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Title: Natural History of the Prairie Vole (Mammalian Genus Microtus) - [KU. Vol. 1 No. 7]
Author: Jameson, E. W.
Language: English
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NATURAL HISTORY OF THE
PRAIRIE VOLE

(~Mammalian Genus~ Microtus)

BY

E. W. JAMESON, Jr.


University of Kansas Publications
Museum of Natural History

Volume 1, No. 7, pp. 125-151
October 6, 1947


UNIVERSITY OF KANSAS
LAWRENCE
1947



UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

Editors: E. Raymond Hall, Chairman; Donald S. Farner, H. H. Lane,
Edward H. Taylor

Volume 1, No. 7, pp. 125-151
October 6, 1947


UNIVERSITY OF KANSAS
Lawrence, Kansas


PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1947

21-6957



NATURAL HISTORY OF THE PRAIRIE VOLE
(MAMMALIAN GENUS MICROTUS)

By
E. W. JAMESON, JR.



CONTENTS


                                       PAGE

Introduction                            128

Methods                                 130

Molt                                    131

Food and Habitat                        132
  Types of cover                        132
  Cuttings                              133
  Food caches                           134
  Plants used as food and as cover      135

Associates                              137

Nest and Burrows                        137

External Parasites                      138
  Fleas (Siphonaptera)                  139
  Lice (Anoplura)                       141
  Mites (Acari except Ixodoidea)        142
  Ticks (Ixodoidea)                     143

Reproduction                            144
  Age classes                           144
  Fecundity                             144
  Size of litters                       146
  The breeding season                   147

Summary                                 149

Literature Cited                        150



INTRODUCTION


The prairie vole (_Microtus ochrogaster_) at Lawrence, Kansas, is
approximately 5-1/2 inches in length, of which the tail comprises 1-1/4
inches, and weighs approximately 1-1/2 ounces. The color on the dorsum
is dark gray with a grizzled appearance from the mixture of black and
fulvous on the long hairs; the venter is paler, sometimes pale fulvous
or cinnamon. The animal is compactly built much as are the other
microtine rodents. The short legs and short tail, small eyes and partly
hidden ears, and heavy and flattened head all suggest its semifossorial
mode of life. The prairie vole spends most of its time in an elaborate
system of tunnels (some entirely below the ground) and in almost hidden
galleries in the dense grass.

_Microtus ochrogaster_ can be separated from other voles in its
geographic range by a combination of several characters. The plantar
tubercles usually number five, although a few individuals with six
tubercles were found at Lawrence, Kansas. _Microtus pennsylvanicus_,
normally with six plantar tubercles, as Bole and Moulthrop (1942:156)
pointed out, sometimes has only five. Therefore, the number of plantar
tubercles alone is not a certain means for separating _pennsylvanicus_
from _ochrogaster_. The color of the venter of _ochrogaster_ is usually
fulvous or cinnamon instead of grayish as in _pennsylvanicus_, but there
is variation in this respect too; some prairie voles also have a grayish
venter. The shorter tail of _ochrogaster_ will assist in establishing
its identity where it occurs with _pennsylvanicus_. The third upper
molar has two closed triangles in _ochrogaster_ and usually three in
_pennsylvanicus_. The pelage of _ochrogaster_ is coarse whereas
_pennsylvanicus_ has fine fur. Prairie voles may be separated from pine
mice (_Pitymys nemoralis_ and _P. pinetorum_) with which they are
sometimes found, by the larger eyes, less rusty color, and longer tail.
The Cooper lemming mouse (_Synaptomys cooperi_) differs from the prairie
vole in having the upper incisors grooved, and in possessing a shorter
tail which approximates the hind foot in length.

Of _Microtus ochrogaster_ from Lawrence, Douglas County, Kansas, average
measurements of twenty-five adult males are: total length, 143 (121-167)
mm.; tail, 32 (25-42) mm.; hind feet, 20 (17-22) mm.; weight, 43 (38-55)
grams. Twenty-five adult females from the same place average: total
length, 150 (131-170) mm.; tail, 33 (31-41) mm.; hind foot, 19 (17-21)
mm.; weight, 45 (38-58) grams.

The prairie vole is found in suitable habitats in the central part of
North America. It has been recorded from Edmonton, Alberta, in the
northwest (Bailey, 1900:76), southeastward to Chesapeake, Ohio (Bole and
Moulthrop, _op. cit._:156), and in the southwest as far as Ft. Reno,
Oklahoma (Bailey, _op. cit._:74). See figure 1 showing the known range
of _Microtus ochrogaster_. _Microtus ludovicianus_, a close relative of
_ochrogaster_, has been taken along the southern part of the boundary
between Texas and Louisiana (Lowery, 1943:247).

The activities of voles, especially those of the genus _Microtus_,
attracted the attention of naturalists even in early times. Aristotle
(translated by Thompson, 1910) wrote: "The rate of propagation of field
mice in country places, and the destruction that they cause, are all
beyond telling. In many places their number is so incalculable that but
very little of the corn-crop is left to the farmer; and so rapid is
their mode of proceeding that sometimes a small farmer will one day
observe that it is time for reaping, and on the following morning, when
he takes his reapers afield, he finds his entire crop devoured. Their
disappearance is unaccountable: in a few days not a mouse will be there
to be seen."

[Illustration: FIGURE 1. Range of the Prairie Vole (_Microtus
ochrogaster_).]

Several early naturalists in this country commented on the fluctuations
in numbers of individuals, and on the breeding and feeding habits of
voles. Kennicott (1857) in an agricultural report on the mammals of
Illinois wrote about the breeding of the prairie vole. He described its
stores of plants and commented on the behavior of some captives. Quick
and Butler (1885) discussed the habits of _Microtus ochrogaster_ as
well as those of _M. pennsylvanicus_, _Pitymys pinetorum_, and
_Synaptomys cooperi_ in Indiana, and described the feeding and breeding
habits of these species. Criddle (1926) gave an account of the feeding
and breeding habits of _Microtus ochrogaster_ in Manitoba, and Fisher
(1945) published a short description of the food and reproduction of the
same species as he observed it in Missouri. Stone investigated the fauna
in the nests of this vole in the same state, but has not yet, as of
March, 1946, published his findings.



