.
Columns of Drierite(R) and Ascarite(R) removed water vapor and carbon
dioxide, respectively, from air entering and leaving the chamber. Dry
carbon-dioxide-free room air was pumped into the chamber (Gilman model
13152 pressure/vacuum pump) at a rate of 3.0 L/min (Gilmont model
K3203-20 flow meter). Downstream from the chemical absorbents, an
aliquot (0.1 L/min) of dry carbon-dioxide-free air was drawn off the
chamber exhaust line and analyzed for oxygen content (Applied
Electrochemistry model S-3A oxygen analyzer, model 22M analysis cell,
and model R-1 flow control). All gas values were corrected to standard
temperature and pressure for dry gas. Oxygen consumption was
calculated from the difference in oxygen content between inlet and
outlet air using Eq. 8 of Depocas and Hart (1957).
Each raccoon was fasted for at least 12 hours before oxygen
consumption measurements began. At the start and end of each metabolic
trial the animal was weighed to the nearest 10 g (Doctors Infant
Scale, Detecto Scales, Inc., Brooklyn, N.Y., U.S.A.). The body mass
used in calculating minimum oxygen consumption and evaporative water
loss was estimated from timed extrapolations of the difference between
starting and ending weights, and the time at which these variables
were measured.
_Evaporative Water Loss_
During metabolic measurements at temperatures above freezing,
evaporative water loss was determined gravimetrically. Upstream from
the chemical columns, an aliquot of air (0.1 L/min) was drawn off the
exhaust line and diverted for a timed interval through a series of
preweighed (0.1 mg) U-tubes containing Drierite(R). The aliquot then
passed through a second series of U-tubes containing Ascarite(R)
before entering the oxygen analysis system. Evaporative water loss was
calculated using Eq. 1
[.E] = (m_{w}·[.V]_{e})/([.V]_{a}·t·m) Eq. 1
where [.E] is evaporative water loss (mg·g^{-1}·h^{-1}), m_{w} is mass
of water collected (mg), [.V]_{e} is rate of air flow into the chamber
(3.0 L/min), [.V]_{a} is rate of air flow through the U-tubes
(0.1 L/min), t is length of the timed interval (h), and m is the
estimated mass of the raccoon at the time of sampling (g).
_Body Temperature_
Veterinarians at the National Zoological Park surgically implanted
calibrated temperature-sensitive radio transmitters (Telonics, Inc.,
Mesa, AZ, U.S.A.) into abdominal cavities of two female and two male
raccoons. Transmitter pulse periods were monitored with a digital
processor (Telonics TDP-2) coupled to a receiver (Telonics
TR-2-164/166). During some metabolic measurements, body temperatures
of these animals were recorded to the nearest 0.1°C at 30-minute
intervals. The daily cycle of body temperature of these raccoons also
was measured once a month.
CALIBRATIONS
_Calorimeter_
At the conclusion of these experiments, the accuracy of our
calorimetry apparatus was tested by burning an ethanol lamp in the
metabolism chamber. During these tests a CO_{2} analyzer was
incorporated into the system (Beckman, LB-2). Results demonstrated
that we measured 84% of the oxygen consumed by the lamp as well as 84%
of the water and CO_{2} it produced; standard deviation = ±2.6, ±5.0,
and ±3.6, respectively (n = 27). Average respiratory quotient (RQ)
calculated from these data was O.657 ±0.008 (n = 27), which is 99.5%
of that predicted (0.66). McNab (1988b) reports that the accuracy of
open-flow indirect calorimetry systems, such as ours, depends on the
rate of air flow through the animal chamber. If flow rates are too
low, there is inadequate mixing of air within the chamber, and the
rate of oxygen consumption, as calculated from the difference in
oxygen content of air flowing into and out of the chamber (Depocas and
Hart, 1957), is underestimated. At some critical rate of air flow,
which is unique to each combination of chamber and animal, this
situation changes such that measured rates of oxygen consumption
become independent of any further increase in flow rate (McNab,
1988b). In recent tests of our system, where we burned the ethanol
lamp at a variety of chamber flow rates, the efficiency of
measurement increased linearly as flow rate increased, and the
critical rate of air flow was about 6.7 L/min. This appeared to
explain why a flow rate of 3.0 L/min underestimated oxygen consumption
of the ethanol lamp.
Our earlier tests of the efficiency of our system indicated that
although we underestimated actual oxygen consumption of the ethanol
lamp, we did so with a fair degree of precision; probably because flow
rates were closely controlled. During our metabolic measurements,
chamber flow rates also were closely controlled at 3.0 L/min, and we
believe, therefore, that these measurements also were carried out with
a high degree of precision. Consequently, all measured values of
oxygen consumption and water production were considered to be 84% of
their actual value and were adjusted to 100% before being included in
this report.
_Body Temperature Transmitters_
The calibration of all temperature-sensitive radio transmitters
drifted over time. Transmitters were calibrated before they were
surgically implanted and again after they were removed from the
animals. Although the drift of each transmitter was unique, it was
also linear (S. Tomkiewicz, Telonics, Inc., pers. com.). All body
temperature measurements were corrected from timed extrapolations of
the difference between starting and ending calibrations.
STATISTICAL METHODS
Values of oxygen consumption, evaporative water loss, and body
temperature were plotted as a function of chamber air temperature.
Linear regressions of oxygen consumption at temperatures below the
thermoneutral zone (T_{n}), and evaporative water loss at temperatures
above freezing, were determined with the SAS (1982) GLM procedure.
Lower critical temperature (T_{lc}) was determined graphically from
intersection of the line representing [.H]_{b} and the regression line
representing oxygen consumption below T_{n}. Slopes and intercepts of
regression lines, as well as other mean values, were compared with
_t_-tests (Statistical Analysis System, 1982; Ott, 1984:138-175).
Unless indicated otherwise, data are expressed as mean ± standard
deviation (s.d.).
ESTIMATING INTRINSIC RATE OF NATURAL INCREASE
We employed the method first described by Cole (1954) to calculate
r_{max}:
1 = e^{-r_{max}} + b·e^{-r_{max}(a)} - b·e^{r_{max}(n+1)} Eq. 2
where a is potential age of females first producing young, b is
potential annual birth rate of female young, and n is potential age of
females producing their final young. After life-history data were
substituted into Eq. 2, r_{max} was determined by trial and error
substitution (Hennemann, 1983).
Because r_{max} represents the genetically fixed, physiologically
determined maximum possible rate of increase, data on earliest
possible age of female reproduction, highest possible birth rate of
female young, and longest possible female reproductive life span were
used for a, b, and n, respectively. Calculated values, therefore,
represent physiologically possible, not ecologically possible,
intrinsic rates of increase (Hennemann, 1983, 1984; Hayssen, 1984;
McNab, 1984b). Values of n were derived from longevity records for
captive animals, and as these were all large values of similar
duration (14-16 years), they had very little effect on r_{max}. All
species considered have one litter per year, and because their sex
ratios at birth are about 50:50, variation in b was due to differences
in litter size. Therefore, age of first reproduction and litter size
had the greatest effect on r_{max}. Intrinsic rate of increase scales
to body mass (Fenchel, 1974), and we removed this effect by comparing
each calculated r_{max} with the value expected (r_{maxe}) on the
basis of body mass (Hennemann, 1983).
COMPARISON OF ADAPTIVE UNITS
Dimensionless numbers for each of the four variables used in
calculating composite scores were derived as follows. Ratios of
measured to predicted values were used for basal metabolism (H_{br})
and minimum wet thermal conductance (C_{mwr}). Thermoregulatory
ability at low temperatures is closely related to the ratio
H_{br}/C_{mwr} (McNab, 1966). This ratio was used, therefore, to gauge
each species' cold tolerance. For D_{d} we used the ratio of food
categories actually used by a species to the total number of food
categories taken by all species tested (D_{dr}). The ratio of
calculated to expected intrinsic rates of natural increase was used to
derive r_{maxr}. Composite scores were calculated as
Composite score = [(H_{br}/C_{mwr}) + D_{dr} + r_{maxr}]/3 Eq. 3
The correlation between number of climates these species occupy and
their composite scores was tested by linear regression.
$Results$
BODY MASS
According to monthly live-trapping records, the body mass of
free-ranging female raccoons increased from 3.6 ±0.6 kg during summer
to 5.6 ±0.8 kg in early winter, and the mass of free-ranging males
increased from 4.0 ±0.5 to 6.7 ±0.9 kg during the same interval. These
seasonal changes in body mass were due to fluctuations in the amount
of body fat and represent a mechanism for storing energy during fall
for use in winter. In summer, captive and trapped male and captive
female raccoons had the same body mass (4.73 ±0.61, 4.41 ±0.70, and
4.67 ±0.88 kg, respectively, Table 2). Mass of captive females did
not change between seasons, whereas captive males were heavier in
winter than summer (p<0.005; Table 2). This seasonal change in mass of
our captive males was of a much smaller magnitude (0.6 kg) than that
observed for wild males (2.7 kg). During winter, captive males (5.34
±1.39 kg) were heavier than captive females (4.49 ±0.98 kg; p<0.005;
Table 2). Thus, our captive animals maintained a body mass throughout
the year that was intermediate to the range of values found for wild
raccoons in the same area.
TABLE 2.--Body mass in kg and basal metabolism
(mL O_{2}·kg^{-0.75}·h^{-1}) of _Procyon lotor_
in summer and winter (s.d. = standard deviation
and n = number of observations).
----------------+-----------------------------------------------------
Season and sex | Body mass, ±s.d., (n) Basal metabolism, ±s.d., (n)
----------------+-----------------------------------------------------
Summer |
Trapped male | 4.41 ±0.70 (52) 780 ±112 (20)
Captive male | 4.73 ±0.61 (22) 680 ±102 (8)
Captive female| 4.67 ±0.88 (41) 618 ± 92 (13)
Winter |
Captive male | 5.34 ±1.39 (31) 704 ± 81 (19)
Captive female| 4.49 ±0.98 (42) 667 ±139 (25)
----------------+-----------------------------------------------------
BASAL METABOLIC RATE
Within thermoneutrality, [.H]_{b} (mL O_{2}·g^{-1}·h^{-1}) was
0.54 ±0.09 for trapped males in summer, 0.46 ±0.07 for captive males
in summer, 0.42 ±0.07 for captive females in summer, 0.47 ±0.06 for
captive males in winter, and 0.46 ±0.10 for captive females in winter
(Figures 2, 3). Ratios of these measured values to those predicted by
the Kleiber (1932, 1961:206) equation are 1.28, 1.12, 1.02, 1.17, and
1.09, respectively. To minimize the effect of body size (Mellen, 1963)
and to facilitate comparisons between sexes and seasons and between
captive and trapped animals, basal metabolism also was calculated as a
function of metabolic body size (mL O_{2}·kg^{-0.75}·h^{-1}; Table 2).
Based on this analysis, trapped summer males had a higher basal
metabolism than captive males (p<0.025) or females (p<0.005) in either
season (Table 2). There was no difference in basal metabolism between
captive males and females in either summer or winter, and there was no
seasonal difference in their basal metabolic rates (Table 2).
MINIMUM THERMAL CONDUCTANCE
Minimum wet and dry thermal conductances were calculated using Eqs. 4
and 5
C_{mw} = [.H]_{r} / (T_{b} - T_{a}) Eq. 4
C_{md} = ([.H]_{r} - [.E]_{eq}) / (T_{b} - T_{a}) Eq. 5
where C_{mw} is wet and C_{md} is dry conductance
(mL O_{2}·g^{-1}·h^{-1}·°C^{-1}); [.H]_{r} is the lowest resting
metabolic rate measured at each temperature (mL O_{2}·g^{-1}·h^{-1});
[.E]_{eq} is oxygen equivalent for heat lost by evaporation
[[.E]_{eq} = mL O_{2}·g^{-1}·h^{-1} = [.E]·[lambda]/[gamma], where
[.E] is evaporative water loss (mg·g^{-1}·h^{-1}), [lambda] is heat of
vaporization for water (2.43 J/mg), and [gamma] is heat equivalent for
oxygen (20.097 J/mL)]; T_{b} is body temperature (°C); and T_{a} is
chamber air temperature (°C). Only data from animals equipped with
temperature-sensitive radio transmitters were used for these
calculations.
