Title: Genera and Subgenera of Chipmunks
Author: White, John A.
Language: English
As this book started as an ASCII text book there are no pictures available.


*** Start of this LibraryBlog Digital Book "Genera and Subgenera of Chipmunks" ***


 Genera and Subgenera of Chipmunks
 BY
 JOHN A. WHITE

 University of Kansas Publications
 Museum of Natural History

 Volume 5, No. 32, pp. 543-561, 12 figures in text

 December 1, 1953

 University of Kansas
 LAWRENCE
 1953



 UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

 Editors: E. Raymond Hall, Chairman, A. Byron Leonard,
 and Robert W. Wilson

 Volume 5, No. 32, pp. 543-561, 12 figures in text

 December 1, 1953

 UNIVERSITY OF KANSAS
 Lawrence, Kansas

 PRINTED BY
 FERD VOILAND, JR., STATE PRINTER
 TOPEKA, KANSAS
 1953



 Genera and Subgenera of Chipmunks
 By
 JOHN A. WHITE


CONTENTS


                                                      PAGE
 Introduction                                          546
 Historical                                            546
 Methods, Materials, and Acknowledgments               547
 Evaluation of Characters                              548
   Characters in which the Subgenera _Eutamias_
       and _Neotamias_ Agree, but Differ from the
       Genus _Tamias_                                  548
     Structure of the Malleus                          548
     Structure of the Baculum                          549
     Structure of the Hyoid Apparatus                  552
     Presence or Absence of P3                         553
     Length of Tail in Relation to Total Length        553
     Color Pattern                                     554
   Characters in which the Subgenus _Eutamias_
       and the Genus _Tamias_ Agree, but Differ
       from the Subgenus _Neotamias_                   554
     Shape of the Infraorbital Foramen                 554
     Width of the Postorbital Process at Base          554
     Position of the Supraorbital Notch                554
     Degree of Convergence of the Upper Tooth-rows     554
     Degree of Constriction of the Interorbital Region 555
     Shape of the Pinna                                555
   Structural Features that are too Weakly Expressed
     to be of Taxonomic Use                            555
   Discussion                                          555
 Genera and Subgenera                                  557
   Genus _Eutamias_ Trouessart                         557
     Subgenus _Eutamias_ Trouessart                    558
     Subgenus _Neotamias_ Howell                       558
   Genus _Tamias_ Illiger                              559
 Discussion                                            559
 Conclusions                                           560
 Literature Cited                                      561


ILLUSTRATIONS

 Figs. 1-3. Dorsomedial Views of Malleus               549
 Figs. 4-10. Lateral Views of Baculum                  551
 Figs. 11-12. Ventral Views of Hyoid Apparatus         552



INTRODUCTION


The supraspecific categories of the chipmunks, as in most other groups
of squirrels, have been a source of controversy for many years. Before
presenting new evidence and a review of older evidence bearing on the
problem, it seems desirable to review briefly in chronological order,
the taxonomic history of the genera and subgenera of the chipmunks.


HISTORICAL

Linnaeus (1758:64) described the eastern North American chipmunks under
the name _Sciurus striatus_ and based his description on that of Catesby
(1743:75). The Asiatic chipmunk was first described, under the name
_Sciurus sibiricus_, by Laxmann (1769:69). Schreber (1785, 4:790)
separated the Asiatic and North American chipmunks into the Asiatic and
American varieties. Gmelin (1788:50) followed Schreber and, employing
trinomials, used the names _Sciurus striatus asiaticus_ and _S. s.
americanus_. Illiger (1811:83) proposed _Tamias_ as the generic name of
the chipmunk of eastern North America. Say (1823:45) described _Sciurus
quadrivittatus_, the first species of chipmunk known from western North
America.

Trouessart (1880:86-87) proposed _Eutamias_ as the subgeneric name to
include the western North American and Asiatic chipmunks.

Merriam (1897:189-190) raised _Eutamias_ to full generic rank. In so
doing he neither listed nor described any characters but wrote that "it
will be observed that the name _Eutamias_, proposed by Trouessart in
1880 as a subgenus of _Tamias_ is here adopted as a full genus. This is
because of the conviction that the superficial resemblance between the
two groups is accidental parallelism, in no way indicative of affinity.
In fact the two groups, if my notion of their relationship is correct,
had different ancestors, _Tamias_ being an offshoot of the
ground-squirrels of the subgenus _Ictidomys_ of Allen, and _Eutamias_
of the subgenus _Ammospermophilus_, Merriam."

Howell (1929:23) proposed _Neotamias_ as the subgeneric name for the
chipmunks of western North America, of the genus _Eutamias_.