METHODS


The information in the present account was obtained by observing animals
in the field, and by examining trapped animals that were brought into
the laboratory. Five hundred individuals were caught in snap-traps, and
forty additional voles that were marked were captured a total of 157
times. More than 90 per cent of the specimens were trapped at Lawrence,
Douglas County, Kansas, but voles were examined also in Ellsworth,
Atchison, and Jefferson counties, Kansas, and in Douglas County,
Illinois. My data pertain to _Microtus ochrogaster_ in the above named
areas from October, 1945, until August, 1946. The findings may not be
typical of this species in other areas and in other years.

The museum special traps were used both with and without bait. The bait
consisted of a piece of walnut meat on the treadle. By placing the trap
crosswise in the runway, voles were captured whether or not the treadle
was baited. Immediately upon removal from the trap, each vole was placed
in a white flannel sack, one sack sufficing for several voles when
necessary. In this way the loss of ectoparasites was kept to a minimum.
The fleas were counted, and the numbers of lice and mites were
estimated; some specimens of ectoparasites were saved for
identification.

The voles taken in live traps were marked and released. The marking was
done by cutting off one or more toes in such a manner that the vole
could later be identified. From left to right, the toes were assigned
numbers from one to five on the left hind foot, and by tens from ten to
fifty on the right hind foot. Number 33, therefore, was assigned to the
one vole of which the middle toe of each hind foot had been cut off.
Each time an animal was captured alive, it was weighed, specimens of
fleas, lice and mites were preserved, and the external appearance of the
reproductive organs was noted. The extent of the molt line, if the vole
was molting, was recorded. Corresponding data were kept for each dead
vole caught in a snap trap.

Assistance is acknowledged from Professors E. Raymond Hall, A. Byron
Leonard, Worthie H. Horr, and Donald F. Hoffmeister; and I have had also
much helpful advice from Professors W. J. Hamilton, Jr., and P. C.
Stone.



MOLT


The skins of 44 molting prairie voles were pinned out flat. The flesh
sides clearly show the areas of molt. Various stages in the molt process
were observed also in animals caught in live traps. The molt begins when
the animal is three or four weeks old; at this time the juvenal pelage
is replaced by the subadult pelage. The second molt occurs when the
prairie vole is between eight and twelve weeks old, and is the means by
which the adult pelage replaces the subadult pelage. These same two
molts were found by Hatfield (1935) to occur in captive _Microtus
californicus_. Molting voles of the species ochrogaster were trapped in
each month of the year.

The molting processes of juveniles and subadults follow the same
pattern. The first area of molting is in the pectoral region. The molt
patch extends caudad toward the tail and cephalad toward the chin. New
pelage separates this area of active molt into two strips on the fourth
or fifth day. By this time each strip has spread laterad to the legs and
sides, and is 10 to 20 mm. wide. Ultimately each strip unites with its
opposite, usually at the center of the dorsum. This area of molt then
spreads cephalad and caudad. Fourteen to fifteen days after the
beginning of the molt, the entire dorsum is in process of being covered
with new pelage. Shortly before the completion of the molt, the dorsal
area of molt divides into two patches, one on the rump and one on the
nape. The areas last to be covered with new pelage are the crown and
that between the ears and the eyes. A slight variation in the above
process occurred in some specimens in which the lateral strips joined
immediately cephalad of the tail instead of at the center of the dorsum.
The entire process takes approximately three weeks.

Large voles (45 grams or more) grow hair in irregular patches that
measured 5 to 15 mm. In these large voles the molt is accomplished
slowly and does not cover large areas of the body at any one time. The
small areas of molt are visible for 7 to 10 days, and were found on
approximately three quarters of the large voles examined.



FOOD AND HABITAT


The diet of the prairie vole reflects both its environment and its
choice of food. The plants eaten are usually green and succulent, but
some dry, hard seeds and small stems of woody plants are also eaten. The
vegetation, which supplies the food for the vole, is important as cover
or nesting material. For this reason food and habitat are discussed
together.


TYPES OF COVER

Prairie voles inhabit areas where the dominant plants in summer are
clover or grasses or both. The lawn on the campus at the University of
Kansas consists mostly of several kinds of grasses, but in some places
alfalfa (_Medicago sativa_) replaces clover (_Trifolium_ sp.), and in
other places sedges (_Scirpus_ spp.) are found in addition to the
grasses. The grass is short; it is mowed to a length of 4 to 6 inches.
Bluegrass (_Poa pratensis_) and crabgrass (_Digitaria ischaemum_) form
most of the sod. Bluejoint (_Andropogon furcatus_) is common in a
sparsely wooded part of the campus, an area which has many voles.
Foxtail (_Setaria lutescens_ and _S. viridis_) and prairie threeawn
(_Aristida oligantha_) are also common on the lawn, but these become dry
in late summer, and at that time supply neither food nor cover for the
voles. The voles make well-beaten depressions in the sod, and the grass
arches over them to form canopies.

In the winter, when the snow flattened the grass on the campus so that
there were no longer protective canopies of blades over the runways of
the voles, they migrated into areas of Japanese honeysuckle (_Lonicera
japonica_). At this season the honeysuckle was their main food. In areas
where this vine was not available, the voles abandoned their surface
runways and remained below the ground, coming to the surface only under
the protection of a blanket of snow. The voles returned to the grass and
clover habitat in March and April in 1946.

One pure stand of Ladino clover in Jefferson County, Kansas, was studied
in late November and early December of 1945. The clover was 2 to 4
inches high, and although it was the sole food of the voles, it
furnishes but little cover. They were common here; 300 traps yielded 111
voles in two nights.


CUTTINGS

The voles seek particularly the tender heads of grasses and the terminal
leaves of sweet clover (_Melilotus alba_). To obtain these parts, the
voles begin by cutting through the base of the plant. The surrounding
plants are often near enough to support the freshly cut piece in an
upright position. The vole makes successive cuttings, 40 or 50
millimeters from the ground, until the desired parts of the plant are
within reach. The cuttings that have accumulated at the base of the
plant may be eaten, but frequently they remain as evidence of the vole's
feeding activity.

On May 12, 1946, an analysis was made of the cuttings found in an area
of alfalfa, grasses, and weeds. From table 1 it may be seen that
quackgrass, alfalfa, wild lettuce, and cleavers were common. In three
nights 70 traps caught 8 prairie voles and 3 deer mice; no pine mice or
cotton rats were caught on the area. The stomachs of the voles and the
deer mice were examined, and only the stomachs of the voles contained
green material. Analysis of the cuttings (see table 2) indicates that
alfalfa was eaten in greater quantity than any other plant; it made up
almost three quarters of the cuttings although but one quarter of the
cover. All other plants occurred less commonly in the piles of cuttings
than they did in the estimated composition of the cover. Grasses and
wild lettuce were next to alfalfa in importance.