TABLE 3.--Minimum wet and dry thermal conductances
(mL O_{2}·g^{-1}·h^{-1}·°C^{-1}) of _Procyon lotor_ in summer
and winter. Means of values were calculated from equations 3
and 4 (s.d. = standard deviation and n = number of observations).
----------------------+----------------------------------------
| Thermal conductance
Season and sex |----------------------------------------
| Wet ±s.d. (n) Dry ±s.d. (n)
----------------------+----------------------------------------
Summer |
Captive, both sexes | 0.0256 ±0.0028 (18) 0.0246 ±0.0019 (12)
Winter |
Captive, female | 0.0172 ±0.0023 (10) 0.0161 ±0.0027 (6)
----------------------+----------------------------------------
[Illustration: FIGURE 2.--Relationship between oxygen consumption
and chamber air temperature for raccoons in summer: captive
females, open circles; captive males, closed circles; trapped
males, open squares. Sloping lines represent regressions of oxygen
consumption on chamber air temperature, and horizontal lines, basal
metabolism.]
[Illustration: FIGURE 3.--Relationship between oxygen consumption
and chamber air temperature for raccoons in winter: captive
females, open circles; captive males, closed circles. Solid sloping
line represents regression of oxygen consumption on chamber air
temperature for males and females, and the horizontal line, basal
metabolism for males and females.]
C_{mw} was calculated for each season from metabolic measurements made
at all air temperatures below T_{lc} (Table 3). Because evaporative
water loss was not measured at temperatures below freezing, C_{md}
was calculated only from metabolic determinations made at air
temperatures between T_{lc} and 0°C. There was no difference
between males and females in summer for either C_{mw} or C_{md}
(mL O_{2}·g^{-1}·h^{-1}·°C^{-1}). Data for each sex were combined
to give a summer average of 0.0256 ±0.0028 for C_{mw}, and 0.0246
±0.0019 for C_{md} (Table 3). These summer conductances were 49% higher
(p<0.005) than those calculated for winter females (0.0172 ±0.0023, and
0.0161 ±0.0027 for C_{mw} and C_{md}, respectively; Table 3). C_{mw} and
C_{md} were not different from each other in either summer or winter,
which indicated that in both seasons evaporative water loss contributed
very little to heat dissipation at temperatures below T_{n}. Comparisons
of thermal conductances calculated on the basis of metabolic body size
(Mellen, 1963) gave the same results.
EVAPORATIVE WATER LOSS
Evaporative water loss increased as chamber temperature increased in
both summer and winter (Figures 4, 5). In summer, the pattern of
increase was different for females and males. Polynomial regressions
for trapped and captive males produced equations that describe a
concave relationship between T_{a} and evaporative water loss, whereas
the equation for females describes a sigmoid curve (Table 4; Figure 4).
For females, water loss increased rapidly at temperatures above 25°C
(Figure 4). The intercepts and coefficients of the X, X², and X³ terms
of the polynomial regression equations (Table 4) were compared
(_t_-tests) to determine if they differed from each other. The
coefficients in the equation for trapped males differed from those for
captive females in the X² (p<0.05) and X³ (p<0.025) terms. The
intercept and coefficients of the equation for captive males, however,
were not different from those for either captive females or trapped
males. Although this lack of difference is understandable in the case
of trapped males, where the shape of the two curves is similar
(concave), it is not so clear for the sigmoid curve of captive females
(Figure 4). Perhaps the lack of difference in this case is simply due
to the small number of observations available for captive males (n = 10;
Table 4). Nonetheless, in summer at 35°C, both captive and trapped
males relied less on evaporative cooling than did captive females
(Figure 4).
In winter, males and females had similar rates of evaporative water
loss across the full range of temperatures tested (Figure 5).
Therefore, data for both sexes were combined. The intercept and
coefficients of this equation (Table 4) did not differ from those for
summer females, but they did differ from those in the regression for
trapped males in the X² (p<0.05) and X³ (p<0.025) terms. As was
the case for females in summer, rates of water loss for winter animals
increased most rapidly at temperatures above 25°C (Figure 5).
[Illustration: FIGURE 4.--Relationship between evaporative water
loss and chamber air temperature for raccoons in summer: captive
females, open circles; captive males, closed circles; trapped
males, open squares. Lines represent polynomial regressions of
evaporative water loss on chamber air temperature.]
[Illustration: FIGURE 5.--Relationship between evaporative water
loss and chamber air temperature for raccoons in winter: captive
females, open circles; captive males, closed circles. Lines
represent polynomial regressions of evaporative water loss on
chamber air temperature.]
TABLE 4.--Polynomial regression equations describing evaporative
water loss (mg·g^{-1}·h^{-1}) of _Procyon lotor_ in summer and
winter (X = chamber temperature (°C), Y = evaporative water loss,
n = number of observations, R² = coefficient of determination, and
SEE = standard error of estimate).
--------------+--------------------------------------------------------
Season and sex| Equation (n) R²
--------------+--------------------------------------------------------
Summer |
Trapped male |Y = 0.1899 + 0.0114·X + 0.0011·X² - 0.00002·X³ (32) 0.86
SEE | 0.0885 0.0223 0.0015 0.00003
Captive male |Y = 0.2174 + 0.0192·X + 0.0009·X² - 0.00003·X³ (10) 0.73
SEE | 0.3983 0.0834 0.0048 0.00008
Captive |
female |Y = 0.0127 + 0.0943·X - 0.0060·X² + 0.00013·X³ (31) 0.64
SEE | 0.2218 0.0547 0.0036 0.00006
Winter |
Captive, |
both sexes |Y = 0.1550 + 0.0426·X - 0.0025·X² + 0.00006·X³ (57) 0.80
SEE | 0.0734 0.0192 0.0013 0.00002
--------------+--------------------------------------------------------
THERMOREGULATION AT LOW TEMPERATURES
_Body Temperature_
Body temperatures in Figure 6 are those recorded during metabolic
measurements from animals equipped with surgically implanted,
temperature-sensitive radio transmitters. Each point was recorded
during the lowest level of oxygen consumption at each T_{a}. In both
summer and winter, T_{b}'s were lowest during metabolic measurements
at T_{a}'s around T_{lc}. At T_{a}'s below T_{lc}, T_{b}'s increased
(Figure 6), which is an unusual response. Under similar conditions,
other procyonids either maintain a nearly constant T_{b} or allow it
to fall slightly (Müller and Kulzer, 1977; Chevillard-Hugot et al.,
1980; Müller and Rost, 1983; Chevalier, 1985). For our raccoons,
confinement in the metabolism chamber at low temperatures must have
stimulated a greater than necessary increase in metabolic rate such
that heat production exceeded heat loss, which caused T_{b} to become
elevated.
[Illustration: FIGURE 6.--Relationship between body temperature and
chamber air temperature in summer (panel A), and winter (panel B):
captive females, open circles and solid lines; captive males, solid
circles and dashed lines. Solid vertical lines represent lower
critical temperatures.]
TABLE 5.--Regression equations describing oxygen consumption
(mL O_{2}·g^{-1}·h^{-1}) of _Procyon lotor_ at temperatures below
their lower critical temperature (I = x-intercept (°C), n = number
of observations, R² = coefficient of determination, SEE = standard
error of estimate for the y-intercept (a) and slope (b), X = chamber
temperature (°C), and Y = oxygen consumption).
----------------+------------------------------------------------------
Season | SEE
and sex | -----------
| Equation (n) R² a b I
----------------+------------------------------------------------------
Summer |
Trapped male | Y = 1.09 - 0.0281·X (30) 0.64 0.0353 0.0040 38.8
Captive male | Y = 0.97 - 0.0258·X (12) 0.91 0.0235 0.0025 37.6
Captive female| Y = 1.04 - 0.0251·X (29) 0.78 0.0288 0.0026 41.1
Winter |
Captive, |
both sexes | Y = 0.68 - 0.0193·X (36) 0.68 0.0157 0.0023 35.2
----------------+------------------------------------------------------
_Summer_
During summer, T_{lc} for male raccoons was 20°C, whereas for females
it was 25°C (Figure 2). Regression equations calculated to describe
oxygen consumption at T_{a}'s below T_{lc} are presented in Table 5.
For three groups of summer animals, slopes of regressions are
identical. This indicates that minimum conductances of these three
groups were equivalent. Intercepts of these equations are different,
which suggests a difference in metabolic cost of thermoregulation
between these groups (Figure 2); captive males had a lower intercept
than either trapped males (p<0.005) or captive females (p<0.05), but
there was no difference in intercepts of captive females and trapped
males. These regression equations, therefore, also were derived using
values of oxygen consumption expressed in terms of metabolic body mass
(Mellen, 1963). Relationships between intercepts of these equations
are different than those for regressions in Table 5. Intercept for
females was intermediate to, and not different from, those of the two
groups of males. However, captive males still had a lower intercept
than trapped males (p<0.025). Thus, in summer, thermoregulatory
metabolism was less expensive for captive than for trapped males, and
in spite of a 5°C difference in their T_{lc}'s (Figure 2), captive
males and females had similar thermoregulatory costs.
Regression lines for three groups of animals in summer extrapolate to
zero metabolism at values equivalent to, or greater than, normal
T_{b}; 38.8°C for trapped males, 37.6°C for captive males, and 41.1°C
for captive females (Table 5). Thus, all three groups had minimized
thermal conductance at T_{a}'s below T_{lc} (Scholander et al., 1950b;
McNab, 1980b). Minimum wet thermal conductance calculated for raccoons
in summer with Eq. 4 (Table 3) is numerically similar to these "slope"
values (Table 5), and it was, therefore, considered to be the best
estimate of C_{mw} for _Procyon lotor_ during that season
(0.0256 mL O_{2}·g^{-1}·h^{-1}·°C^{-1}).
_Winter_
During winter T_{lc} for both sexes decreased to 11°C (Figure 3).
Regression equations of thermoregulatory metabolism for males and
females in winter are not different from each other in either slope or
intercept. These data, therefore, were combined into a single equation
(Table 5). Slope and intercept of this equation are both lower
(p<0.005 and p<0.05, respectively) than those for summer animals
(Table 5). Identical results were obtained from comparisons using
regressions derived from oxygen consumption expressed in terms of
metabolic body mass (Mellen, 1963). Thermoregulatory costs at any
temperature below 20°C were lower for winter than summer animals
(Figures 2, 3).
TABLE 6.--Regression equations describing oxygen consumption
(mL O_{2}·g^{-1}·h^{-1}) of _Procyon lotor_ at temperatures below
their lower critical temperature in winter (A = females with radio
transmitters, B = females without radio transmitters, C = males,
I = x-intercept (°C), n = number of observations, R² = coefficient
of determination, X = chamber temperature (°C), and Y = oxygen
consumption).
-----+--------------------------------------
Group| Equation (n) R² I
-----+--------------------------------------
A | Y = 0.63 - 0.0158·X (10) 0.66 40.1
B | Y = 0.72 - 0.0226·X (11) 0.71 32.1
C | Y = 0.69 - 0.0200·X (15) 0.79 34.7
-----+--------------------------------------
[Illustration: FIGURE 7.--Relationship between body temperature and
time of day at various months of the year: captive females, open
circles; captive males, closed circles. Vertical cross-hatched
areas represent civil twilight.]