Ellerman (1940, 1:426) gave _Eutamias_ and _Neotamias_ equal subgeneric
rank with _Tamias_ under the genus _Tamias_; on pages 427-428 he quoted
Merriam, as I have done above, and later, after quoting the key to the
genera and subgenera of chipmunks of Howell (1929:11), Ellerman wrote
(_op. cit._: 428-429), "This key convinces me that all these forms
must be referred to one genus only. The characters given to separate
'_Eutamias_' from _Tamias_ are based only on the absence or presence of
the functionless premolar, and on the colour pattern. If colour pattern
is to be used as a generic character, it seems _Citellus suslicus_ will
require a new name when compared with _C. citellus_, etc." And again,
"The Asiatic chipmunk is intermediate between typical _Tamias_ and the
small American forms in many characters." To substantiate this, Ellerman
(_loc. cit._) quotes Howell (_loc. cit._), in comparing the subgenera
_Eutamias_ and _Neotamias_, as follows: "'the ears [of subgenus
_Eutamias_] are broad, rounded, of medium height, much as in _Tamias_;
postorbital broad at base, tapering to a point, much as in _Tamias_;
interorbital constriction slight, as in _Tamias_; upper molariform tooth
rows slightly convergent posteriorly, as in _Tamias_.'" Ellerman (_loc.
cit._) again quotes Howell (_loc. cit._), "'_Eutamias_ of Asia resembles
_Tamias_ of North America and differs from American _Eutamias_ in a
number of characters, notably the shape of the anteorbital foramen,
the postorbital process, the breadth of the interorbital region, the
development of the lambdoidal crest, and the shape of the external ears.
On the other hand, American _Eutamias_ agrees with the Asiatic members
of the genus in the shape of the rostrum, the well-defined striations of
the upper incisors, the presence of the extra peg-like premolar, and in
the pattern of the dorsal stripes.'"

Bryant (1945:372) wrote, "I am convinced that Ellerman's interpretation
of the relationships of the chipmunks is correct." After commenting
that the presence or absence of P3, "is of significance only in
distinguishing between species of squirrels," Bryant adds that "The
other differences between the eastern and the western chipmunks do not
appear to be of sufficient phylogenetic importance to warrant the
retention of the two groups as genera."


METHODS, MATERIALS, AND ACKNOWLEDGMENTS

     Characters previously mentioned in the literature as having
     taxonomic worth for supraspecific categories of chipmunks were
     checked by me on specimens old enough to have worn permanent
     premolars. Some structural features not previously used were
     found to have taxonomic significance. The baculum in each of
     the supraspecific categories of sciurids of North America was
     examined; the bacula were processed by the method described by
     White (1951:125) to obviate "variation" caused by shriveling of
     the smaller bacula or breaking of the more delicate parts of
     the larger bacula. Mallei and hyoid bones of the genera and
     subgenera of the chipmunks were mostly studied in the dry
     state. Part of the hyoid musculature in these same groups of
     chipmunks was dissected.

     In all, I studied more than 1,000 skulls and skins of the
     subgenus _Neotamias_, approximately 50 skulls and skins of
     _Tamias striatus_, and 15 skulls and skins of the subgenus
     _Eutamias_ (_Eutamias sibiricus asiaticus_ from Manchuria).
     Numerous other specimens were examined but not in such detail.

     I am grateful to Professor E. Raymond Hall for guidance in
     the study. For encouragement and advice I am grateful also
     to Doctors Robert W. Wilson, Cecil G. Lalicker, Edwin C.
     Galbreath, Keith R. Kelson, E. Lendell Cockrum, Olin L. Webb,
     and others at the Museum of Natural History, and in the
     Department of Zoology of the University of Kansas. My wife,
     Alice M. White, made the drawings and helped me in many other
     ways. For lending specimens I thank Dr. David H. Johnson of the
     United States National Museum, and Dr. George C. Rinker of the
     Department of Anatomy, University of Michigan.

     Assistance with field work is acknowledged from the Kansas
     University Endowment Association, the National Science
     Foundation, and the United States Navy, Office of Naval
     Research, through contract No. NR161 791.


EVALUATION OF CHARACTERS

The following paragraphs treat the characters listed by Howell,
Ellerman, and Bryant, and such additional characters as I have found
useful in characterizing the genera and subgenera of chipmunks. Some of
the findings, I think, illustrate how study of such mammalian structures
as the baculum, malleus, and hyoid apparatus--structures that seem to
be little influenced by the changing external environment--clarifies
relationships, if these previously were estimated only from other parts
of the anatomy of Recent specimens.

The structural features and characters to be discussed, or listed, below
may be arranged in three categories as follows: 1) Characters in which
the subgenera _Eutamias_ and _Neotamias_ agree but are different from
the genus _Tamias_; 2) Characters in which the subgenus _Eutamias_
and the genus _Tamias_ agree but are different from the subgenus
_Neotamias_; 3) Structural features that are too weakly expressed to
be of taxonomic use.


CHARACTERS IN WHICH THE SUBGENERA EUTAMIAS AND NEOTAMIAS AGREE, BUT
DIFFER FROM THE GENUS TAMIAS

_Structure of the Malleus._--The malleus in chipmunks is composed of
a head and neck, a manubrium which has a spatulate process at the end
opposite the head, and a muscular process situated about halfway between
the spatulate process and the head of the malleus. An articular facet
begins on the manubrium near the neck and spirals halfway around the
head of the malleus. A lamina extends from the anterior edge of the head
and neck, tapers to a point and joins the tympanic bulla anteriorly
where there is a suture between the lamina and bulla. The lamina is one
half as long as the rest of the malleus (see figs. 1-3).

The head of the malleus in _Tamias_ is clearly more elongated than in
_Eutamias_. The plane formed by the lamina in _Eutamias_ makes an angle
of approximately 90 degrees with the plane formed by the manubrium; in
_Tamias_ the two planes make an angle of approximately 60 degrees.