TABLE 1.--_The Relative Abundance of Plants in an Area of Alfalfa,
Grasses, and Weeds_[A]

                                      Percentage by number
    Species                                of plants

Quackgrass (_Agropyron repens_)                 30
Speargrass (_Poa annua_)                         1
California brome (_Bromus carinatus_)            1
Smooth brome (_Bromus inermis_)                  1
Alfalfa (_Medicago sativa_)                     25
Peppergrass (_Lepidium densiflorum_)             2
Cleavers (_Galium aparine_)                     15
Wild lettuce (_Lactuca scariola_)               25

TABLE 2.--_Composition of Ten Piles of Cuttings_[B]

                                                               Frequency of
    Species                      Ten piles of cuttings          occurrence

_Agropyron repens_       1   0   0   2   0   0   6  19   0   4      13
_Poa annua_              0   0   0   0   0   0   0   0   0   0      00
_Bromus carinatus_       0   0   0   0   0  10   0   0   0   0      04
_Bromus inermis_         0   0   0   0   0   0   0   0   0   0      00
_Medicago sativa_       40  14  30  30  31   5   0   0  21   4      73
_Lepidium densiflorum_   0   0   0   0   0   0   0   0   0   0      00
_Galium aparine_         0   0   0   0   1   0   1   0   0   0      01
_Lactuca scariola_       6   2   1   2   5   0   0   0   2   4      09

[Footnote A: Analysis made on May 12, 1946, on an area 20 × 80 yards, at
Lawrence, Kansas.]

[Footnote B: Each of the first ten vertical columns gives the
composition of one pile of cuttings. The last column gives the
percentage of occurrence in the piles of cuttings of each species of
plant in the area. Place and date for data in table 2 same as for table
1.]

Approximately one out of every ten voles caught in snap traps had a
piece of plant in its mouth. Occasionally a vole took a piece of food
into a live trap. Evidently the food is not always eaten where it is
procured. Grasses of the genus _Poa_ are the kinds most frequently found
in the mouths of dead voles. _Bromus carinatus_, _B. inermis_ and sweet
clover (_Melilotus alba_) were found in the runways. The pulpy fruit of
the horse nettle (_Solanum carolinense_) was found partly eaten,
especially near the entrances to underground passages.


FOOD CACHES

Caches of seeds and underground parts of plants are stored in
subterranean chambers. One lot of food was found stored on the surface
of the ground. Four times, piles of seeds in runways indicated the
species of plants which the voles were storing.

One underground cache was found on May 27, 1946, on the University
campus, by John Evans, Richard Edgar, and the writer. This cache was in
a large chamber in a tunnel system of the prairie vole, on an
east-facing hillside of walnut trees, catalpas, and Kentucky coffee
trees. The oval chamber was 250 mm. wide, 400 mm. long, and 200 mm.
high. The roof, at its highest point, was 30 mm. below the surface of
the ground. There were two entrances to the cavity, both on the downhill
side. The cache consisted of eight quarts of seeds (approximately 2,800)
of the Kentucky coffee tree (_Gymnocladus dioica_). The seeds were
packed with earth and all were well preserved. The site of this cache
was in an area which was shaded by a small coffee tree. A seed of this
tree is spheroidal, measures 17 mm. in width, and weighs 2 grams.

Several times in the fall of 1945, in the above-mentioned grove, the
writer found pods of the coffee tree lying in the runs of the voles.
These pods were sometimes entire, but more often they had been gnawed;
frequently only part of a pod remained, indicating that the voles were
storing or feeding upon the seeds, although the possibility that the
mice were storing food did not occur to the writer at the time. Three
times, seeds of other plants were found piled at the entrances of the
burrows of voles. Twice these piles consisted of from 50 to 70 seeds of
the common dandelion (_Taraxacum officinale_). The third pile was
composed of 20 seeds of the giant ragweed (_Ambrosia trifida_).

A pasture of Canadian bluegrass (_Poa compressa_), wild millet
(_Echinochloa crusgalli_), sedges (_Scirpus_ spp.), and clover
(_Trifolium_ sp.) in Atchison County, Kansas, was examined in November,
1945. This area was the home of a dense population of prairie voles.
Wherever a path of the voles crossed a deep imprint of a horse's hoof,
there was a collection of cuttings from the horizontal stems of the
clover which bordered the runways. Some of the cuttings may have been
made by lemming mice (_Synaptomys cooperi_) which were also common in
the area.

Several kinds of voles store food. Bailey (1920) wrote of the caches of
_Microtus pennsylvanicus_ in North Dakota, where, in one locality, this
vole was known as the bean mouse. He stated that the Indians dug up
beans (_Falcata comosa_) and the tubers of the Jerusalem artichoke
(_Helianthus tuberosus_) which the voles had stored. Lantz (1907:17)
found a cache of the roots of wild morning glory (_Convolvulus sepium_)
laid away by _Microtus pennsylvanicus_. Nelson (1893:140) wrote that, as
winter approached, _Microtus operarius_ gathered small bulbous roots,
sometimes storing a peck or more in a single cavity. Fisher (1945) in
Missouri found a gallon of the fruits of the horse nettle (_Solanum
carolinense_) stored in a hollow stump by the prairie vole. Kennicott
(1857:99) found five or six quarts of roots of two species of
spike-flower (_Liatrus_), _Helianthus_, and various grasses among the
winter provisions of the prairie vole in Illinois.


PLANTS USED AS FOOD AND AS COVER

Table 3 lists, according to their families, the species of plants which
the prairie vole was observed to use for food. The same species are
sometimes used as cover. The majority of the plants are in three
families: the grass family (Graminae), the pulse family (Leguminosae),
and the composite family (Compositae).

The grasses that supply the voles' food and cover are mostly _Poa_ (the
bluegrasses) and _Bromus_ (bromegrass, chess, or cheat). _Poa pratensis_
is a common lawn and pasture grass, _P. annua_ is a weed species. The
bluegrasses begin to grow in late winter about Lawrence, Kansas, and
they remain green until late in the fall. During this time, the voles
eat the blades and heads of bluegrass, and make their runways under the
culms. The prairie voles utilize several species of _Bromus_. _Bromus
inermis_ and _B. carinatus_ are important range and pasture grasses, but
_japonicus_ is a weed of little or no economic value. These are soft,
tender grasses, but, in contrast to the bluegrasses, they become dry in
midsummer, and are then unsuitable as food. However, they continue to
form a protection over the runways of the voles.