The regression line for _Procyon lotor_ in winter (Table 5)
extrapolates to zero metabolism at 35.2°C, which is below normal T_{b}
(Figures 6, 7). This suggests that not all raccoons measured in winter
minimized thermoregulatory metabolism or conductances at T_{a}'s below
T_{lc} (Scholander et al., 1950b; McNab, 1980b). To assess this
possibility, data for these animals were divided into three groups:
(A) females with radio transmitters, (B) females without radio
transmitters, and (C) males (Table 6). Regression equations of
metabolism below T_{lc} were derived for each group, and based on
extrapolated T_{b}'s at zero metabolism, only the two females with
implanted radio transmitters (group A) minimized thermoregulatory
metabolism and conductance. Had animals in groups B and C also
minimized their thermal conductances, while retaining their measured
metabolic rates, their rates of heat production would have been
disproportionately higher than their rates of heat loss. Equation 4
predicts that under these conditions their body temperatures would
have been elevated to 42.0°C and 40.4°C, respectively. Thus, in order
to avoid such a large increase in body temperature, animals in groups
B and C increased their thermal conductances in preference to lowering
their metabolic rates. The regression equation of thermoregulatory
metabolism for all winter animals (Table 5), therefore, overestimates
minimum metabolic cost of temperature regulation below T_{lc}, and its
slope underestimates C_{mw}. Consequently, the best estimate of C_{mw}
for _Procyon lotor_ in winter is the value calculated for group A
animals with Eq. 4 (0.0172 mL O_{2}·g^{-1}·h^{-1}·°C^{-1}; Table 3),
and the minimum cost of thermoregulatory metabolism at any T_{a} below
T_{lc} is best estimated by substituting this value into Eq. 4 and
solving for [.H]_{r}.
THERMOREGULATION AT HIGH TEMPERATURES
_Body Temperature_
In both summer and winter, T_{b}'s increased during metabolic
measurements at T_{a}'s above T_{lc} (Figure 6). This response also
was seen during metabolic measurements conducted on other procyonids
(Müller and Kulzer, 1977; Chevillard-Hugot et al., 1980; Müller and
Rost, 1983; Chevalier, 1985).
_Summer_
During summer our data suggested that the upper critical temperature
(T_{uc}) was higher than 35°C. The lowest rates of oxygen consumption
at T_{a} = 35°C occurred after 1.5 to 2.5 hours of exposure to that
temperature. Prolonged exposure to this temperature in summer did not
make animals restless, and their rate of oxygen consumption was very
stable throughout each measurement. Body temperature responses at
T_{a} = 35°C were recorded from two males and two females that had
implanted radio transmitters. With the exception of one male, T_{b}'s
were maintained near 38°C (Figure 6). The one exception (a male)
maintained its T_{b} at 39.3°C. At T_{a} = 35°C, summer males had
rates of evaporative water loss that were lower than those of summer
females (Figure 4). At this temperature, males dissipated 35% ±6% and
females 56% ±18% of their metabolic heat via evaporative water loss.
Thus, at T_{a} = 35°C, males must have utilized modes of heat transfer
other than evaporative cooling (convective and conductive heat
transfer) to a greater extent than females.
_Winter_
Body temperature, evaporative water loss, and metabolic data indicated
that, in winter, T_{uc} was very close to 35°C. In winter, the lowest
level of oxygen consumption was recorded during the first hour after
the chamber had reached T_{a} = 35°C. Unlike summer, animals became
restless after the first hour at 35°C, at which point their oxygen
consumption increased and showed a high degree of variability. Body
temperature responses at 35°C were recorded from both females that had
implanted radio transmitters. In one case, T_{b} rose from 37.9°C at
the end of the first hour to 40.5°C by the end of the second hour, and
as it did not show signs of leveling off, we terminated the
experiment. We exposed that same animal to T_{a} = 35°C one other time
during winter. In that instance, its T_{b} rose to 40.0°C during the
first 30 minutes and was maintained at that level for three hours with
no apparent distress. The other female elevated its T_{b} from 37.3°C
to 39.0°C during the second hour at T_{a} = 35°C and maintained its
T_{b} at that level for two hours. Thus, during winter, prolonged
exposure to T_{a} = 35°C stimulated more of an increase in T_{b} than
it did in summer. During winter, both males and females increased
evaporative water loss at T_{a} = 35°C (Figure 5) but only to the
extent that they dissipated 35% ±10% of their metabolic heat
production. Thus, even in winter, convective and conductive heat
transfers were still the most important modes of heat loss at this
temperature.
DAILY CYCLE OF BODY TEMPERATURE
The daily cycle of raccoon T_{b}'s during summer and winter are
presented in Figure 7. In general, T_{b}'s showed a marked circadian
cycle in phase with photoperiod. T_{b}'s rose above 38°C for several
hours each night but remained below 38°C during daytime. During
summer, with the exception of one female whose record was not typical
(Figure 7), T_{b}'s rose above 38°C shortly after sunset, whereas in
winter T_{b}'s did not rise above 38°C until several hours after
sunset. Once T_{b} was elevated it usually remained so until just
before or after sunrise (Figure 7). During summer, T_{b} was above
38°C for 85% or more of the time between sunset and sunrise (87% for
the female with the typical body temperature pattern, and 85% and 98%
for males), whereas in winter it was elevated for only 47%-78% of the
time between sunset and sunrise (47% and 61% for females, and 67% and
78% for males). During night, T_{b} would oscillate between 38°C and
about 39°C, such that two peak values occurred. These peak values
presumably corresponded to two periods of heightened nighttime
activity. During summer, one of these peaks occurred before and the
other after 24:00 hours, whereas in winter both peaks occurred after
24:00 hours. With the exception of one female in winter (Figure 7),
the lowest T_{b} of the day for both sexes was near 37°C, and this
typically occurred during daytime (Figure 7).
$Discussion$
BASAL METABOLIC RATE
_Background_
Basal metabolism represents the minimum energy required by a mammal to
maintain endothermy and basic homeostasis (Lusk, 1917:141; Kleiber,
1932, 1961:251; Benedict, 1938:191-215; Brody, 1945:59; Robbins,
1983:105-111). Mammals with lower than predicted [.H]_{b} maintain
endothermy and enjoy its attendant advantages at a discount, whereas
others, with rates that are higher than predicted, pay a premium
(Calder, 1987). Such variation in [.H]_{b} appears to be tied to
ecological circumstances rather than taxonomic affinities (Vogel,
1980; McNab, 1986a, 1988a, 1989), and depending on environmental
conditions, each rate provides an individual with various advantages
and limitations. During the course of evolution, therefore, each
species' [.H]_{b} evolves to provide it with the best match between
its energy requirements for continuous endothermy, its food supply,
and the thermal characteristics of its environment.
_Captive versus Wild Raccoons_
Male raccoons trapped in summer had higher [.H]_{b}'s than our captive
animals in any season (Table 2). The higher rate of metabolism of
these trapped males could have been due to the stress of captivity or
to the fact that "wild" animals actually may have higher metabolic
rates than those that have adjusted to captivity. If the latter is
true, then our data for captive animals underestimated the actual
energy cost of maintenance metabolism for _Procyon lotor_ in the wild.
At present, we have no way of determining which of these alternatives
is true.
_Seasonal Metabolism of Raccoons_
In some temperate-zone mammals, [.H]_{b} is elevated in winter, which
presumably increases their "cold-hardiness." Conversely, lower summer
metabolism is considered to be a mechanism that reduces the potential
for heat stress. Such seasonal variation in [.H]_{b} has been found in
several species: collard peccary, _Tayassu tajacu_ (Zervanos, 1975);
antelope jackrabbit, _Lepus alleni_ (Hinds, 1977); desert cottontail,
_Sylvilagus audubonii_ (Hinds, 1973); and, perhaps, cold-acclimatized
rat, _Rattus norvegicus_ (Hart and Heroux, 1963). Unlike these
species, our captive raccoons showed no seasonal variation in [.H]_{b}
(Table 2). Instead, raccoons achieved "cold-hardiness" in winter and
reduced their potential for heat stress in summer with a large
seasonal change in thermal conductance (Table 3).
TABLE 7.--Metabolic characteristics of several procyonid species.
---------------------+------------------------------------------------
|Body Basal[a] Minimum[b]
Species |mass metabolism conductance T_{b}[c]
|(g) ------------ ------------- -------------
| Meas H_{br} Meas C_{mwr} [alpha] [rho]
---------------------+------------------------------------------------
_Bassariscus astutus_| 865 0.43 0.68 0.0288[e] 0.85 37.6 23
_Procyon cancrivorus_|1160 0.40 0.69 0.0368[e] 1.25
_Potos flavus_ |2030 0.36 0.51
_Potos flavus_ |2400 0.32 0.65 38.1 36.0
_Potos flavus_ |2600 0.34 0.71 0.0200[f] 1.02
_Nasua nasua_ |3850 0.26 0.60 0.0200[f] 1.24 38.3 36.4
_Nasua nasua_ |4847 0.33 0.79 0.0238[e] 1.65 39.1 37.9
_Nasua narica_ |5554 0.25 0.62 0.0208[e] 1.55 38.9 37.4
_Nasua narica_ |4150 0.42 1.20 0.0341[e] 2.20
| 0.0224[g] 1.45
_Procyon lotor_ |
Summer |
Trapped male |4400 0.54 1.28
Captive male |4790 0.46 1.07 0.0256[f] 1.77 38.4 37.5
Captive female |4670 0.42 1.02 0.0256[f] 1.79 38.2 37.6
Winter |
Captive male |5340 0.47 1.17 38.6 37.6
Captive female |4490 0.46 1.10 0.0172[f] 1.15 38.3 37.3
---------------------+------------------------------------------------
---------------------+-----------------------------------------------
|
Species | T_{n}[d]
|---------------
| T_{lc} T_{uc} References
---------------------+-----------------------------------------------
_Bassariscus astutus_| 35.5 Chevalier (1985)
_Procyon cancrivorus_| 26 Scholander et al. (1950b, c)
_Potos flavus_ | McNab (1978a)
_Potos flavus_ | 23 30 Müller and Kulzer (1977)
_Potos flavus_ | 23 33 Müller and Rost (1983)
_Nasua nasua_ | 25 33 Chevillard-Hugot et al. (1980)
_Nasua nasua_ | 30 35 Mugaas et al. (in prep.)
_Nasua narica_ | 25 35
_Nasua narica_ | Scholander et al. (1950b, c)
|
_Procyon lotor_ | This study
Summer |
Trapped male | 20
Captive male | 20
Captive female | 25
Winter |
Captive male | 11
Captive female | 11
---------------------+-----------------------------------------------
[a] Meas is measured basal metabolism (mL O_{2}·g^{-1}·h^{-1}). H_{br}
is the ratio of measured to predicted basal metabolism where the
predicted value is calculated from [.H]_{b} = 3.42·m^{-.25}
(Kleiber, 1932, 1961:206) and m is body mass in grams.
[b] Meas is measured minimum thermal conductance
(mL O_{2}·g^{-1}·h^{-1}·°C^{-1}). C_{mwr} is the ratio of measured
to predicted minimum thermal conductance where the predicted value
is calculated from C_{m} = 1.0·m^{-0.5} (McNab and Morrison, 1963;
Herreid and Kessel, 1967), and m is body mass in grams.
[c] T_{b} is body temperature during the active ([alpha]) and rest
([rho]) phases of the daily cycle (°C).
[d] T_{n} is the thermoneutral zone as defined by the lower (T_{lc})
and upper (T_{uc}) critical temperatures (°C).
[e] Conductance calculated as the slope of the line describing oxygen
consumption at temperatures below the lower critical temperature.
[f] Conductance calculated from C_{mw} = [.H]_{r}/(T_{b} - T_{a}),
where [.H]_{r} is resting metabolic rate at temperatures below
T_{lc}, and other symbols are as described elsewhere.
[g] Inactive-phase thermal conductance: estimated from Scholander et
al. (1950b), assuming that active-phase thermal conductance is 52%
higher than values determined during the inactive phase (Aschoff,
1981).
_Comparison of Procyon lotor with Other Procyonids_
_Procyon lotor_ has a much higher mass-specific [.H]_{b} than other
procyonids (Table 7). To quantify the magnitude of this difference,
we compared the measured value for _Procyon lotor_ with one calculated
for it from a mass-specific least-squares regression equation (Eq. 6;
R² = 0.78) derived from data for those procyonids with lower than
predicted [.H]_{b}: _Potos flavus_, _Procyon cancrivorus_,
_Nasua nasua_, _Nasua narica_, and _Bassariscus astutus_ (Table 7).