Examination of series of mallei of _Eutamias_ and _Tamias_ indicate that
there is slight individual variation, slight variation with age, and no
secondary sexual variation. Intraspecific variation in the subgenus
_Neotamias_ is slight, consisting of differences in size. Specimens
of the subgenus _Eutamias_ from Manchuria have mallei which are
morphologically close to the mallei of the subgenus _Neotamias_.

 [Illustration: FIGS. 1-3. Dorsomedial views of left malleus.

 FIG. 1. _Tamias striatus lysteri_, No. 11920 sex?;
 from Carroll Co., New Hampshire.

 FIG. 2. _Eutamias sibiricus asiaticus_, No. 199637 male NM;
 from I-mien-po, N. Kirin, Manchuria.

 FIG. 3. _Eutamias townsendii senex_, No. 165 male;
 from Lake Tahoe, California.]

_Structure of the Baculum._--In discussing the baculum in _Eutamias_ and
_Tamias_, it seems desirable to do so in the light of the structure of
the baculum in other sciurids.

The bacula of North American sciurids are divisible into six distinct
types represented by those of the genera _Spermophilus_, _Marmota_,
_Sciurus_, _Tamiasciurus_, _Eutamias_, and _Glaucomys_.

The type of baculum in _Spermophilus_ is spoonshaped with a ventral
process that is spinelike or keellike. Also, spines usually are present
along the margin of the "spoon." The base (proximal end) of the baculum
is broad, and some species have a winglike process extending dorsally
and partly covering a longitudinal groove. The shaft is more or less
curved downward in the middle (see figs. 7, 10).

In _Marmota_ the baculum is greatly enlarged at the posterior end and
forms a shieldlike surface. The ventral surface of the base is flattened
and the ventral surface of the shaft curves slightly ventrally then
dorsally to the tip. The dorsal region of the base culminates in a
point, from which there is a ridge that extends anteriorly and that
tapers rapidly into the shaft near the tip. The tip, dorsally, has a
slight depression surrounded by knobs, which are more or less well
defined, and which resemble, topographically, the spines described for
_Spermophilus_ (see fig. 8).

In _Sciurus_ the baculum is semispoonshaped and asymmetrical. There
is a winglike process on one side and a spine, which projects
lateroventrally, on the other side of the tip. The base of the baculum
is broad but not so broad as in most species of _Spermophilus_.
Extending posteriorly from the region of the tip, at which point a
spine projects lateroventrally, there is a ridge, which is often
partly ossified and that extends to a point near the base (see fig. 4).

In _Tamiasciurus_ the baculum is absent or vestigial (Layne,
1952:457-459).

In _Eutamias_ the baculum is broad at the base and the shaft tapers
distally to the junction of the shaft and tip, or the base is only
slightly wider than any part of the shaft. The tip often forms an abrupt
angle with the shaft and there is a keel on the dorsal surface of the
tip (see figs. 5, 6).

The baculum in _Glaucomys_ is the most distinctive of that of any
American sciurid. According to Pocock (1923:243-244), "The baculum [of
_G. volans_] is exceedingly long and slender, slightly sinuous in its
proximal third, and inclined slightly upwards distally. The extreme apex
is bifid, the lower process being rounded, the upper more pointed. On
the left side there is a long crest running from the summit of the
upper terminal process and ending abruptly behind the left side about
one-third of the distance from the proximal end of the bone. It lies
over a well-marked groove, and there is a second shallower groove on the
right side of the bone." The baculum of _G. sabrinus_ is markedly wider,
more flattened and shorter than in _G. volans_. The crest, which is also
present in _G. volans_, starts from the upper terminal process and
extends to the base of the baculum on the left side. There is a knoblike
process on the crest at a point three fourths the length of the baculum
from its base. The distal one third of the baculum curves sharply but
smoothly upwards (see fig. 9).

Keeping in mind that the baculum in the North American sciurids can be
classified into six structural groups, as given above, the baculum in
each of the subgenera _Eutamias_ and _Neotamias_ and in the genus
_Tamias_ is briefly described.

In the subgenus _Neotamias_ the baculum resembles a leg and foot of man,
with a narrow ridge (keel) in the center of the "instep" of the foot
(Howell 1929:27). The tip (=foot) curves dorsally at the distal end
(see figs. 5, 6).

 [Illustration: FIGS. 4-10. Lateral views of right side
 (except left-lateral view in fig. 9) of baculum.

 FIG. 4. _Sciurus aureogaster aureogaster_,
 No. 37000; from 70 km. S C. Victoria (by highway), and 6 km. W of
 highway, Tamaulipas.

 FIG. 5. _Eutamias quadrimaculatus_, No. 95780 BS;
 from Mountains near Quincy, Plumas Co., California.

 FIG. 6. _Eutamias sibiricus asiaticus_, No. 199632 NM;
 from 120 mi. up the Yalu River, Korea.

 FIG. 7. _Tamias striatus lysteri_, No. 193493 NM;
 from Locust Grove, New York.

 FIG. 8. _Marmota flaviventer dacota_, No. 41641;
 from 1-1/2 mi. E Buckhorn, 6,150 ft., Weston Co., Wyoming.

 FIG. 9. _Glaucomys sabrinus bangsi_, No. 15079;
 from 10 mi. NE Pinedale, 8,000 ft., Sublette Co., Wyoming.