The legumes, which appeared to be most important to the prairie vole,
are clover (_Trifolium_ spp. and _Melilotus alba_) and alfalfa
(_Medicago sativa_). These plants are common in both cultivated and
feral states. They form a different type of cover from that made by
grasses. Voles, living in clover and alfalfa, do not make runways as
distinct as they do in grasslands. The clover and alfalfa plants are
branched and of a spreading growth form, whereas the grasses have leaves
which are appressed to the main stem. The individual grass plants grow
close together, and a vole cannot run through grass without trampling
some of it. As voles use the same paths repeatedly, the grass in their
runs becomes flattened and dies. There is sufficient room between the
stems of the clover and alfalfa plants to allow the voles to pass
through without treading on the stems. In such a habitat, vole runways
are poorly developed, and are difficult to find. Voles in grasslands
feed in runways, as attested by the piles of cuttings found in the
runways and the nibbled grass which borders them. Voles in clover or
alfalfa feed at the bases of the plants wherever the plants may grow. In
the latter type of cover the cuttings are rather evenly distributed.

Compositae formed a minor part of the cover in most of the habitats
studied. Many grasslands have a stand of dandelions; sow thistle, wild
lettuce, and ragweed were also common in some grasslands. The voles ate
the leaves and sometimes the seeds and underground parts of these
plants.

TABLE 3. _Plants Used for Food by the Prairie Vole_

Graminae
  _Poa annua_
  _P. compressa_
  _P. pratensis_
  _Bromus inermis_
  _B. carinatus_
  _B. japonicus_
  _Andropogon furcatus_
  _Agropyron repens_
  _Setaria lutescens_
  _S. viridis_

Leguminosae
  _Melilotus alba_
  _Medicago sativa_
  _Trifolium_ spp.
  _Gymnocladus dioica_

Solanaceae
  _Solanum carolinense_

Boraginaceae
  _Galium aparine_

Caprifoliaceae
  _Lonicera japonica_

Compositae
  _Lactuca scariola_
  _Sonchus arvensis_
  _Ambrosia trifida_
  _A. artemisiifolia_
  _Taraxacum officinale_



ASSOCIATES


In the mixed areas of grassland and clover that were described above,
the cotton rat (_Sigmodon hispidus_), the deer mouse (_Peromyscus
maniculatus_), and the little short-tailed shrew (_Cryptotis parva_)
were commonly caught in the runways of the prairie vole. Less frequently
trapped were the common mole (_Scalopus aquaticus_), the large
short-tailed shrew (_Blarina brevicauda_), the Cooper lemming mouse
(_Synaptomys cooperi_), the pine mouse (_Pitymys nemoralis_), and the
harvest mouse (_Reithrodontomys megalotis_). In the dense growth of
Japanese honeysuckle, the prairie vole shared runways with the
white-footed mouse (_Peromyscus leucopus_), the large short-tailed
shrew, and the pine mouse.



NEST AND BURROWS


The prairie vole makes a tortuous network of paths through the grass and
honeycombs the topsoil with its tunnels. The underground passages lead
to nests or to chambers where food is sometimes stored. The runways
through the grass are 40 to 50 mm. wide, and usually lie slightly below
the surface of the ground. By using the same path repeatedly, the voles
create little ruts in which they run. The bottom of the runways are bare
soil or are covered with only a thin layer of trampled grass. Cotton
rats, on the other hand, apparently do not use their runs over long
periods, for they are not well-beaten runways, but are made merely by
parting the grass and not by trampling it down or cutting it off. Voles
were trapped in runways of the cotton rats, but no cotton rat was caught
in a typical runway of a vole.

The burrows of the prairie vole are 40 to 50 mm. in diameter, and the
shallowest part is usually 50 to 100 mm. below the surface of the
ground. Burrows leading to nests or food chambers may descend deeper
than the others. Some prairie voles were trapped in tunnels of the
common mole (_Scalopus aquaticus_). The voles make their own burrows,
and are especially active at this task when a hard rain has loosened the
previously hard, dry soil. The rain in the first two weeks of October,
1945, made the soil much more friable than it had been at the beginning
of the month, and the voles took advantage of the favorable opportunity
to construct many new burrows. In October, particles of soil were packed
beneath the toenails of many specimens.

In this time fifteen nests were found. They were 6 to 18 inches below
the surface of the ground, and two tunnels led from each nest to the
surface runway. The nest cavities were spheroidal, and measured 150 to
200 mm. horizontally, and 80 to 100 mm. vertically. The floors were
slightly concave and were covered with loose dirt and a mixture of dried
grass and one or two leaves. The remainder of the cavity was filled with
the dry grass of which the nest was composed. Criddle (1926) stated that
at Treesbank, Manitoba, this vole makes its nests in the burrow systems
of the pocket gopher (_Thomomys talpoides_); and Kennicott (1857:98)
found nests of the prairie vole in old ant hills.

Each of two nests that had been recently occupied was placed in a
Berlese funnel, and in this way the arthropod fauna of the nests was
collected. The most common arthropods in the nests were mites
(parasitic, predaceous, and free-living) and springtails. Sowbugs,
centipedes, spiders, and fleas were also present. Of these arthropods,
the laelaptid mites, one kind of tick, and one kind of flea have a
direct relationship with the vole. These parasites are the same species
which are found on the vole itself. The mites were _Eulaelaps
stabularis_ (Koch) and _Atricholaelaps glasgowi_ (Ewing). One adult
tick, _Ixodes sculptus_ Newman, was in one nest. The fleas, about a
dozen in each nest, were _Ctenophthalmus pseudagyrtes_ Baker, the flea
most frequently found on the prairie vole.



EXTERNAL PARASITES


The pelage of prairie voles, pine mice, deer mice, and shrews forms a
habitat for many kinds of parasitic arthropods. The fleas, lice, and
mites from the prairie vole were collected, counted, and identified. The
ectoparasites from the other small mammals living in the same habitat as
the prairie vole were also considered. Some ectoparasites begin to leave
the host when it dies, and any counts of ectoparasites made from
snap-trapped voles may fall short of the number which was on the animal
when it was alive. The average number of fleas recorded from live voles
exceeds that found on snap-trapped voles (see table 4). The numbers of
lice and mites were estimated, but selected voles were examined to
obtain absolute numbers of these kinds of ectoparasites.