[.H]_{b} = 2.39·m^{-0.25} Eq. 6
[.H]_{b} in Eq. 6 is basal metabolism (mL O_{2}·g^{-1}·h^{-1}) and m
is body mass (g). Measured values of [.H]_{b} for _Procyon lotor_ were
1.45 to 1.86 times greater than those predicted for it by Eq. 6
(Table 8).
TABLE 8.--Basal metabolism (mL O_{2}·g^{-1}·h^{-1}) of _Procyon
lotor_ as predicted by Eq. 6 ([.H]_{b} = 2.39·m^{-0.25}). Body
masses, used to calculate predicted values, and measured values
were taken from Table 7.
----------------+--------------------------------
Season and sex | Predicted Measured/Predicted
----------------+--------------------------------
Summer |
Trapped male | 0.29 1.86
Captive male | 0.29 1.59
Captive female| 0.29 1.45
Winter |
Captive male | 0.28 1.68
Captive female| 0.29 1.59
----------------+--------------------------------
_Influence of Diet on Basal Metabolism_
BACKGROUND.--With respect to [.H]_{b}, McNab (1986a:1) maintains that
"the influence of climate is confounded with the influence of food
habits," and that departures from the Kleiber (1961) "norm" are best
correlated with diet. Although this does appear to be the case for
diet specialists, the analysis is not so clear-cut for omnivorous
species (McNab, 1986a). His analysis also indicates that an animal's
"behavior" (i.e., whether it is terrestrial, arboreal, subterranean,
aquatic, etc.), secondarily modifies the influence of food habits on
[.H]_{b}. For example, terrestrial frugivores have [.H]_{b}'s that are
very near predicted values, whereas arboreal frugivores have rates
that are much lower than predicted (McNab, 1986a).
TABLE 9.--Food habits of some Procyonids. References for foods were
as follows: _Potos flavus_, _Procyon cancrivorus_, and _Nasua nasua_
taken from Bisbal (1986); _Nasua narica_ taken from Kaufmann
(1962:182-198); _Bassariscus astutus_ taken from Martin et al.
(1951), Taylor (1954), Wood (1954), Toweill and Teer (1977), and
Trapp (1978); _Procyon lotor_ taken from Hamilton (1936), Stuewer
(1943:218-220), Stains (1956:39-51), and Greenwood (1981). Symbols
represent either qualitative (#) or quantitative (+,|) assessments
of feeding habits: # indicates that the animal was observed eating
the food; + and | represent volume and frequency, respectively, of
food utilization. No attempt was made to account for seasonal
variation in the use of these foods.
+ <20% by volume when found. | 1%-19% frequency of occurrence.
++ >20% by volume when found. || 20%-50% frequency of occurrence.
||| >50% frequency of occurrence.
-----------+--------------------------------------------------------------
|_Potos_ _Procyon_ _Nasua_ _Nasua_ _Bassariscus_ _Procyon_
Food |_flavus_ _cancrivorus_ _nasua_ _narica_ _astutus_ _lotor_
-----------+--------------------------------------------------------------
Mammalia | + | # ++ ||| ++ ||
Aves | ++ | + ||
Birds' eggs| |||
Reptilia | + | + ||| # + | + |
Amphibia | + | # + |
Pices | ++ || ++ ||
Insecta |++ | + ||| ++ ||| # + || ++ ||
Arachnida | ++ ||| # + | + |
Chilopoda | ++ |||
Diplopoda | # + |
Crustacea | ++ ||| # ++ |||
Mollusca | + || # + ||
Annelida | # + |
Nuts | ++ ||
Grains | ++ ||
Buds | + |
Fruit |++ ||| ++ # || ++ |||
Leaves | + |
Grass | + |
-----------+--------------------------------------------------------------
FOOD HABITS OF PROCYONIDS.--Food habits of six procyonids for which
metabolic data are available are presented in Table 9. All six species
clearly have mixed diets. Compared to other species, _Procyon lotor_
is highly catholic in its diet, taking food from almost twice as
many categories as _Nasua narica_, three times as many as _Procyon
cancrivorus_, _Nasua nasua_, and _Bassariscus astutus_, and nine times
as many as _Potos flavus_.
For those species for which food habit data are quantified, we used
Eisenberg's (1981:247-251) substrate/feeding matrix method, where
"substrate" is analogous to McNab's (1986a) "behavior," to construct
the following feeding categories that are based on the major food
groups utilized by each species (Table 9).
1. _Potos flavus:_ (1) arboreal/frugivore, insectivore.
2. _Procyon cancrivorus:_ (1) semiaquatic/crustacivore,
molluscivore, insectivore, piscivore, carnivore.
3. _Nasua nasua:_ (1) terrestrial/insectivore, arachnidivore,
carnivore, frugivore.
4. _Bassariscus astutus:_ (1) terrestrial/carnivore, insectivore,
frugivore.
5. _Procyon lotor:_ (1) terrestrial/carnivore, granivore,
frugivore, insectivore; and (2) semiaquatic/crustacivore,
molluscivore, insectivore, piscivore, carnivore.
FOOD HABITS AND BASAL METABOLISM.--The most important foods in the
diet of _Procyon lotor_ are vertebrates, nuts, seeds, and fruits
(Table 9). These are the same foods that are eaten by those dietary
specialists that have [.H]_{b}'s equivalent to, or higher than, values
predicted for them by the Kleiber equation (McNab, 1986a). The most
important foods in the diets of _Potos flavus_, _Procyon cancrivorus_,
and _Nasua nasua_ are invertebrates and fruit (Table 9), and these
foods are eaten by dietary specialists that have lower than predicted
[.H]_{b}'s (McNab, 1986a). Major foods in the diet of _Bassariscus
astutus_ are terrestrial vertebrates, insects, and fruit (Table 9).
Dietary specialists that eat terrestrial vertebrates have higher than
predicted [.H]_{b}'s, whereas those that feed on insects have [.H]_{b}'s
that are lower than predicted (McNab, 1986a). Year-round utilization of
vertebrates by _Bassariscus astutus_ suggests that it also should have
a metabolic rate that is equivalent to or higher than predicted, rather
than lower (McNab, 1986a). However, perhaps year-round inclusion of
insects in its diet (Martin et al., 1951; Taylor, 1954; Wood, 1954;
Toweill and Teer, 1977; Trapp, 1978), plus water- and energy-conserving
advantages of a low metabolic rate, each exert a stronger selective
influence on [.H]_{b} than do vertebrates in its diet.
SUMMARY.--The basal metabolic rate of these procyonids does appear to
be influenced by diet. But, it is apparent from this family's
evolutionary history and tropical origins that climate also has had a
profound influence on its member's metabolism. The history of the
family and the data presented here (Table 7) suggest that lower than
predicted [.H]_{b} is a feature that evolved very early as the primary
metabolic adjustment to a tropical climate. From this perspective, it
could be argued that climate would have been the major selective force
determining [.H]_{b}, whereas food habits would have had a secondary
influence.
_Basal Metabolism and Intrinsic Rate of Natural Increase_
BACKGROUND.--McNab (1980a) suggested that if food is not restricted
during an animal's reproductive period, the factor that will limit
growth and reproduction will be the rate at which energy can be used
in growth and development. Under these conditions, an increase in
[.H]_{b} would actually increase r_{max} because it would provide a
higher rate of biosynthesis, a faster growth rate, and a shorter
generation time. Hennemann (1983) tested McNab's (1980a) premise and
found a significant correlation between r_{max} and metabolic rate,
independent of body size, for 44 mammal species. A low correlation
coefficient for this relationship, however, indicated to him
(Hennemann, 1983) that factors such as (1) food supply, (2) thermal
characteristics of the environment, and (3) brain size also contribute
toward shaping a species' reproductive potential, particularly when
these factors strongly influence rates of biosynthesis or growth or
for some reason alter generation time. Results of our estimates of
r_{max} for procyonids are presented in Table 10.
_Procyon lotor._--This species had the highest [.H]_{b} and D_{d}, and
also had the highest r_{max} (1.34; Table 10). Such a high r_{max} may
infer that this trait evolved under conditions where food and
temperature were not limiting to reproduction. Under these conditions
selection could have favored those reproductive characteristics
sensitive to a higher [.H]_{b} (biosynthesis, growth, and generation
time; McNab, 1980a). _Procyon lotor_'s high reproductive potential is
due to its early age of first female reproduction and its large litter
size, characteristics that may reflect metabolically driven increases
in both biosynthesis and growth.
_Bassariscus astutus._--This species has a low [.H]_{b} but an r_{max}
that was 124% of expected (Table 10). This suggests that r_{max}
evolved under conditions where food and temperature were not
limiting to reproduction. Reduced litter size should restrict this
species' reproductive potential and may be a reflection of its low
[.H]_{b}. The factor that is responsible for increasing its
reproductive potential, however, is its early age of first female
reproduction. _Bassariscus astutus_ is the smallest of these
procyonids, and even though it has a low [.H]_{b}, its small mass
may contribute to its ability to reach adult size and sexual maturity
in its first year. The high quality of its diet (a high proportion of
small vertebrates; Table 9) also may be a factor that is permissive to
early female reproduction. Thus, small body size and diet may be
factors that have allowed this species to evolve a higher than expected
reproductive potential in spite of its low [.H]_{b}.
_Nasua narica._--This species is one of the largest procyonids
(Table 7), and it possesses characteristics that should limit its
reproductive potential: lower than predicted [.H]_{b} (Table 7), a
relatively low-quality diet (Kaufmann, 1962:182-198; Table 9), and
delayed time of first reproduction (Table 10). In spite of this,
_Nasua narica_ has a higher than expected r_{max} (111% of predicted;
Table 10). The life history feature that enhances _Nasua narica_'s
reproductive potential, and increases r_{max} beyond expected, is its
large litter size. In this species females live in bands. Each year
just before their young are born these bands break up, and each female
seeks out a den for herself and her litter. Once the young are able to
leave the den (approximately five weeks), bands reform. In this
situation, females not only care for their own young but also for those
of other females in the band (Kaufmann, 1962:157-159, 1982, 1987;
Russell, 1983). This social structure may contribute to this species'
ability to produce large litters and in this way increase its
reproductive potential.
TABLE 10.--Intrinsic rate of natural increase (r_{max}) of several
procyonids. (a = potential age of females producing first young;
b = potential annual birth rate of female young (= average litter
size/2; average litter size was calculated from the published range
of litter sizes for each species); n = potential age of females
producing their final young; r_{maxe} = intrinsic rate of natural
increase expected from body mass (Hennemann, 1983); r_{maxr} = ratio
of calculated to expected intrinsic rate of natural increase
(r_{max}/r_{maxe}).)
---------------------+------------------------------------------------
|
Species |Body mass a b n r r [a] r [b]
| (g) max maxe maxr
---------------------+------------------------------------------------
_Procyon lotor_ | 4940 0.83 2.25 16 1.34 0.53 2.52
|
|
|
_Bassariscus astutus_| 900 0.83 1.50 14 1.02 0.82 1.24
|
|
|
_Nasua narica_ | 3900 2.50 2.25 14 0.62 0.56 1.11
|
_Nasua nasua_ | 3850
_Procyon cancrivorus_| 1160 0.83 1.50 15 1.02[c] 0.77 1.32
| 1.75 0.65[c] 0.84
_Potos flavus_ | 2490 1.75 0.50 12 0.30 0.63 0.48
|
_Bassaricyon gabbii_ | 1600 1.75 0.50 15 0.32 0.71 0.45
|
---------------------+------------------------------------------------
---------------------+------------------------------------------------
|
Species | References
---------------------+------------------------------------------------
_Procyon lotor_ | Dunn and Chapman (1983); Eisenberg (1981:489);
| Kaufmann (1987); Lotze and Anderson (1979);
| Nowak and Paradiso (1983:981); Sanderson
| (1987); Stains (1956:28-31); This study
_Bassariscus astutus_| Kaufmann (1982, 1987); Nowak and Paradiso
| (1983:979, 980); Poglayen-Neuwall and
| Poglayen-Neuwall (1980); Poglayen-Neuwall
| and Toweill (1988); Russell (1983)
_Nasua narica_ | Kaufmann (1982, 1987); Nowak and Paradiso
| (1983:983); Sanderson (1983)
_Nasua nasua_ | Chevillard-Hugot et al. (1980)
_Procyon cancrivorus_| Crandall (1964:312); Poglayen-Neuwall (1987)
|
_Potos flavus_ | Ford and Hoffmann (1988); Nowak and Paradiso
| (1983:984)
_Bassaricyon gabbii_ | Eisenberg (1981:489); Nowak and Paradiso
| (1983:985)
---------------------+------------------------------------------------
[a] r_{maxe} = 4.9·m^{0.2622}, where m is body mass in grams.