 FIG. 10. _Spermophilus armatus_, No. 14888;
 from W end Half Moon Lake, 7,900 ft., Sublette Co., Wyoming.]

In the subgenus _Eutamias_, the baculum "tapers gradually from base
to tip, the distal portion upturned in an even curve and slightly
flattened ..." (_op. cit._:26). Microscopic examination reveals that
there is a faint keel on the dorsal surface of the tip.

_Eutamias_, like _Callosciurus_, _Menetes_, _Dremomys_, _Lariscus_,
_Rhinosciurus_, and _Nannosciurus_, has a keel on the dorsal surface of
the tip of the baculum (compare figures 5 and 6 with the descriptions
and figures in Pocock, 1923:217-225).

In _Tamias_ the baculum is "a slender bone 4.5-5 millimeters in length,
nearly straight, upturned at the tip and slightly expanded into the
shape of a narrow spoon or scoop, with a slight median ridge on the
under surface." (Howell _op. cit._:13.) The "median ridge" is a keel on
the ventral surface. In having a keel on the ventral surface of the tip,
the baculum of _Tamias_ is comparable to that of _Spermophilus_.

Examination of series of bacula of the subgenus _Neotamias_ and the
genus _Tamias_ indicates, as in the case of the mallei, that there is
slight individual variation and slight variation with age. In the
subgenus _Neotamias_ interspecific variation in the baculum is
considerable, but the general plan of structure remains constant. From
this study of variation of the baculum in American chipmunks, it can be
extrapolated that the baculum in the Asiatic _Eutamias_ would show
little individual variation in structure. I have seen only two bacula
of the Asiatic _Eutamias_.

 [Illustration:
 FIGS. 11-12. Ventral views of the hyoid apparatus in
 _Tamias_ and _Eutamias_.

 FIG. 11. _Tamias striatus venustus_, No. 11072 female;
 from Winslow, Washington Co., Arkansas.

 FIG. 12. _Eutamias minimus operarius_, No. 5376 male;
 from 14 mi. N El Rito, Rio Arriba Co., New Mexico.]

_Structure of the Hyoid Apparatus._--The hyoid apparatus in the
chipmunks is made up of an arched basihyal with a thyrohyal attached to
each limb of this "arch." To each junction between the "arch" and the
thyrohyals, a hypohyal is attached by ligaments to a flat articular
surface. A ceratohyal then is attached posteriorly to the hypohyal and
a stylohyal ligament is attached to each ceratohyal posteriorly. The
stylohyal is loosely attached along its sides to the tympanic bulla and
finally attached, at the posterior end, to the bulla at a point slightly
ventral and posterior to the auditory meatus.

In the genus _Eutamias_ the hypohyal and ceratohyal are completely fused
in adults, the suture between these two bones being visible in juvenal
specimens (see fig. 12).

In the genus _Tamias_ the hypohyal and ceratohyal remain distinct
throughout life. The hypohyal may frequently be divided into two parts,
a variation which is also present in _Marmota_.

The musculature associated with the hyoid apparatus in _Eutamias_ and
_Tamias_ is as described by Bryant (1945:310, 316) for the Nearctic
squirrels. However, the conjoining tendon of the anterior and posterior
pairs of digastric muscles is ribbonlike in _Eutamias_ and rodlike
(rounded in cross section) in _Tamias_.

_The presence or absence of P3 and the projection of the anterior root
of P4 in relation to the masseteric knob._--Only rarely is P3 absent
in _Eutamias_ or present in _Tamias_. P3 in specimens of old adult
_Eutamias_, shows wear, thus suggesting that P3 is functional in older
chipmunks. In _Eutamias_, which normally has a P3, the anterior root of
P4 projects to the outside of the masseteric knob, whereas in _Tamias_,
which normally lacks a P3, the anterior root of P4 projects directly
to the masseteric knob or to the lingual side of this structure. The
projection of the anterior root of P4 seems to be correlated with the
presence or absence of P3. However, in a specimen of _Tamias striatus
rufescens_ (No. 11117 KU), the left P3 is present, yet the anterior
root of P4 still projects to the lingual side of the masseteric knob.

Ellerman (1940:48) and Bryant (1945:368-369, 372) think that the
presence or absence of P3 is not of generic significance in chipmunks,
since P3 is vestigial and probably is in the process of being lost, and
since this character is rarely used as a generic character in other
sciurids. I think that the presence or absence of P3, together with the
projection of the anterior root of P4 in relation to the masseteric
knob, is of generic significance, for, squirrels in general have
retained the dentition and dental formula of a primitive rodent, and
any change in the pattern of the teeth or in dental formula is, in my
opinion, of a fundamental nature.

_Length of tail in relation to total length._--The tail in _Eutamias_ is
more than 40 per cent of the total length, whereas in _Tamias_ the tail
is less than 38 per cent of the total length. In this respect _Tamias_
resembles most ground squirrels of the genus _Spermophilus_.

_Color pattern._--The chipmunks vary but little in color pattern, for,
even in _Eutamias dorsalis_, which is one of the most aberrant of the
chipmunks in color pattern, the pattern is characteristic of _Eutamias_.