The fleas, lice, and mites were mounted on one inch by three inch glass
slides; the ticks were preserved in 70 per cent alcohol. Dr. E. W. Baker
identified the mites; Dr. R. A. Cooley and Dr. Glen M. Kohls, the ticks;
Dr. G. W. Wharton, the chiggers; and Dr. Gordon F. Ferris, the lice. To
each of these gentlemen I am grateful. The fleas were identified by
myself.


FLEAS (SIPHONAPTERA)

The information on the average numbers of fleas on voles was obtained
from live-trapped and some snap-trapped voles. Fleas were counted only
on voles which were removed from the traps within twenty-four hours
after the traps had been last examined. The average numbers of fleas
found on prairie voles in this study are given in table 4.

TABLE 4. _Average Numbers of Fleas on Prairie Voles_[C]

                        Subadults      Adults

Live-trapped voles      1.9 (73)      3.4 (29)
Snap-trapped voles      1.1 (26)      1.3 (27)

[Footnote C: The fleas on the live-trapped voles are all _Ctenophthalmus
pseudagyrtes_ Baker, and those on snap-trapped voles represent several
species (see table 2). The numbers in parentheses are the numbers of
voles examined.]

Table 5 shows the average degree of infestation for ten months of an
eleven month period. The monthly averages for the most part show no
variations. The latter half of February provides an exception in that a
series of 22 snap-trapped voles and 11 live-trapped voles taken at that
time had on the average, 9.7 and 5.3 fleas respectively. Pine mice
(_Pitymys nemoralis_) occurred in small numbers in the area where
_Microtus ochrogaster_ was live-trapped, and _Ctenophthalmus
pseudagyrtes_ was the flea found to be common on both of these voles.

TABLE 5.--_Monthly Averages of Fleas on Prairie Voles_

Jan.  Feb.  Mar.  Apr.  May  June  July  Aug.  Sept.  Oct.  Nov.  Dec.

 .6   5.1   5[D]  ...    3    1.8  1.4   1.7    ...   1.1     2    2
 (6)  (11)  (6)   ...   (6)  (88)  (26)  (6)    ...   (8)   (14)  (2)

[Footnote D: This figure is high because one vole had the high number of
19 fleas. The numbers in parentheses show the number of live voles
examined for each month. All fleas were _Ctenophthalmus pseudagyrtes_
Baker.]

Some fleas have a habitat preference as well as a host specificity. As
voles from different areas were examined, different kinds of fleas were
encountered. A population of free-living voles under observation on the
Campus at Lawrence was parasitized only by _Ctenophthalmus
pseudagyrtes_. From 90 prairie voles collected in a field of clover 4
miles northwest of Lawrence, the only species of flea recovered was
_Orchopeas leucopus_. In both places the prairie vole was the most
common mammal, but in the field of clover three deer mice (_P.
maniculatus_) also were trapped. In a third field, one mile west of
Lawrence, the prairie vole was host to both the above mentioned fleas.
Here both the prairie vole and the cotton rat (_Sigmodon hispidus_) were
common.

The host distribution of fleas on seven small mammals which lived in the
same habitats as the prairie vole is given in table 6.

TABLE 6.--_Frequency of Occurrence of Fleas on Seven Species of Small
Mammals_[E]

Column headings:

A: _Cryptotis parva_
B: _Blarina brevicauda_
C: _Peromyscus maniculatus_
D: _Peromyscus leucopus_
E: _Sigmodon hispidus_
F: _Microtus ochrogaster_
G: _Pitymys nemoralis_

                                      A   B   C   D   E   F   G
----------------------------------------------------------------
_Orchopeas leucopus_ (Baker)          0   0  53  31  37   6  10
_Orchopeas howardii_ (Baker) =
  _O. wickhami_ (Baker)               0   0   0   0   0   1   0
_Nosopsyllus fasciatus_ (Bosc)        0   0   0   0   0   1   0
_Epitedia wenmanni_ (Rothschild)      0   0   0   9   0   2   0
_Rectofrontia fraterna_ (Baker)       0   0   0   0   0   1   0
_Corrodopsylla hamiltoni_ (Traub)    47   8   0   0   0   0   0
_Ctenophthalmus pseudagyrtes_ Baker   0  38   0   0   4  25  53
_Peromyscopsylla scotti_ I. Fox       0   0   0   6   0   0   0
                                     --------------------------
  Total number examined              34  13  34  35  57 414  21
----------------------------------------------------------------

[Footnote E: The numbers represent the percentage of each species which
was parasitized by fleas. The mammals were collected at Lawrence,
Douglas County, Kansas, between October, 1945, and June, 1946. These
data are entirely from snap-trapped animals with the exception of those
from _Microtus_ and _Pitymys_ which are from both snap-trapped and
live-trapped animals.]

It is seen that some fleas are rather specific in their choice of hosts,
and that others are commonly found on two or more small mammals in the
same habitat. In each of these groups there are fleas which have a
habitat preference, that is to say, the flea lives on the host when the
host lives in a given habitat, but is absent when the host lives in
another habitat.


GROUP 1: FLEAS WITH A HOST PREFERENCE

_Epitedia wenmanni_ was found on the white-footed mouse (_Peromyscus
leucopus_) and only rarely on the prairie vole. _Corrodopsylla
hamiltoni_ was taken only from the two kinds of shrews (_Blarina
brevicauda_ and _Cryptotis parva_). Fleas on shrews may have a
well-developed host preference. At any rate, Elton, Baker, Ford, and
Gardner (1931) found that _Doratopsylla dasycnemus_ rarely strayed from
its normal host (_Sorex araneus_) to other small mammals.
_Peromyscopsylla scotti_ was taken from the white-footed mouse
(_Peromyscus leucopus_), and had a habitat preference also. It was found
only on those white-footed mice which were trapped in the woodlands at
various places in Douglas County; white-footed mice which were trapped
in areas of brush were free of this parasite.