[b] Regression of r_{max} on body mass (m). Assume r_{max} = 1.02 for
_Procyon cancrivorus_: r_{max} = 0.00005·m + 0.623; R = 0.19;
R² = 0.03; Regression of r_{maxr} (Table 10) on H_{br} (Table 7);
assume _Nasua nasua_ has the same r_{maxr} as _Nasua narica_:
r_{maxr} = 3.35·H_{br} - 1.11; R = 0.93; R² = 0.86.
[c] Estimate based on females reproducing in their first (a = 0.83) or
second (a = 1.75) year.
_Nasua nasua._--Unfortunately, there is not enough reproductive data
to allow calculation of r_{max} for _Nasua nasua_ (Table 10), therefore,
it is not possible to compare the reproductive potential of this South
American coati with its North American relative, _Nasua narica_. Given
its low [.H]_{b} and relatively low-quality diet of fruit and
terrestrial invertebrates (Table 9), however, r_{max} of _Nasua nasua_
may be very similar to that of _Nasua narica_.
_Procyon cancrivorus._--The age of first female reproduction for
_Procyon cancrivorus_ has not been reported. However, if one assumes
females can reproduce in their first year, r_{max} for _Procyon
cancrivorus_ would be 1.02 (132% of expected; Table 10). If, on the
other hand, first female reproduction is delayed until the second year,
r_{max} would be 0.65 (84% of predicted; Table 10). _Procyon
cancrivorus_ has a low [.H]_{b}, reduced litter size, and small body
mass. Its low [.H]_{b} may limit litter size, but as with _Bassariscus
astutus_, the quality of its diet (a high percentage of small
vertebrates; Table 9) and its small body size may make it possible for
females to reproduce in their first year and thus increase the species'
reproductive potential. This reasoning would argue that _Procyon
cancrivorus_ probably enjoys higher, rather than lower, than expected
r_{max}.
_Potos flavus._--In addition to a low [.H]_{b}, this species possesses
other characteristics that limit its reproductive potential:
low-quality diet, delayed reproduction, and birth of a single young
each year. Because there does not appear to be any other feature of
its life history that can counteract the influence of these factors,
r_{max} in _Potos flavus_ has evolved to be only 48% of expected
(0.30; Table 10). Its close relative, the olingo, _Bassaricyon gabbii_,
appears to share the same condition (Table 10).
SUMMARY.--This brief survey illustrates that, with the exception of
_Potos flavus_, procyonids tend to have values of r_{max} that are
higher than those predicted for them on the basis of mass (Table 10).
Regression analysis indicates that, within the family, body mass
accounts for only a small amount (3%) of the variation in r_{max},
whereas the positive slope of the correlation between r_{maxr} and
H_{br} (R = 0.93) suggests that low metabolism has a limiting effect
on r_{max} (see Table 10, footnote f). The implication here is that
low [.H]_{b} would be associated with a lower rate of biosynthesis,
a slower growth rate, and a longer generation time. Procyonids with
low [.H]_{b} but higher than expected r_{max} must possess other
traits that serve to offset the effects of low metabolism. Our survey
indicates that the following features compensate for low [.H]_{b} and
help increase r_{max}: (1) a high-quality diet may make biosynthesis
and growth more efficient, thus optimizing the time element associated
with each of these processes; (2) larger litter sizes and cooperation
in care of the young may increase survivorship in spite of a slower
growth rate; and (3) an early age of first reproduction, a long
reproductive life span, and moderate-size litters (two to four young)
may in the long run add as many individuals to the population as a
shortened generation time. Our survey also suggests that, at the other
extreme, factors such as a low-quality diet, reduced litter size,
absence of cooperative care of the young, delayed age of first
reproduction, and shortened reproductive life span all serve to
decrease r_{max}. Thus, it is obvious that diet, litter size, social
structure, reproductive strategy, and reproductive life span can operate
synergistically with [.H]_{b} to magnify its influence on r_{max} (as
with _Procyon lotor_ and _Potos flavus_), or they can function in
opposition to [.H]_{b} to change the direction of its influence on
r_{max} (as with _Bassariscus astutus_, _Procyon cancrivorus_, _Nasua
narica_, and perhaps _Nasua nasua_).
_Basal Metabolism and Climatic Distribution_
_Procyon lotor._--The evolution of a higher [.H]_{b} (Tables 7, 8) may
have been the physiological cornerstone that enabled _Procyon lotor_
to break out of the mold being exploited by other procyonids and to
generalize its use of habitats and climates. Once this basic
physiological change was in place, selection for appropriate
alterations in thermal conductance, capacity for evaporative cooling,
diversity of diet, and energy storage would have provided this species
with the suite of adaptations needed to extend its distribution into
other habitats and climates. Support for this concept follows from the
fact that high levels of [.H]_{b} are associated with (1) cold-hardiness
in mammals that live in cold-temperate and arctic climates (Scholander
et al., 1950c; Irving et al., 1955; Irving, 1972:115, 116; Shield, 1972;
Vogel, 1980; Golightly and Ohmart, 1983); (2) the ability to utilize a
wide variety of food resources and to occupy a large number of different
environments and habitats (McNab, 1980a); and (3) a high intrinsic rate
of natural increase (McNab, 1980a; Hennemann, 1983; Lillegraven et al.,
1987; Nicoll and Thompson, 1987; Thompson, 1987).
OTHER PROCYONIDS.--Other procyonids (_Potos flavus_, _Procyon
cancrivorus_, _Nasua narica_, and _Nasua nasua_) have lower than
predicted [.H]_{b}'s (Table 7), a characteristic that is considered to
be an energy-saving adaptation for those that live in relatively stable
tropical and subtropical habitats (Müller and Kulzer, 1977;
Chevillard-Hugot et al., 1980; Müller and Rost, 1983). However,
_Bassariscus astutus_ is found in tropical, subtropical, and temperate
climates. This species is found from tropical Mexico to temperate
regions of the western United States (Kaufmann, 1982, 1987; Nowak and
Paradiso, 1983:979). In the northern part of its distribution,
_Bassariscus astutus_ lives in habitats that are unstable (arid
regions), that are low in productivity, and that characteristically
have marked seasonal changes in temperature. Its lower than predicted
[.H]_{b} could be an important water-conserving adaptation at times when
temperatures are high (McNab and Morrison, 1963; McNab, 1966; MacMillen
and Lee, 1970; Noll-Banholzer, 1979) and an important energy-conserving
mechanism when cold weather may limit food availability and hunting time
(Scholander et al., 1950c; Wang et al., 1973). As will be seen later,
_Bassariscus astutus_ is unique among procyonids with lower than
predicted [.H]_{b}'s in that it also has a lower than predicted
C_{mw} (Table 7). This allows it to use less energy than expected for
thermoregulation at low temperatures. Another species with a similar set
of adaptations (lower than predicted [.H]_{b} and C_{mw}) is the arctic
hare, _Lepus arcticus_ (Wang et al., 1973), which lives in one of
the coldest and least-productive regions on earth. Wang et al. (1973)
suggest that this combination of adaptations allows _Lepus arcticus_
to better match its energy requirements to the low productivity of its
environment. A similar relationship may hold for _Bassariscus astutus_,
particularly in colder arid portions of its distribution, and may be the
reason that it, but not other procyonids with low [.H]_{b}'s, has been
able to inhabit temperate climates.
MINIMUM THERMAL CONDUCTANCE
_Background_
Thermal conductance is a measure of the ease with which heat is
passively transferred to or from a body through its tissues and pelt.
Within T_{n}, a mammal is able to vary its thermal conductance over a
wide range of values by changing heat transfer characteristics of both
of these layers. Minimum thermal conductance occurs when total heat
transfer through these layers is reduced to its lowest possible rate.
This minimum value, which is the reciprocal of maximum resistance,
occurs, theoretically, but not always practically (see McNab, 1988b),
at the animal's T_{lc} and is best estimated under standard conditions
in a metabolism chamber (McNab, 1980b; Aschoff, 1981). Minimum thermal
conductance scales to body mass (McNab and Morrison, 1963; Herreid and
Kessel, 1967; McNab, 1970, 1979b; Bradley and Deavers, 1980; Aschoff,
1981). Therefore, to make comparisons between species of various sizes,
we scaled out body mass by expressing C_{mw} as the ratio of measured
to predicted values (C_{mwr}; Table 7). These ratios were used to make
comparisons of heat-transfer characteristics between species that
occupy different habitats or climates.
_Effect of Molt on Thermal Conductance_
In summer, T_{lc}'s of male and female _Procyon lotor_ (Figure 2) were
very similar to those of other procyonids (22°C-26°C; Table 7). In
winter, T_{lc} of both sexes shifted downward to 11°C (Figure 3). This
seasonal shift in T_{lc} occurred as the result of a seasonal change
in minimum thermal conductance (Table 3). For many northern mammals, a
seasonal change in thermal conductance is partly mediated via cyclic
changes in the insulative quality of their pelt (Scholander et al.,
1950a; Irving et al., 1955; Hart, 1956, 1957; Irving, 1972:165).
_Procyon lotor_ begins to shed its heavy winter coat about the time
its young are born. Molt progresses through summer and by late August
the new coat is complete (Stuewer, 1942). During its summer molt,
_Procyon lotor_'s C_{mw} increased by about 49% over the value for
female raccoons in winter (Table 3). In summer, therefore, it had the
highest mass specific C_{mw} of those procyonids considered
(C_{mwr} = 1.77 and 1.79; Table 7). An increase in thermal conductance
facilitates passive heat loss for temperate and arctic species, and
this serves as an important thermoregulatory adaptation during warm
summer months (Scholander et al., 1950c; Irving et al., 1955;
Hart, 1956, 1957; Irving, 1972:165). This adaptation is particularly
important to those temperate- and arctic-zone species (including
raccoons) whose [.H]_{b}'s do not decrease during summer (Irving et al.,
1955). From August on, the fur of _Procyon lotor_ becomes increasingly
longer and heavier, with peak, or prime, condition occurring in late
fall and early winter (Stuewer, 1942). Minimum conductance of our
captive raccoons was lowest in winter (C_{mwr} = 1.15) when their pelts
were in prime condition (Tables 3, 7). Because "primeness" of raccoon
pelts varies geographically, thicker pelts being associated with colder
climates (Goldman, 1950:21; Whitney and Underwood, 1952:24-41), the
degree of seasonal change in C_{mw} must also vary geographically.
The only other procyonid for which a seasonal molt has been described
is _Bassariscus astutus_. Molt in this species extends from late summer
to late fall (Toweill and Toweill, 1978). How molt effects thermal
conductance in _Bassariscus astutus_ is not known because metabolic
data for this species (Table 7) apparently were collected only when
their pelts were in prime condition (Chevalier, 1985).