The width of the longitudinal stripes is uniform in _Eutamias_ whereas
in _Tamias_ the dorsal, longitudinal light stripes are more than twice
as wide as the other stripes.

In _Eutamias_, only the two lateralmost dark stripes are short, whereas
in _Tamias_ all four of the lateral dark stripes are short; none extends
to the rump or to the shoulder.

The dark median stripe is present in both _Eutamias_ and _Tamias_ as
well as in other genera such as _Callosciurus_ and _Menetes_ (Ellerman
1940:390).


CHARACTERS IN WHICH THE SUBGENUS EUTAMIAS AND THE GENUS TAMIAS AGREE,
BUT DIFFER FROM THE SUBGENUS NEOTAMIAS

_Shape of the infraorbital foramen._--In the subgenus _Eutamias_ and
in the genus _Tamias_ the infraorbital foramen is rounded, whereas in
most species of the subgenus _Neotamias_ the foramen is slitlike. In
_Eutamias townsendii_, however, the infraorbital foramen is rounded
as much as in the subgenus _Eutamias_ and in the genus _Tamias_.

_Width of the postorbital process at base._--The postorbital process is
broader at the base in the subgenus _Eutamias_ and in the genus _Tamias_
than in most species of the subgenus _Neotamias_. In _E. townsendii_,
however, this process is relatively as broad as in the subgenus
_Eutamias_ and in the genus _Tamias_.

_Position of the supraorbital notch in relation to the posterior notch
of the zygomatic plate._--In the subgenus _Eutamias_ and in the genus
_Tamias_ the supraorbital notch is distinctly anterior to the posterior
notch of the zygomatic plate, whereas in the subgenus _Neotamias_, the
supraorbital notch is only slightly anterior to the posterior notch of
the zygomatic plate. This difference may be correlated with differences
in size, since specimens of the subgenus _Eutamias_ and the genus
_Tamias_ are larger than specimens of the subgenus _Neotamias_.

_Degree of convergence of the upper tooth-rows._--The rows of upper
cheek-teeth converge posteriorly in the subgenus _Eutamias_ and in
the genus _Tamias_, except that in some specimens of _E. sibiricus
asiaticus_ the rows of upper cheek-teeth are nearly parallel to each
other. In most species of the subgenus _Neotamias_ the rows of upper
cheek-teeth are nearly parallel to each other, although in the specimens
that I have seen of _E. townsendii_, the upper rows of cheek-teeth
converge posteriorly.

_Degree of constriction of the interorbital region._--The interorbital
region is more constricted in most species of the subgenus _Neotamias_
than in the subgenus _Eutamias_ and the genus _Tamias_. In specimens of
_E. t. townsendii_ of the subgenus _Neotamias_, however, the degree of
constriction of the interorbital region is approximately the same as in
the subgenus _Eutamias_ and the genus _Tamias_.

_Shape of the pinna._--The pinna is narrower and more pointed in the
subgenus _Neotamias_ than in the subgenus _Eutamias_ and the genus
_Tamias_.


STRUCTURAL FEATURES THAT ARE TOO WEAKLY EXPRESSED TO BE OF TAXONOMIC USE

The following alleged characters have been mentioned in the literature.
Since the degree of expression of these features is so slight, or
since there is marked variation within one or more natural groups of
chipmunks, no reliance is here placed on these features. They are as
follows: (1) Degree of the posterior projection of the palate; (2)
relative size of the auditory bullae; (3) position, in relation to P4,
of the notch in the posterior edge of the zygomatic plate; (4) size of
m3 in relation to m2; (5) degree of development of the mesoconid and
ectolophid of the lower molars; (6) shape and length of the rostrum;
(7) degree of distinctness of minute longitudinal grooves on the upper
incisors.

A variation that does not readily fall in any one of the three
categories mentioned above is the degree of development of the
lambdoidal crest. The crest is least developed in the subgenus
_Neotamias_ and most developed in the genus _Tamias_. The larger the
skull, the more the lambdoidal crest is developed; seemingly, therefore,
the degree of development is an expression of size of the skull and may
be determined by heterogonic growth.


DISCUSSION

As shown in table 1, there are ten characters by means of which
_Eutamias_ and _Tamias_ can be separated consistently. The subgenus
_Eutamias_ occurs on the Asiatic side and the subgenus _Neotamias_
occurs on the North American side of Bering Strait, yet the two
subgenera agree in the ten features referred to. Although the subgenus
_Neotamias_ and the genus _Tamias_ occur together in parts of the United
States and Canada, they differ in the ten features, indicating that the
subgenera _Eutamias_ and _Neotamias_ are more closely related to each
other than either is to _Tamias_.