GROUP 2: FLEAS COMMONLY FOUND ON TWO OR MORE KINDS OF SMALL MAMMALS

_Orchopeas leucopus_ was an outstanding example of this group. It was
the most common flea on the deer mouse, the white-footed mouse, and the
cotton rat. In certain areas it was common on the two voles (_Pitymys
nemoralis_ and _Microtus ochrogaster_). _Ctenophthalmus pseudagyrtes_ is
the most abundant flea on the two kinds of voles and on the large shrew
(_Blarina brevicauda_), and was found sparingly on the cotton rat.

Several kinds of fleas do not belong in either of the above groups. Some
fleas were accidental strays from mammals not included in table 6; and
one flea (_Rectofrontia fraterna_) may prove to be a common nest
parasite. _Orchopeas howardii_ is common on tree squirrels (_Sciurus
niger_ and _S. carolinensis_). _Nosopsyllus fasciatus_ is a cosmopolitan
flea on _Rattus norvegicus_. _Rectofrontia fraterna_ was taken once from
a prairie vole. Since the only specimens in the University of Kansas
Entomological Collections are from "mouse nests," this flea may be found
to be a nest inhabiting parasite.

Some fleas are possible bridges by which a blood parasite could be
transmitted from one kind of a mammal to another. If _Ctenophthalmus
pseudagyrtes_ acted as the intermediate host of a disease-causing
organism, an epizootic from _Microtus ochrogaster_ might be transmitted
to _Pitymys nemoralis_ or to _Sigmodon hispidus_ or _Blarina
brevicauda_. There are several other such potential bridges for blood
parasites. Although table 6 does not prove that individual fleas wander
from one host to another, the frequency with which the several kinds of
fleas are removed from live mice suggests that the fleas occasionally do
so.


LICE (ANOPLURA)

Lice collected from the prairie vole were all of one species,
_Hoplopleura acanthopus_ (Burmeister). Of 59 voles examined for the
presence of lice, 33 were found to be parasitized; the 59 voles had an
average of 3.4 lice each. Other mice which used the same runways as the
prairie vole had their own species of Anoplura. The cotton rat was host
to _Hoplopleura hirsuta_ Ferris, and the two species of _Peromyscus_
were parasitized by _Hoplopleura hesperomydis_ (Osborn).

The writer collected _Hoplopleura acanthopus_ from _Microtus
californicus_ at Calaveras Dam, Alameda County, California, and from _M.
pennsylvanicus_ at Ithaca, Tompkins County, New York. Elton, Ford,
Baker, and Gardner (1931) recorded this same species from _M. argestis_
in England.

Lice on the prairie vole are the same species as those found on other
species of _Microtus_ in other areas, but since Anoplura of the prairie
vole do not parasitize the cotton rat, the white-footed mouse, and the
deer mouse, this host specificity of lice makes it unlikely that lice
would carry blood parasites from the prairie vole to any of the latter
named rodents.


MITES (ACARINA EXCEPT IXODOIDEA)

Many of the small mammals examined in this study had mites, some of
which were collected and identified. Mites were collected from other
species of voles in several localities in the United States and in one
locality in Canada; as voles in widely separated regions are sometimes
hosts to the same species of mites, these records will be presented
here.

The frequency of some kinds of mites in the identified material suggests
that they are more abundant than other kinds. The occurrence of mites on
small mammals from Lawrence, Kansas, is presented in table 7.

The following comments can be made concerning the specificity and
geographic ranges of several species of mites:

_Liponyssus occidentalis_ Ewing was found only on _Cryptotis parva_.

_Eulaelaps stabularis_ (Koch) was one of the more common kinds found on
the prairie vole. This mite is rather large (about 1 mm. long) and is
frequently (with the following species) seen running through the pelage
of its host. In addition to the records for this species in table 1, it
was found to be a common parasite on _Pitymys pinetorum_ at Point Abino,
Welland County, Ontario. Elton, Ford, Baker and Gardner (1931) found
this same mite on _Apodemus sylvaticus_ and _Clethrionomys glareolus_ in
England.

_Atricholaelaps glasgowi_, like the preceding species, was one of the
commoner mites on the prairie vole. It was found also on _Pitymys
pinetorum_ at Point Abino, Welland County, Ontario; on _Microtus
pennsylvanicus_ at Ithaca, Tompkins County, New York; and on _M.
californicus_ at Calaveras Dam, Alameda County, California.

_Atricholaelaps sigmodoni_ occurred only on the cotton rat.

_Laelaps kochi_ was less commonly found than _Eulaelaps stabularis_ and
_Atricholaelaps glasgowi_. In Kansas the prairie vole and the cotton rat
were hosts to _Laelaps kochi_, and it occurred on _Microtus
pennsylvanicus_ at Ithaca, New York, and on _M. californicus_ at
Berkeley, California.

Trombiculidae are commonly known by their larvae which are called
chiggers or harvest mites. The white-footed mouse, the cotton rat, and
the prairie vole were parasitized at Lawrence. In the winter these mites
live in the ears of these small mammals, but in the summer they were
found both in the ears and on the rump. Those obtained in winter were
_Ascoschöngastia brevipes_ (Ewing); other species may be involved.

Listrophoridae was represented on the prairie vole by a species of
_Myocoptes_ and a species of _Listrophorus_. These mites cling to the
hairs of their host, and do not occur on the skin of the voles.

No evidence was seen that mites had any ill effect on the health of
their hosts. No voles had scabs on the skin; and the ears were not
swollen and disfigured as they sometimes are by chiggers. Although the
identity of a specimen of mite could not be determined until it was
mounted, a person could tell whether or not it was one of the larger,
very active Laelaptidae, one of the hair-clinging Listrophoridae, or one
of the tiny, orange Trombiculidae.

On July 12, 1946, three prairie voles were examined to determine the
number of mites they supported. The voles were freshly caught, no one of
them having been dead for more than five minutes before they were
examined. These three voles had an average of 25 Laelaptidae, 22
Listrophoridae, and 53 Trombiculidae.

Six species of mites (Ixodoidea excepted) were found on the prairie
vole. Four of these were collected also from other small mammals living
in the same habitat as this vole. Two species of mites were found to
occur on voles in New York, Kansas, and California.


TICKS (IXODOIDEA)

Two kinds of ticks were found. One adult specimen of _Ixodes sculptus_
Neumann was clinging to the head of a vole, just in front of its eye.
This species of tick was taken also from the thirteen-lined ground
squirrel (_Citellus tridecimlineatus_) at Lawrence. One nymph of
_Dermacentor variabilis_ (Say) was found attached to the scapular region
of a prairie vole. Both of these specimens were taken in June.