Goldman (1950:20) reports that _Procyon cancrivorus_ does not have a
seasonal molt. Like other tropical procyonids, _Procyon cancrivorus_
lives in an environment that has the following characteristics: high
even temperatures throughout the year (1°C-13°C difference in monthly
mean temperature), a greater range in temperature between day and
night than in mean monthly temperature throughout the year, uniform
lengths of day and night, seasonal variation in rainfall, and lowest
temperatures during the rainy season(s) (Kendeigh, 1961:340). In such
a stable environment there would be no advantage to a sharply defined
seasonal molt cycle that could place an animal in thermoregulatory
jeopardy by increasing its thermal conductance. This would be
particularly true for animals like tropical procyonids that have lower
than predicted [.H]_{b}'s but that maintain typical eutherian body
temperatures (Table 7). Consequently, molt in all tropical procyonids
may either be prolonged or continuous. This is a feature of their
biology that needs to be examined in more detail.
_Comparison of Thermal Conductances_
_Procyon lotor_ VERSUS TROPICAL PROCYONIDS.--C_{mwr} for _Procyon
lotor_ in winter was 1.15, which is similar to the values for _Potos
flavus_ and _Procyon cancrivorus_, 1.02 and 1.25, respectively
(Table 7). These two tropical species, therefore, have C_{mw}'s that
are similar on a mass specific basis to the value for _Procyon lotor_
in winter. However, at their T_{lc}'s, the thermal gradient sustained
by these tropical animals is only about 11°C, whereas for _Procyon
lotor_ in winter it was 26.5°C. Examination of Eq. 4 with respect to
these thermal gradients suggests that tropical procyonids achieve such
low C_{mw}'s by virtue of their lower than predicted [.H]_{b}'s rather
than by having pelts that are exceptionally good insulators. In fact,
the insulation afforded by the pelts of these tropical procyonids is
about the same as that of the 50 g arctic lemming, _Dicrostonyx
groenlandicus rubricatus_, whose coat has an insulative value that is
about half that of the hare, _Lepus americanus_, red fox, _Vulpes fulva
alascensis_, and pine martin, _Martes americana_, animals comparable in
size to these procyonids (Scholander et al., 1950a). Therefore, pelts
of these tropical procyonids do not have the same insulative value as
the prime winter coat of _Procyon lotor_.
_Nasua narica_ and _Nasua nasua_ have tropical and subtropical
distributions and they are the only procyonids that are diurnal
(Kaufmann, 1962:103-105, 1982, 1987). Because they are active during
the day they experience a more extreme thermal environment (higher
T_{a}'s and solar radiation) than their nocturnal cousins. Values of
C_{mwr} for _Nasua narica_ (1.45 and 1.55) and _Nasua nasua_ (1.24 and
1.65) are higher than those for _Procyon cancrivorus_ or _Potos flavus_
(Table 7). Thus, these coatis have higher mass specific C_{mw}'s than
their nocturnal tropical cousins. A high C_{mw} reduces the cost of
thermoregulation in hot environments because it increases an animal's
ability to lose excess heat passively. The higher C_{mw}'s of these
coatis serve as an adaptation that contributes to the success of their
diurnal life style as well as their ability to expand their habitat use
to areas with less thermal stability, such as oak and pine woodlands
and deserts.
_Bassariscus astutus._--This species has the lowest mass specific
C_{mw} of these procyonids (C_{mwr} = 0.85; Table 7), which indicates
that its pelt has a greater insulative value than the coats of _Potos
flavus_, _Procyon cancrivorus_, _Nasua nasua_, or _Nasua narica_. This,
coupled with a lower than predicted [.H]_{b}, allows _Bassariscus
astutus_ to maintain T_{b} with less energy expenditure than is
possible for any other procyonid of comparable size; and this
combination of adaptations provides _Bassariscus astutus_ with a
distinct energy advantage in environments that have low productivity
(Wang et al., 1973). The evolution of a pelt that provides better
insulation must be considered an important contributing factor for
the spread of this species into desert regions of the western United
States.
THERMOREGULATION AND USE OF STORED FAT AT LOW TEMPERATURES
_Background_
THERMOREGULATION.--At temperatures below a mammal's T_{n}, heat loss
exceeds [.H]_{b}. To maintain T_{b} under these conditions, metabolic
rate must be increased (Eq. 4). _Procyon lotor_ in summer during its
annual molt (Table 5; Figure 2), _Bassariscus astutus_ (Chevalier,
1985), _Nasua nasua_ (Chevillard-Hugot et al., 1980; Mugaas et al.,
in prep.), _Nasua narica_ (Scholander et al., 1950b; Mugaas et al.,
in prep.), and _Potos flavus_ (Müller and Kulzer, 1977; Müller and Rost,
1983) all are able to elevate their metabolic rates by 130% above basal
when they are exposed to T_{a} = 0°C. _Procyon cancrivorus_ responds to
0°C with an increase in metabolic rate of 257% above basal (Scholander
et al., 1950b). All animals listed have about the same T_{lc} and T_{b},
so the temperature differential producing this response is about the
same for each species. Metabolic ability to defend body temperature
against low ambient temperatures, therefore, is well developed in these
procyonids. Such large increases in metabolic rate are energetically
expensive, and if these animals were routinely exposed to T_{a} = 0°C,
it would be difficult for them to acquire enough food each day to
maintain endothermy. Raccoons in winter pelage, however, need only
elevate their metabolic rate by 47% above basal to maintain endothermy
at T_{a} = 0°C (Table 5; Figure 3). Each year at the completion of its
molt, the raccoon's highly insulative pelt is renewed. This lowers
their T_{lc} by 9°C to 15°C below that measured for them in summer
(Figure 3) and decreases their cost of thermoregulation at low
temperatures. The increased insulative capacity of their pelt is one
of the primary adaptations that has allowed _Procyon lotor_ to extend
its distribution into cold climates.
STORED FAT.--Cyclic fattening is an integral and important part of a
raccoon's annual cycle (Mugaas and Seidensticker, ms); however, it has
not been reported for other procyonids. During winter in parts of the
United States and Canada, raccoons are confined to their dens for
variable periods of time (days to months) depending on the severity of
the weather (Stuewer, 1943:223-225; Whitney and Underwood, 1952:108-116;
Sharp and Sharp, 1956; Mech et al., 1968; Schneider et al., 1971).
During this confinement, they do not hibernate but rather enter a state
of "dormancy" and become inactive. While dormant they remain endothermic
(T_{b} > 35°C; Thorkelson, 1972:87-90) and derive most of their energy
requirement from fat reserves accumulated during fall. The rate at which
fat stores are consumed during winter dormancy depends on the
thermoregulatory requirement imposed on them by local weather
conditions, the insulative quality of their pelt, and any advantage
they may gain by seeking shelter in a den.
_Thermal Model of the Raccoon and Its Den_
Heat transfer between an animal and its environment is a function of
the interaction of its body temperature and thermal conductance with
various environmental variables (air temperature, wind speed, vapor
pressure, and thermal radiation). When a raccoon is outside its den,
its thermal conductance (C_{mw}) is the only barrier to heat transfer
with the external environment. However, when it enters a tree den, a
raccoon imposes two other thermal barriers between itself and the
external environment: (1) conductance of the air space between its fur
and the den's walls (C_{a}) and (2) conductance of the den's walls
(C_{d}; Thorkelson, 1972:59-63; Thorkelson and Maxwell, 1974).
Thorkelson and Maxwell (1974) modeled heat transfer of a simulated
raccoon (a water-filled aluminum cylinder equipped with a heater and
covered with a raccoon pelt) in a closed tree den. In their system,
65% of resistance to heat flux was attributable to the pelt, whereas
the remainder (35%) was due to C_{a} and C_{d}. Because resistance is
the inverse of conductance, and resistances for the raccoon and its
den are arranged in series, we can estimate total conductance (C_{t})
of this system with Eq. 7.
1/C_{t} = 1/C_{mw} + 1/C_{a} + 1/C_{d} Eq. 7
Minimum thermal conductance C_{mw} for raccoons in winter was
0.0172 mL O_{2}·g^{-1}·h^{-1}·°C^{-1} (Table 3). Based on Thorkelson
and Maxwell's (1974) model we let 1/C_{mw} = 0.65(1/C_{t}) =
1/0.0172 mL O_{2}·g^{-1}·h^{-1}·°C^{-1}, and 1/C_{a} + 1/C_{d} =
0.35(1/C_{t}). Substituting these values into Eq. 7 and solving
for C_{t} yields 0.0112 mL O_{2}·g^{-1}·h^{-1}·°C^{-1}, a value
that is 35% lower than that of the animal alone. Substituting
this value and the value for basal metabolism of winter raccoons
(0.47 mL O_{2}·g^{-1}·h^{-1}; Table 7) into Eq. 4 and solving for
(T_{b} - T_{a}) yields a new temperature differential of 42°C.
Therefore, by using tree dens, raccoons in north central Virginia,
with T_{b} = 37°C (Figure 7), could effectively reduce their T_{lc}
from 11°C to -5°C and markedly reduce their metabolic cost of
thermoregulation.
_Metabolic Advantage of the Den_
Given prevailing winter temperatures in north central Virginia (see
"Materials and Methods"), adult raccoons in that area should be able
to sustain endothermy most of the time they are in their dens by simply
maintaining [.H]_{b}. Depending on the mass of their stored fat, they
could remain in their dens for several weeks without eating (Mugaas and
Seidensticker, ms). The thermal advantage of a den could be further
enhanced during colder temperatures if two or more raccoons occupied it
at the same time and huddled together, and/or if these animals could
reduce C_{mw} even more by lowering T_{b} and cooling their extremities.
Although we do not have any data to verify the second mechanism, there
are many accounts in natural history literature that document raccoons
occupying dens together (Lotze and Anderson, 1979). This habit could be
particularly important for the young of the year and may be one reason
why they often continue to den with their mothers during winter (Lotze
and Anderson, 1979; Seidensticker et al., 1988). Raccoons that live in
colder climates, such as Minnesota, undoubtedly obtain the same
advantage from a den as Virginia animals, but because of their greater
body mass, longer fur, and potentially lower C_{mw}, T_{lc} of a
Minnesota raccoon in a den could be even lower than what we calculated
for Virginia raccoons. Therefore, when they are in their dens, raccoons
living in very cold climates also may be able to maintain homeothermy
with a basal level of metabolism.
THERMOREGULATION AT HIGH TEMPERATURES
_Background_
In hot environments mammals depend on behavior to minimize their
thermal load (escape to shaded or cooler microclimates, use posture
and orientation to wind and sun, restrict activity, become nocturnal,
etc.) and on evaporative water loss to rid themselves of excess heat.
With regard to evaporative heat loss, Calder and King (1974:326)
arbitrarily subdivided the response to various T_{a}'s as follows:
"(1) cool temperatures at which water loss should be minimized, both
to reduce heat loss and as an adaptation to terrestriality; (2) an
intermediate temperature range wherein evaporation is gradually
increased as dry heat losses are proportionately reduced with smaller
thermal gradients; and (3) warm to hot temperatures at which
evaporation must be actively increased to dispose of metabolic and
exogenous heat loads." Some mammals are able to thermoregulate very
well at high ambient temperatures via panting or sweating, whereas
others have a very limited capacity. Hence, there is no general
approach to calculating evaporative water loss under these conditions
(Campbell, 1977:85). However, the ratio of evaporative heat lost to
metabolic heat produced can be used to quantify a species' capacity
for evaporative cooling and to make comparisons between species.
_Comparison of Procyonid Responses to Heat Stress_
_Potos flavus._--This species lives in Neotropical forests of Central
and South America. It is nocturnal, arboreal in habit, and appears to
be the most heat-sensitive of these procyonids. Its T_{uc} is at 30°C
to 33°C (Table 7; Müller and Kulzer, 1977; Müller and Rost, 1983). It
begins to pant at about 30°C, but its efforts at evaporative cooling
are very ineffective. At 33°C _Potos flavus_ can dissipate 33% of its
metabolic heat via evaporative water loss, but at 35°C the efficiency
of this mechanism falls to 20% (Müller and Rost, 1983). Consequently,
when exposed to T_{a}'s above 33°C, any kind of excitement causes its
T_{b} to rise rapidly in an uncontrolled manner (Müller and Kulzer,
1977; Müller and Rost, 1983). These animals rely on their nocturnal
and arboreal habits to keep them out of situations that could lead to
hyperthermia (Müller and Kulzer, 1977; Müller and Rost, 1983).