TABLE 1.--CHARACTERS BY MEANS OF WHICH THE GENERA EUTAMIAS AND TAMIAS
CAN BE DISTINGUISHED

=======================================================================
            Character                |   _Eutamias_   |   _Tamias_
-------------------------------------+----------------+----------------
Shape of head of malleus.            | not elongated. | elongated.
-------------------------------------+----------------+----------------
Angle formed by planes of lamina and | approximately  | approximately
manubrium of malleus.                | 90 degrees.    | 60 degrees.
-------------------------------------+----------------+----------------
Position of keel on tip of baculum.  | dorsal.        | ventral.
-------------------------------------+----------------+----------------
Relation of hypohyal and ceratohyal  | fused in       | never fused.
bones of hyoid apparatus.            | adults.        |
-------------------------------------+----------------+----------------
Appearance in cross section of       | flattened.     | rounded.
conjoining tendon of anterior and    |                |
posterior digastric muscles.         |                |
-------------------------------------+----------------+----------------
Presence or absence of P3.           | present.       | absent.
-------------------------------------+----------------+----------------
Projection of anterior root of P4 in | buccal.        | lingual.
relation to masseteric knob.         |                |
-------------------------------------+----------------+----------------
Length of tail in relation to total  | more than      | less than
length.                              | 40 per cent.   | 38 per cent.
-------------------------------------+----------------+----------------
Width of longitudinal stripes.       | subequal.      | median pair of
                                     |                | light stripes
                                     |                | twice as wide
                                     |                | as others.
-------------------------------------+----------------+----------------
Length of lateral longitudinal light | outermost      | both pairs
stripes.                             | pair short.    | short.
-----------------------------------------------------------------------

It must be pointed out here that the subgenus _Neotamias_ always differs
from both the subgenus _Eutamias_ and the genus _Tamias_ in pointed
_versus_ rounded pinna of ear (see table 2) and in the supraorbital
notch being slightly posterior to or even with, instead of distinctly
anterior to, the posterior notch of the zygomatic plate. The relative
position of these two notches, however, seems to be a matter of relative
(heterogonic) growth. Further, the base of the postorbital process of
the frontal usually is narrower (relative to the length of the process)
in the subgenus _Neotamias_ but there is a gradation in this feature in
_Neotamias_ culminating in the species _E. townsendii_ in which the
bases of the processes are relatively as broad as in the subgenus
_Eutamias_ and the genus _Tamias_. The same condition obtains in the
shape of the infraorbital foramen which is subovate to rounded in the
subgenus _Neotamias_ and always rounded in the other chipmunks.

TABLE 2.--CHARACTERS BY MEANS OF WHICH THE SUBGENUS EUTAMIAS AND THE
GENUS TAMIAS MAY BE DISTINGUISHED FROM THE SUBGENUS NEOTAMIAS

========================================================================
        Character              | subgenus     | subgenus    | genus
                               | _Neotamias_  | _Eutamias_  | _Tamias_
-------------------------------+--------------+-------------+-----------
Shape of infraorbital foramen. | subovate to  | always      | always
                               | rounded.     | rounded.    | rounded.
-------------------------------+--------------+-------------+-----------
Relative width of the          | narrow to    |             |
postorbital process at base.   | broad.       | broad.      | broad.
-------------------------------+--------------+-------------+-----------
Position of supraorbital notch | even with or |             |
in relation to posterior       | slightly     | anterior.   | anterior.
notch of zygomatic plate.      | posterior.   |             |
-------------------------------+--------------+-------------+-----------
Convergence, posteriorly,      | not always.  | not always. | always.
of upper tooth-rows.           |              |             |
-------------------------------+--------------+-------------+-----------
Degree of constriction of      | slight       | marked.     | marked.
interorbital region.           | to marked.   |             |
-------------------------------+--------------+-------------+-----------
Shape of pinna.                | long and     | broad and   | broad and
                               | pointed.     | rounded.    | rounded.
------------------------------------------------------------------------

These differences of _Neotamias_ are so slight in comparison with the
similarities (ten features mentioned above) that _Neotamias_ here is
accorded only subgeneric rank under the genus _Eutamias_, instead of
generic rank.

Howell's (1929) arrangement of the genera and subgenera of chipmunks is
judged to be correct as indicated by the following arrangement that I
propose.


GENERA AND SUBGENERA

Genus #Eutamias# Trouessart

 _Eutamias_ Trouessart, E. L. Catal. Mamm. viv. et foss., Rodentia,
   in Bull. Soc. d'Etudes Sci. d'Angers, 10:86-87, 1880. Type
   _Sciurus striatus asiaticus_ Gmelin.

 _Eutamias_, Merriam, C. H., Proc. Biol. Soc. Washington,
   11:189-190, July 1, 1897.

 _Eutamias_, Howell, A. H., N. Amer. Fauna, 52:26, November 30, 1929.

 _Tamias_, Ellerman, J. R., The families and genera of living rodents.
   British Mus. (Nat. Hist.), 1:426, June 8, 1940.

 _Tamias_, Bryant, M. D., Amer. Midland Nat., 33:732, March 1945.

_Diagnosis._--Skull lightly built, narrow; postorbital process light
and weak; lacrimal not elongated; infraorbital foramen lacks canal,
relatively larger than in most sciurids; P3 present; head of malleus not
elongated; plane of manubrium of malleus 90 degrees to plane of lamina;
hypohyal and ceratohyal bones of hyoid apparatus fused in adults;
conjoining tendon between anterior and posterior sets of digastric
muscles ribbonlike; keel on dorsal side of tip of baculum; tail more
than 40 per cent of total length; five longitudinal dark stripes evenly
spaced and subequal in width; two lateral dark stripes short.



Subgenus #Eutamias# Trouessart

 _Eutamias_ Trouessart, E. L. Catal. Mamm. viv. et foss., Rodentia, in
   Bull. Soc. d'Etudes Sci. d'Angers 10:86-87, 1880. Type _Sciurus
   striatus asiaticus_ Gmelin.