TABLE 7. _Host Distribution of Mites on Seven Small Mammals_[F]

Column headings:

A: _Scalopus aquaticus_
B: _Cryptotis parva_
C: _Blarina brevicauda_
D: _Peromyscus maniculatus_
E: _Peromyscus leucopus_
F: _Sigmodon hispidus_
G: _Microtus ochrogaster_

                                        A    B    C    D    E    F    G
-------------------------------------------------------------------------
_Ascoschöngastia brevipes_ (Ewing)      ..   ..   ..   ..   X    X    X
_Liponyssus occidentalis_ Ewing         ..   X    ..   ..   ..   ..   ..
_Eulaelaps stabularis_ (Koch)           X    X    X    ..   X    ..   X
_Atricholaelaps glasgowi_ (Ewing)       ..   ..   ..   X    ..   ..   X
_Atricholaelaps sigmodoni_ Strandtmann  ..   ..   ..   ..   ..   X    ..
_Laelaps kochi_ Oudemans                ..   ..   ..   ..   ..   X    X
_Myocoptes_ sp.                         ..   ..   ..   ..   ..   ..   X
_Listrophorus_ sp.                      ..   ..   ..   ..   ..   ..   X
-------------------------------------------------------------------------

[Footnote F: These data are from material collected at Lawrence, Douglas
County, Kansas.]



REPRODUCTION


AGE CLASSES

Each prairie vole was assigned to one of three age classes (juvenile,
subadult, or adult) principally on the basis of weight, but partly on
the quality and color of the pelage. The three age classes are
characterized in table 8.

TABLE 8. _Characters of Juvenile, Subadult, and Adult Prairie Voles_

    Juvenile              Subadult               Adult

Less than 21 grams       21-38 grams          38 grams or more

Weight usually less      Average weight       Average weight
than 20 grams            30-32 grams          40-45 grams

Entire pelage dull       Pelage of rump       Pelage usually
                         dull; rest of        entirely glossy
                         pelage glossy        (rump sometimes dull)

Dorsal color black       Dorsal color         Entire dorsal color
                         grizzled except      grizzled except
                         on rump              sometimes on rump


FECUNDITY

Hamilton (1941:4) found for _Microtus pennsylvanicus_ that macroscopic
tubules of the cauda epididymis were an indication of fecundity. By
noting the size of the tubules (whether macroscopic or not) and by
making smears from them in approximately every 25th male caught, I found
that the presence of sperm was positively correlated with large-sized
tubules of the cauda epididymis in _Microtus ochrogaster_.
Inferentially, males with sperm were fecund.

There is a relationship almost positive between the size of the tubules
of the cauda epididymis and the length of the testes. Testes longer
than 7 mm. have macroscopic tubules in the cauda, and in testes shorter
than 7 mm. these tubules cannot be seen with the naked eye, Hamilton
(1937b) found that in _M. pennsylvanicus_ testes smaller than 8 × 4 mm.
did not contain sperm. The testes of the prairie vole descend into the
scrotum in the breeding season. In the two winter months, when the voles
did not bring forth young, the testes decreased in size (see figure 3)
and were withdrawn into the body cavity. The presence of the testes in
the body cavity does not mean that a vole is not in breeding condition,
for many specimens with abdominal testes were fecund.

The females were considered to be fecund if they were gravid, or if
there were placental scars in the horns of the uteri.

[Illustration: FIGURE 2. Fecundity of Prairie Voles by Months. Adults
and Subadults are Considered Together.]

[Illustration: FIGURE 3. Seasonal Changes in the Length of Testes.]


SIZE OF LITTERS

The number of mammae characteristic of a species of vole may be a rough
guide to the average size of a litter for that species. The prairie vole
has fewer mammae (three pairs) than some other voles in North America,
and might, therefore, be expected to have smaller litters. Fifty-eight
gravid females of _Microtus ochrogaster_ examined by me had an average
of 3.4 embryos each; the number of embryos ranged from one to seven.
Hamilton (1936a) gave 5.07 as the average number of young per litter in
_M. pennsylvanicus_. Hatfield (1935) stated that _M. californicus_ has
an average of 5.7 young in a litter. Both _pennsylvanicus_ and
_californicus_ normally have four pairs of mammae. The expectation as to
the size of the litter seems to be realized. In the prairie vole one
pair of mammae is pectoral and two pairs are abdominal. Usually a
lactating vole showed evidence of only the abdominal mammae having been
in use.

The size of litters was found to vary with the season of the year (see
table 9). Gravid females were collected in every breeding month except
September.

TABLE 9. _Average Size of Litters of Microtus ochrogaster by Months_[G]

Jan.  Feb.  Mar.  Apr.  May  June  July  Aug.  Sept.  Oct.  Nov.  Dec.

 0    2.8   3.9   3.2   3.4   3.1   2.8   3.0   ..    3.2   2.6    0
 ..   (4)   (10)  (6)   (8)   (9)   (5)   (2)   ..    (5)   (5)    ..

[Footnote G: These months are from October, 1945, until August, 1946.
The numbers in parentheses indicate the number of gravid females
collected each month.]

Table 9 shows that the prairie vole produced the largest litters in
March. A comparison of table 9 with figure 2 shows that the largest
litters were produced at the height of the breeding season. Baker and
Ransom (1933), studying _Microtus agrestis_, also found that larger
litters were characteristic of the height of the breeding season; and
that at the beginning and at the end of the breeding season the litters
averaged smaller.

The size of litters varied also with the age of the female. To place a
gravid female in its proper age class, the weight of the embryos was
subtracted from the total weight, and the remaining weight was used as
the body weight. The average size of the litters of 14 subadults was
2.9, and in 35 adults it was 3.4. Hatfield (_op. cit._) found that the
younger females of _M. californicus_ gave birth to smaller litters than
did the adults.

Not included in either of the above analyses are nine gravid females
collected in November in a pasture watered by an artesian spring in
Atchison County, Kansas. In this pasture there was a high concentration
of prairie voles, and the percentage of fecundity was much higher than
in Douglas County at the same time. In November only 29 per cent of the
female prairie voles in Douglas County were fecund, as against 59 per
cent in Atchison County. The average number of embryos of these nine
voles was 4.1. Data from Atchison County are not included in table 9.