_Nasua nasua_ and _Nasua narica_.--_Nasua nasua_ is abundant in
tropical and subtropical South America, whereas _Nasua narica_ occupies
the same climates in North America from southern Arizona and New Mexico
south through Panama and on into Colombia and Ecuador (Hall and Kelson,
1959:892; Ewer, 1973:391, 392; Poglayen-Neuwall, 1975). Both coatis
are diurnal and forage primarily on the ground (Kaufmann, 1962:185-188,
1987; Poglayen-Neuwall, 1975; Nowak and Paradiso, 1983:982),
consequently they are exposed to a more severe thermal environment
while active (higher T_{a}'s and solar radiation) than are nocturnal
procyonids. Both coatis are more heat-tolerant than _Potos flavus_;
their T_{uc}'s are higher (33°C-35°C; Table 7), they can tolerate
T_{a}'s of 35°C without raising their T_{b}'s (Chevillard-Hugot et al.,
1980; Mugaas et al., in prep.), and they have a greater capacity for
evaporative cooling than _Potos flavus_ (Mugaas et al., in prep.). The
greater heat tolerance of these coatis is compatible with their diurnal
habits and widespread distribution in a variety of forest habitats in
both tropical and subtropical areas of the western hemisphere.
_Bassariscus astutus._--In addition to living in Neotropical forests of
Mexico, _Bassariscus astutus_ also flourishes in hot arid climates, and
it has extended its range much farther north than _Nasua narica_ (Hall
and Kelson, 1959:881,892; Poglayen-Neuwall, 1975; Kaufmann, 1982). Its
T_{uc} is higher (35.5°C; Table 7) than that of _Potos flavus_, but it
is comparable to those of _Nasua nasua_ and _Nasua narica_. Its capacity
for evaporative cooling is well developed; at 40°C _Bassariscus astutus_
is able to dissipate 100% of its resting metabolic heat via evaporative
water loss, and at 45°C it is able to dissipate 172% (Chevalier, 1985).
In spite of its great capacity for evaporative cooling, this species is
nocturnal, a habit that, along with its low [.H]_{b}, should allow it
to keep thermoregulatory water requirements to a minimum.
_Procyon lotor._--Our data suggested that T_{uc} for _Procyon lotor_ in
winter was comparable to that for _Bassariscus astutus_ (35°C), and
that in summer it was even higher. When exposed to temperatures near
the upper end of its T_{n}, _Procyon lotor_ increased the gradient for
passive heat loss with a controlled rise in T_{b} (Figure 6). In summer
its capacity for passive heat loss was enhanced by the molt of its
heavy winter fur. _Procyon lotor_'s capacity for evaporative cooling
also appeared to be well developed, although our animals were not
heated to the point that evaporative cooling was fully expressed
(Figures 4, 5). However, _Procyon lotor_ is nocturnal, and this may
allow it to eliminate, or at least reduce, the need for evaporative
cooling, even in hot climates. Thus, _Procyon lotor_ appears to be
well equipped physiologically and behaviorally to cope with thermal
demands of hot environments in its distribution.
_Procyon cancrivorus._--Unfortunately, data for the crab-eating
raccoon are not complete enough at high temperatures to include it in
this survey.
SUMMARY.--This comparison demonstrates that capacity for evaporative
cooling, tolerance of an elevated T_{b} to enhance passive heat loss,
and behavioral avoidance of thermal stress are the primary methods used
by procyonids to thermoregulate at high temperatures. _Procyon lotor_
and _Bassariscus astutus_, whose distributions extend into temperate
regions, have developed these abilities to a greater extent than other
procyonids. _Potos flavus_, whose distribution is confined to
lowland tropical forests, has the least ability in this regard. _Nasua
nasua_ and _Nasua narica_ appear to have thermoregulatory abilities
that are intermediate to those of _Bassariscus astutus_ and _Potos
flavus_. This suggests that ancestral procyonids may have had poor to
modest ability to thermoregulate at high temperatures, a condition that
would have limited their ability to leave the thermal stability
afforded by tropical forests. Dispersal into temperate climates,
therefore, required not only increased cold tolerance but also
selective enhancement of those mechanisms used in thermoregulation at
high temperatures.
TABLE 11.--Distribution by climate of selected procyonid species.
-----------------------+---------------------------------------------
| Mild[a] Cold[b]
Species | Tropics Subtropics temperate temperate
-----------------------+---------------------------------------------
_Procyon lotor_ | + + + +
_Bassariscus astutus_ | + + +
_Nasua nasua_ | + +
_Nasua narica_ | + +
_Procyon cancrivorus_ | + +
_Potos flavus_ | +
-----------------------+---------------------------------------------
[a] Extends from the subtropics north to the northern limit of
_Bassariscus astutus_' distribution (Hall and Kelson, 1959:881),
which approximates the 10°C isotherm for average annual
temperature in the United States (Kincer, 1941).
[b] Extends northward from the 10°C isotherm for average annual
temperature in the United States.
COMPOSITE SCORES OF ADAPTIVE UNITS AND GEOGRAPHIC DISTRIBUTION
In Table 11, procyonid species are arranged in descending order with
respect to the number of major climates that are included in their
geographic distributions (Hall and Kelson, 1959:878-897;
Poglayen-Neuwall, 1975; Kortlucke and Ramirez-Pulido, 1982; Nowak and
Paradiso, 1983:977-985). Composite scores ranged from a high of 1.47
for _Procyon lotor_ to a low of 0.39 for _Potos flavus_, whereas
_Nasua nasua_, _Nasua narica_, _Procyon cancrivorus_, and _Bassariscus
astutus_ had intermediate values ranging from 0.64 to 0.79 (Table 12).
Figure 8 demonstrates that there is a direct relationship between the
number of climates these species occupy and their composite scores.
Regression analysis (Y = 2.68·X + 0.24; where Y is number of climates,
and X is composite score) demonstrates a high degree of correlation
between these variables (R = 0.94) and indicates that 89% of the
variance in distribution can be explained by composite scores. The
various combinations of adaptations expressed by these species do,
therefore, play a role in delimiting their climatic (latitudinal)
distributions.
_Procyon lotor's_ normalized scores were higher in all categories than
those of other procyonids. _Procyon lotor_, therefore, possesses those
traits that have allowed it to become the premier climate generalist
of the procyonid family. As an adaptive unit, these traits provide
_Procyon lotor_ with the physiological and behavioral flexibility
required to take full advantage of a wide range of climates and
habitats, and its distribution verifies that it has done so. Even so,
it is probably not fair to assume that this species represents a
perfect physiological match with climate over its entire distribution.
_Procyon lotor_ is, in many respects, still a forest-dwelling species,
and its ability to expand its distribution into other habitats such as
prairie and desert may well be due, in part, to its use of behavior to
take advantage of favorable microclimates in otherwise hostile
environments (Bartholomew, 1958, 1987). This feature of _Procyon
lotor's_ biology needs to be further examined.
TABLE 12.--Normalized and composite scores for selected procyonids.
(H_{br} = ratio of measured to predicted basal metabolism (Table 7),
C_{mwr} = ratio of measured to predicted minimum thermal conductance
(Table 7), D_{dr} = ratio of food categories actually utilized by
each species to total food categories eaten by all six species
(calculated from Table 9), r_{maxr} = ratio of calculated to
expected r_{max} (Table 10).)
----------------------+----------------------------------------------
| Normalized scores
Species |---------------------------- Composite[a]
|H_{br}/C_{mwr} D_{dr} r_{maxr} score
----------------------+----------------------------------------------
_Procyon lotor_ | 0.95 0.95 2.52 1.47
_Bassariscus astutus_ | 0.80 0.33 1.24 0.79
_Nasua nasua_ | 0.48 0.33 1.11[b] 0.64
_Nasua nasua_ | 0.48 0.33 1.11[b] 0.64
_Nasua narica_ | 0.40 0.53 1.11 0.68
_Procyon cancrivorus_ | 0.55 0.33 1.32 0.73
_Potos flavus_ | 0.60 0.11 0.48 0.39
----------------------+----------------------------------------------
[a] Composite score = [(H_{br}/C_{mwr}) + D_{dr} + r_{maxr}]/3.
[b] Value calculated for _Nasua narica_ (Table 10) and used with the
assumption that it must be similar to the value for _Nasua nasua_.
All five species with low [.H]_{b}'s have composite scores less than
1.0 (Table 12; Figure 8). Four of these five, _Nasua nasua_, _Nasua
narica_, _Procyon cancrivorus_, and _Potos flavus_, have
H_{br}/C_{mwr} ratios that are 0.6 or less, which indicates they are
the least cold-tolerant procyonids (McNab, 1966). These four species
also are confined to either tropic, or tropic and subtropic climates
(Table 11). This suggests that these species share a common
thermoregulatory adaptation that represents a specialization to these
climates. Attendant with this adaptation, however, is a high cost of
thermoregulation at temperatures below their T_{lc}, and this must be
an important factor in limiting their distributions to tropic and
subtropic climates. Differences in their distributions within these
climates, therefore, must hinge more on differences in their D_{dr}
and r_{maxr} values than on differences in their H_{br}/C_{mwr}
ratios. This is supported by the fact that _Potos flavus_, which has
the lowest D_{dr} and r_{maxr} values, is confined to a single
climate, whereas _Nasua nasua_, _Nasua narica_, and _Procyon
cancrivorus_ each possess larger D_{dr} and r_{maxr} values and are
found in two climates. Thus, _Potos flavus_, with its highly
specialized diet and low reproductive potential, is the most
ecologically specialized of these procyonids, and its distribution is
limited to the single climate that can provide its requirements.
_Nasua nasua_, _Nasua narica_, and _Procyon cancrivorus_ are less
specialized and thus show more ecological flexibility in their
distributions.
[Illustration: FIGURE 8.--Relationship between number of climates
in which a species is found and its composite score. Symbols for
_Nasua nasua_ overlap at coordinates (0.64, 2). Solid line
represents linear regression of climates (Y) on composite scores
(X): Y = 2.68·X + 0.24; R = 0.94.]
_Bassariscus astutus_, the other species with low [.H]_{b}, is found
in three climates, which indicates that it has greater ecological
flexibility than _Nasua nasua_, _Nasua narica_, or _Procyon
cancrivorus_. D_{dr} and r_{maxr} are comparable for these four
species (Table 12). This suggests that the greater ecological
flexibility of _Bassariscus astutus_ is derived largely from its
greater cold tolerance. _Bassariscus astutus_ has a more insulative
pelt than these other procyonids (C_{mwr} = 0.85; Table 7), so its
H_{br}/C_{mwr} ratio is higher (0.80; Table 12). This, and its greater
capacity for evaporative cooling (Chevalier, 1985), allows
_Bassariscus astutus_ to take advantage of a wider range of thermal
environments than these other species. However, even with its higher
H_{br}/C_{mwr} ratio, the composite score for _Bassariscus astutus_
is not much different than those for _Nasua nasua_, _Nasua narica_,
and _Procyon cancrivorus_ (Table 12). Consequently, _Bassariscus
astutus_ is found in more climates than would be predicted for it on
the basis of its composite score (Figure 8). This suggests that either
the H_{br}/C_{mwr} ratio carries greater weight in determining
distribution than is reflected in this analysis, or as has been
described for some other species (Bartholomew, 1958, 1987),
_Bassariscus astutus_ may extend its distribution farther than
expected via use of its behavior. In either case, for procyonids with
low [.H]_{b}, _Bassariscus astutus_ represents the pinnacle of
adaptation for climate generalization.