 _Eutamias_, Howell, A. H., N. Amer. Fauna, 52:26, November 30, 1929.

 _Eutamias_, Ellerman, J. R., The families and genera of living rodents.
   British Mus. (Nat. Hist.), 1:426, June 8, 1940.

 _Eutamias_, Bryant, M. D., Amer. Midland Nat. 33:732, March 1945.

_Diagnosis._--Size large; lambdoidal crest moderately developed;
supraorbital notches distinctly anterior to posterior notch of zygomatic
plate; baculum with faint keel on dorsal surface of tip which curves
upward; pelage coarse; ears broad, rounded, of medium height.

_Geographic range._--Palearctic. West to Dvina and Kama rivers, Vologda,
and Kazan, in European Russia. South to southern Ural Mountains, Altai
Mountains; Kansu, Szechwan, Shensi, Shansi, and Chihli provinces of
China; Manchuria and Korea. East to Hokkaido Island, Japan; Kunashiri
Island, southern Kurile Islands; Sakhalin Island, and Yakutsk, Siberia.
North nearly to Arctic Coast in Siberia and European Russia (Ellerman
and Morrison-Scott 1951:503).



Subgenus #Neotamias# Howell

 _Neotamias_ Howell, A. H., N. Amer. Fauna, 52:26, November 30, 1929.
   Type, _Eutamias merriami_ J. A. Allen [=_Tamias asiaticus merriami_
   J. A. Allen].

 _Neotamias_, Ellerman, J. R., The families and genera of living rodents.
   British Mus. (Nat. Hist.), 1:426, June 8, 1940.

 _Neotamias_, Bryant, M. D., Amer. Midland Nat., 33:372, March, 1945.

_Diagnosis._--Size small to medium; lambdoidal crest barely discernible;
supraorbital notches even with, or posterior to, posterior notch of
zygomatic plate; baculum with distinct keel on dorsal surface of tip
which curves upward; pelage silky; ears long and pointed.

_Geographic range._--Western Nearctic. West to Pacific Coast. South
to Lat. 20°30' in Baja California and to northwestern Durango and
southeastern Coahuila, Mexico. East to eastern New Mexico, westernmost
Oklahoma, eastern Colorado, Wyoming, northwestern Nebraska, western
and northwestern South Dakota, western and northwestern North Dakota,
northeastern Minnesota, northern Wisconsin and Upper Peninsula of
Michigan, and eastern Ontario. North to southwestern shore of Hudson
Bay, southern shore of Great Slave Lake and Yukon River, Yukon.



Genus #Tamias# Illiger

 _Tamias_ Illiger, J. K. W., Prodromus Syst. Mam. Avium, pp. 83, 1811.
   Type, _Sciurus striatus_ Linnaeus.

 _Tamias_, Howell, A. H., N. Amer. Fauna, 52:26, November 30, 1929.

 _Tamias_, Ellerman, J. R., The families and genera of living rodents.
   British Mus. (Nat. Hist.), 1:426, June 8, 1940.

 _Tamias_, Bryant, M. D., Amer. Midland Nat. 33:372, March, 1945.

_Diagnosis._--Skull lightly built, narrow; postorbital process small
and weak; lacrimal not elongated; infraorbital foramen lacks canal,
relatively larger than in most sciurids; P3 absent; head of malleus
elongated; plane of manubrium of malleus forms 60 degree angle with
plane of lamina; hypohyal and ceratohyal bones of hyoid apparatus fused
in adults; conjoining tendon of anterior and posterior digastric muscles
rounded in cross section; keel on ventral surface of tip which curves
upward in baculum; tail less than 38 per cent of total length; five
longitudinal dark and four longitudinal light stripes present but two
dorsal light stripes at least twice as broad as other stripes; four
lateral dark stripes short.

_Geographic range._--Eastern Nearctic. West to Turtle Mountains, North
Dakota; eastern North Dakota, eastern South Dakota, Nebraska, Kansas,
and Oklahoma. South to southern Louisiana, Mississippi, Alabama,
northwestern Georgia. East to Atlantic Coast from South Carolina to Nova
Scotia. North to northeastern Quebec and southern tip of Hudson Bay.


DISCUSSION

Chipmunks are small striped squirrels that inhabit the Holarctic Realm
and that are found in similar niches in each of the three regions:
Palearctic, western Nearctic, and eastern Nearctic. Ellerman (1940) and
Bryant (1945) placed the chipmunks in three subgenera, corresponding to
the regions mentioned above, under the one genus _Tamias_. Critical
examination of new and old evidence reveals, nevertheless, that the
subgenera _Eutamias_ and _Neotamias_ of the genus _Eutamias_ are more
closely related to one another than either is to the genus _Tamias_.
This relationship can be seen clearly in the structure of the malleus,
baculum, hyoid apparatus, hyoid musculature, the presence or absence of
P3, the projection of the anterior root of P4 in relation to the
masseteric knob, and in the color pattern.