THE BREEDING SEASON

In October, 1945, when this study was begun, the prairie vole was
bringing forth young. In the winter of 1945-'46 at Lawrence, Kansas,
there was a cessation of reproduction. The reproductive activity was
measured in terms of the fecundity of the subadults and the adults of
both sexes. Figure 2 suggests that the decline was most marked in
December and January; no gravid females were collected in these two
months, although two females trapped in the first week of December were
lactating. In October, November, and December, 85 per cent of the
breeding females were adults. In October, 85 per cent of the adult
females were fecund, and in November, this figure was 80 per cent.
Reproduction at this season, in the females, it appears, was largely a
function of the adults. The proportion of adults to the rest of the
population was calculated for each month; and the monthly changes in
relative numbers of adults is shown in figure 4. In November, December,
and January there was a scarcity of adult voles in the population. The
autumnal decline in reproduction occurred simultaneously with the
disappearance of these adults, and is thought to have been largely a
result of it.

Reproductive activity began in February; and in this month one-third of
the females contained embryos, and 90 per cent of the males were fecund.
Reproduction reached its height in March when fecundity for the females
and males was 77 per cent and 100 per cent respectively. In April both
sexes showed signs of being less productive, and still later in the
spring the percentage of fecundity remained at slightly over 65 for both
sexes, this figure being higher for the males than for the females for
any one month. From January to February there was a 30 per cent increase
in the percentage of adults in the population; and for this period,
there was a 33 per cent increase in the fecundity of both males and
females. In February, 80 per cent of the fecund females were adults. The
breeding in the late winter, as in the fall, is thought to depend upon
the percentage of adults in the population. Hamilton (1937b) noted a
similar correlation between winter breeding and dominance of adults in
_Microtus pennsylvanicus_ in New York. Fisher (1945) found that the
prairie vole continued to breed throughout the winter of 1943-'44 in
Missouri; in such a case, one would expect to find a large proportion of
adults in the population.

[Illustration: FIGURE 4. Seasonal Changes in the Numbers of Adults in
Relation to the Total Population of Prairie Voles.]

Throughout the winter of 1945-'46, at Lawrence, the majority of males
were fecund; but fecundity in the females was much less, and in January,
no females showed signs of reproductive activity. From this it appears
that the females, not the males, limit the breeding season of this
species.



SUMMARY


In the eleven month period, October, 1945, until August, 1946, in
northeastern Kansas, more than five hundred specimens of the prairie
vole (_Microtus ochrogaster_) were examined in the flesh; and forty
free-living voles were examined 157 times--an average of slightly less
than four times each.

There is a complete molt from juvenal to subadult pelage, and one from
subadult to adult pelage. These molts require three weeks each.
Subsequent molts are irregular and extend over longer periods of time.

This vole, in summer, inhabits areas of grass, clover, and alfalfa. In
winter, habitats with some woody growth may be sought. Twenty-two kinds
of plants were found to be used for food. Although most of these were
succulent plants, seeds and small woody stems were sometimes eaten. The
prairie vole, like some other species of _Microtus_, lays away stores of
food, usually underground; the maximum quantity found in one cache was
two gallons.

Nine other species of small mammals occur in the same habitat with the
prairie vole, and frequently use its runways. The vole makes a network
of paths through the grass, and constructs its own burrows which lead to
its nests and food stores. Each of fifteen nests found were underground.
Most, if not all, of the underground tunnels are dug when the soil is
moist, not when the soil is dry.

The commonest flea on the prairie vole is _Ctenophthalmus pseudagyrtes_;
it averages 1.9 (for subadult voles) to 3.4 (for adult voles) per
individual vole. Other fleas on this vole are _Orchopeas leucopus_,
_Orchopeas howardii_, _Nosopsyllus fasciatus_, _Epitedia wenmanni_, and
_Rectofrontia fraterna_. The two species of fleas which were actually
common on the vole (_C. pseudagyrtes_ and _O. leucopus_), parasitized
also some other small mammals which lived in the same habitat as the
vole. One species of sucking louse (_Hoplopleura acanthopus_) and two
kinds of mites (_Laelaps kochi_ and _Atricholaelaps glasgowi_) which
occur on the prairie vole in Kansas, occur also on _Microtus
californicus_ in California and on _M. pennsylvanicus_ in New York. Only
three ticks (1 _Dermacenter variabilis_ and 2 _Ixodes sculptus_) were
found on the prairie vole.

Fifty-eight gravid females had an average of 3.4 embryos. Litters at the
height of the breeding season are larger than those at the beginning and
at the end of the breeding season. Reproduction in _Microtus
ochrogaster_ ceased in December, 1945, in northeastern Kansas, and the
first evidence of reproduction in 1946 was observed in February.



LITERATURE CITED


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           December 28, 1893.

QUICK, E. W., and A. W. BUTLER.

     1885. The habits of some Arvicolinae. American Nat., 19:113-118,
           February, 1885.

_Transmitted August 13, 1946._

21-6957



       *       *       *       *       *

Transcriber's Notes


Italic text is shown within _underscores_.

Bold text is shown within ~tildes~.

Page 136, Table 3, under Compositae: changed Loctuca to Lactuca
    (_Loctuca scariola_)

  and changed artemsiifolia to artemisiifolia
    (_A. artemsiifolia_)

Page 139: changed trappd to trapped
    (from live-trapped and some snap-trappd voles.)

  and changed rate to rat
    (the prairie vole and the cotton rate)

Page 141: changed Almeda to Alameda
    (at Calaveras Dam, Almeda County, California,)

Page 142: kept section heading: Mites (Acarina except Ixodoidea)
    (the TOC lists the variation Acari instead of Acarina)

  and changed Almeda to Alameda
    (at Calaveras Dam, Almeda County, California.)

Page 143: changed tridecimlineaus to tridecimlineatus
    (ground squirrel (_Citellus tridecimlineaus_) at Lawrence.)
    Note: Another spelling variation is: tridecemlineatus.

Page 146: changed table 2 to table 9
    (A comparison of table 2 with figure 2 shows that the largest)

Page 143: kept spelling variation: Dermacentor variabilis

Page 149: kept spelling variation: Dermacenter variabilis

Page 150: changed LITERAURE to LITERATURE
    (LITERAURE CITED)

  and kept spelling variation: agrestris, being a reference citation
    (1933. Factors affecting ... field mouse (_Microtus agrestris_)).





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