EVOLUTION OF METABOLIC ADAPTATIONS
_Evolution of Low Basal Metabolic Rate_
A radiation of frugivorous and omnivorous Procyoninae (Table 1)
occurred in the middle and late Miocene of North America. It included
origins of such terrestrial genera as _Cyonasua_, _Nasua_, and
_Procyon_ (Webb, 1985b). The earliest procyonid genus to find its way
to South America was _Cyonasua_, an omnivorous carnivore that
presumably split, along with its sister genus _Arctonasua_, from a
common North American ancestor (Baskin, 1982; Webb, 1985b).
_Cyonasua_, about the size of present-day raccoons, was adapted to a
wide range of habitats and was probably comparable to modern raccoons
with respect to the breadth of its feeding habits (Webb, 1985b;
Marshall, 1988). Because North American _Arctonasua_ was about the
same size as _Cyonasua_ (Webb, 1985b) and shared a number of
characters with it (Baskin, 1982), we speculate that it also may have
had similar habits and occupied similar climates and habitats.
_Bassariscus_, another member of Procyoninae, had an even earlier
origin in tropical North America (Webb, 1985b). The origin of the
small arboreal forms _Potos_ and _Bassaricyon_ (subfamily Potosinae)
is obscure but is thought to have occurred in the rainforests of
Central America (Webb, 1985b). What were the metabolic capabilities of
these early procyonids? We do not know, but for several million years,
from middle to late Miocene, procyonids lived in tropical and
subtropical forests of Central and North America (Webb, 1985b;
Marshall, 1988). Then, in the Pleistocene, several modern forms
crossed the Panamanian land bridge into similar habitats and climates
in South America; but none of them appear to have spread far enough
northward to have crossed the Bering land bridge.
Several million years exposure to a tropical environment, with its
continuous high temperatures and modest range of thermal extremes,
would have favored selection of metabolic and thermoregulatory traits
that would minimize energy requirements: a lower than predicted basal
metabolic rate, a prolonged or continuous molt resulting in very
little annual change in minimum thermal conductance, and a modest
capacity for evaporative cooling. In addition, we would expect
selection to have favored a diverse diet, good reproductive potential,
and behavioral flexibility to utilize a variety of habitats within
these climates. Our analysis has shown that such characteristics are
the norm for extant members of this family living in tropical and
subtropical climates, and we speculate that these traits also were
common to early procyonids and served to restrict them to these
climates. Our speculation is supported by the fact that their known
fossil history from the Miocene is confined to geographic areas that
had tropical and subtropical climates.
Later on, during Pleistocene glaciations, tropical and subtropical
forests shrank, savannas expanded, and temperate climate was pushed
toward equatorial regions. The opposite occurred during interglacial
periods (Raven and Axelrod, 1975; Webb, 1977, 1978; Marshall, 1988).
Consequently, mid-latitudes experienced alternating periods of
temperate and tropical, or at least subtropical, climate change.
Selection of characteristics that would have adapted a species with
low [.H]_{b} to temperate as well as tropic or subtropic climates
could have occurred in mid-latitudes at the temperate edge of these
tropical advances and retreats. Our analysis indicates that, for this
purpose, selection would have favored lower than predicted thermal
conductance, seasonal molt, increased capacity for evaporative
cooling, increased tolerance of elevated T_{b}, increased flexibility
of thermoregulatory behavior, food habits that provided for year-round
access to a high-quality diet in all three climates, and a higher than
predicted r_{max}.
_Bassariscus astutus_ is the only species with low [.H]_{b} that has
all these characteristics, and it is the only one of them that has
added temperate climate to its distribution (Table 11). This suggests
that _Bassariscus astutus_ is a species that evolved away from the
norm for procyonids with low [.H]_{b}, toward characteristics that
allowed it to become more of a climate generalist. _Potos flavus_,
with its dietary specialization, low tolerance to high temperatures,
and arboreal mode of existence, has become a highly specialized
species totally dependent on tropical forests for its survival. As
such, it also represents a species that has evolved away from the
procyonid norm and portrays the extreme in climate specialization.
Olingos, _Bassaricyon gabbii_ (Table 1), may be similar to _Potos
flavus_ in this respect (see also Table 10). This suggests that of the
extant procyonids, _Nasua nasua_, _Nasua narica_, and _Procyon
cancrivorus_ have retained metabolic and behavioral characteristics
that are closest to those of their Miocene ancestors.
_Evolution of High Basal Metabolic Rate_
Between the time that _Cyonasua_ appeared and the Panamanian land
bridge was established in the upper Pliocene (4 to 5 million years
ago), northern climates continued their gradual cooling. This, along
with ongoing elevation of the continents and continuous modification
of their mountain ranges, served to shrink the tropical forest and
create pockets of climatic instability within it and on its edges
(Darlington, 1963:578-596; Marshall, 1988). In areas of instability,
selection would have favored traits that provided for a broader range
of thermal tolerance: higher [.H]_{b}, improved insulative quality of
pelt, a more sharply defined molt cycle, improved capacity for
evaporative cooling, greater D_{d}, and higher r_{max}. Consequently,
by the upper Pliocene, two metabolically distinct groups of procyonids
could have been established: those species with low [.H]_{b} living in
climatically stable forests and those with higher [.H]_{b} living in
unstable tropical, subtropical, and perhaps temperate climates.
_Procyon lotor_ is the only extant procyonid with high [.H]_{b}.
_Procyon cancrivorus_ is its congeneric counterpart in Central and
South America (Table 1), and the two species are sympatric in Panama
and Costa Rica. However, in terms of its metabolism, thermal
conductance, molt, diversity of diet, r_{max}, and climatic
distribution, _Procyon cancrivorus_ shares more in common with other
procyonids than it does with _Procyon lotor_ (Tables 7, 11, 12; Figure
8). This suggests that metabolically _Procyon lotor_ portrays a
divergent line of this genus that arose as the result of a series of
mutations that gave rise to different metabolic characteristics. This
view is in keeping with a recent phylogenetic analysis of this family
that shows the genus _Procyon_ to be highly derived (Decker and
Wozencraft, 1991). Consequently, it would be instructive and would add
to our knowledge of the evolution of climatic adaptation to know more
about the genetic relatedness of these two species as well as their
historical relationship.
Genus _Procyon_ appears in the fossil record (Hemphillian and Blancan
ages; Baskin, 1982) prior to Pleistocene glaciations. During the
Pleistocene, there were four different glacial advances and retreats
in a relatively short time period (the first appearing little more
than a million years ago; Darlington, 1963:578-596; Webb, 1985a;
Marshall, 1988). Glacial retreats created pulses of time during which
subtropic and temperate climates advanced toward the poles into areas
with large seasonal differences in light/dark cycles, whereas glacial
advances pushed these climates southward into areas having smaller
seasonal differences in light/dark cycles (Raven and Axelrod, 1975;
Webb, 1977, 1978; Marshall, 1988). Those members of the genus
_Procyon_ caught in these wide latitudinal fluctuations would have
experienced conditions favorable to continued selection for
characteristics conducive to physiologic adaptation to a wide range of
climatic conditions. _Procyon lotor_ is the only member of its genus
to have survived this selective process, and as we have seen, it does
possess traits that adapt it to a wide range of climatic conditions.
Primary among these is its higher [.H]_{b}, which provides it with
advantages not shared with other procyonids (see earlier discussion).
Three other adaptations also have had a profound influence on _Procyon
lotor_'s ability to generalize its use of climate: (1) the increased
insulative quality of its pelt coupled with its sharply defined molt
cycle, which allows for a large annual change in thermal conductance;
(2) its annual cycle of fat storage; and (3) a diverse high-quality
diet. The first two of these adaptations required evolution of
neuroendocrine pathways capable of responding to time-dependent
environmental cues such as changing day length, changing temperature,
etc. Such conditions would have been available as selective stimuli in
high-latitude forests and savannas of interglacial periods. _Procyon
lotor_'s elevated basal metabolic rate would have increased its
overall energy requirement, and it makes good intuitive sense,
therefore, that evolution during the Pleistocene also would have
favored selection of a diverse diet containing many items of high
nutritive value.
SUMMARY
Our analysis has illustrated that within Procyonidae there are two
distinct modes of metabolic adaptation to climate. One is typified by
those species with low [.H]_{b}'s (_Bassariscus astutus_, _Nasua
nasua_, _Nasua narica_, _Procyon cancrivorus_, and _Potos flavus_),
and the other by _Procyon lotor_ with its higher [.H]_{b}. Those with
low [.H]_{b}'s have more restricted geographic distributions, and,
with the exception of _Bassariscus astutus_, they are all confined to
tropical and subtropical areas. The fossil history of this family
indicates that it had its origins in tropical forests of North and
Central America. This indicates that those procyonids whose
distributions are still primarily restricted to tropical forests share
many of the metabolic adaptations characteristic of their ancestors.
We speculate, therefore, that ancestral procyonids had a lower than
predicted [.H]_{b}, a pelt with modest to poor insulative quality,
good thermogenic ability but poor heat tolerance, modest to poor
capacity for evaporative cooling, no well-defined molt cycle, no
cyclic period of fattening, nocturnal habits, and a modestly diverse
diet of high-enough quality to provide for an average reproductive
potential. Although this pedigree contributed to the success of this
family in tropical and subtropical forests, it limited the ability of
its members to expand their distributions into cooler, less stable
climates. Viewed in this perspective, _Procyon lotor_'s high basal
metabolic rate, extraordinarily diverse diet, well-defined cyclic
changes in fat content and thermal conductance, high level of heat
tolerance, high capacity for evaporative cooling, and high
reproductive potential all stand out in sharp contrast to the
condition described for other procyonids. This suggests that the North
American raccoon represents culmination of a divergent evolutionary
event that has given this species the ability to break out of the old
procyonid mold and carry the family into new habitats and climates.
APPENDIX: LIST OF SYMBOLS
a potential age of females first producing young
b potential annual birth rate of female young
C_{a} conductance of air
C_{d} conductance of den walls
C_{m} minimum thermal conductance
C_{md} minimum dry thermal conductance
C_{mw} minimum wet thermal conductance
C_{mwr} ratio of measured to predicted minimum wet thermal
conductance
C_{t} total conductance
D_{d} diversity of diet
D_{dr} ratio of food categories actually used by a species to
the total number of food categories taken by all species
tested
[.E] evaporative water loss
E_{c} ratio of evaporative heat lost to metabolic heat produced
[.E]_{eq} oxygen equivalent for heat lost by evaporation
[.H]_{b} basal metabolic rate
[.H]_{r} lowest resting metabolic rate at each temperature
H_{br} ratio of measured to predicted basal metabolic rate
m mass of animal
m_{w} mass of water
n potential age of females producing their final young
r_{max} intrinsic rate of natural increase
r_{maxe} expected intrinsic rate of natural increase
r_{maxr} ratio of calculated to expected intrinsic rate of natural
increase
RQ respiratory quotient
T_{a} chamber air temperature
T_{b} body temperature
T_{lc} lower critical temperature
T_{n} thermoneutral zone
T_{uc} upper critical temperature
t time
[.V]_{a} rate of air flow through U-tubes
[.V]_{e} rate of air flow into metabolism chamber
[alpha] active phase of the daily cycle
[gamma] heat equivalent of oxygen
[lambda] heat of vaporization of water
[rho] rest phase of the daily cycle
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TRANSCRIBER'S NOTES
With the exception of the typographical corrections listed below
and some minor changes that may have been made in moving tables or
illustrations so that they are rejoined, the text presented is that
published in the original printed media. Also, the second instance
of t-tests was changed to _t_-tests as the letter "t" is usually
italicized by statisticians.
Emphasis Notation
_Text_ = Italics
$Text$ = Bold
a_{b} = a with subscript b
a^{b} = a with superscript b
Typographical Corrections
Page ii, Instituion's => Institution's
Page 1, linages => lineages
Page 4, consumate => consummate
Page 21, Table 10, footnote f => Table 10, footnote b
Page 26, Nassua => Nasua
Page 31, Incoporated => Incorporated
Page 34, Gettleman => Gittleman
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