Because the genera _Eutamias_ and _Tamias_ occupy similar ecological
niches, the structural similarities that permit these animals to be
called chipmunks, show convergence, and thus can be assumed to be
adaptive. These similarities are in the molars, in shape of the skull,
in color pattern and in other features which have been used by many
systematists to interpret the phylogenetic relationships of the
squirrels. Pocock (1923:211), however, reviewed the taxonomic literature
on sciurids and wrote: "The conclusion very forcibly suggested by the
literature of the subject is the untrustworthiness of such characters."
Pocock (_op. cit._), correctly in my opinion, then established a
supraspecific classification of the sciurids based almost exclusively
on the structure of the baculum and glans penis. I have studied the
baculum in chipmunks and in all the major supraspecific groups of
Nearctic squirrels. The bacula of the Nearctic squirrels and those
of the Palearctic and Indian squirrels, other than the chipmunks, are
described and figured by Pocock (_op. cit._).

The baculum in _Eutamias_, in general plan of structure, resembles
the baculum in the genera _Callosciurus_, _Menetes_, _Rhinosciurus_,
_Lariscus_, _Dremomys_, and _Nannosciurus_, of the tribe Callosciurini
Simpson. The baculum in _Tamias_, in general plan of structure,
resembles that in _Spermophilus_ (=_Citellus_) and _Cynomys_ of the
tribe Marmotini Simpson. These tribes, designated by Simpson (1945:79),
are based on the corresponding subfamilies defined by Pocock
(1923:239-240) primarily on differences in the structure of the baculum.
I assign _Tamias_ to the tribe Marmotini. I assign _Eutamias_ to the
tribe Callosciurini, but do so only tentatively because I have not, at
first hand, studied the bacula of most of the Callosciurini. The fossil
record is too incomplete to reveal the time when the two tribes
diverged. The subgenera _Eutamias_ and _Neotamias_ are closely related.
Indications are that the divergence of the two subgenera occurred,
geologically, but a short time ago, possibly in Pleistocene time.


CONCLUSIONS

1. _Eutamias_ and _Tamias_ are distinct genera of chipmunks.

2. The subgenera _Eutamias_ and _Neotamias_ are valid, for, _Eutamias
sibiricus_ differs from all the species of the subgenus _Neotamias_ to a
greater degree than these species differ from one another.

3. The genera _Eutamias_ and _Tamias_ probably evolved from two distinct
lines of sciurids; one line (_Eutamias_) is represented by the tribe
Callosciurini, and the other (_Tamias_) by the tribe Marmotini.


LITERATURE CITED

 CATESBY, M.
 1743. The natural history of Carolina, Florida, and the Bahama Islands,
       etc., 2:i-xliv, 1-20, 1-100.

 ELLERMAN, J. R.
 1940. The families and genera of living rodents. British Mus. (Nat.
       Hist.), Vol. 1, pp. xxvi + 689, 189 figs., June 8.

 ELLERMAN, J. R., and T. C. S. MORRISON-SCOTT.
 1951. Checklist of Palearctic and Indian mammals. British Mus. (Nat.
       Hist.), pp. 1-810, 1 map, November 30.

 GMELIN, J. F.
 1788. Systema Naturae. 1:1-500.

 HOWELL, A. H.
 1929. Revision of the American chipmunks (genera _Tamias_ and _Eutamias_).
       N. Amer. Fauna 52:1-157, 10 pls., 9 maps, November 30.

 ILLIGER, J. K. W.
 1811. Prodromus systematis mammalium et avium additis terminis
       zoographicis utriuque classis, pp. xviii + 302, C. Salfeld.

 LAXMANN, M. E.
 1769. Sibiriche Brief, pp. iv + 104, Gottingen and Gotha.

 LAYNE, J. N.
 1952. The os genitale of the red squirrel, _Tamiasciurus_. Jour. Mamm.
       33:457-459, 1 fig., November 19.

 LINNAEUS, C.
 1758. Systema Naturae. 1:1-824.

 MERRIAM, C. H.
 1897. Notes on the chipmunks of the genus _Eutamias_ occurring west of
       the east base of the Cascade-Sierra system, with descriptions of
       new forms. Proc. Biol. Soc. Washington, 11:189-212, July 1.

 POCOCK, R. I.
 1923. The classification of the Sciuridae. Proc. Zool. Soc. London,
       1923:209-246, June.

 SAY, T. (_in_ Edwin James).
 1823. Account of an expedition from Pittsburgh to the Rocky Mountains,
       performed in the years 1819 and 1820, under the command of Major
       Stephen H. Long. From the notes of Major Long, Mr. T. Say, and
       other gentlemen of the exploring party. Compiled by Edwin James.
       Vol. 1, pp. 1-503; Vol. 2, pp. 1-442 + appendix i-xcviii.

 SCHREBER, J. C. D.
 1785. Saugthiere, 4:790-802.

 SIMPSON, G. G.
 1945. The principles of classification and a classification of mammals.
       Bull. Amer. Mus. Nat. Hist., 85:xvi + 350, October 5.

 TROUESSART, E. L.
 1880. Catalogue des mammiferes vivants et fossiles. Ordre des Rongeurs.
       Bull. Soc. d'Etudes Sci. d'Angers, 10:58-212.

 WHITE, J. A.
 1951. A practical method for mounting the bacula of small mammals.
       Jour. Mamm. 32:125, February 15.


_Transmitted June 26, 1953._





*** End of this LibraryBlog Digital Book "Genera and Subgenera of Chipmunks" ***

Copyright 2023 LibraryBlog. All rights reserved.