University of Kansas Publications

Museum of Natural History

Volume 7, No. 7, pp. 489-506, 2 figures in text

July 23, 1954

Subspeciation in the Meadow Mouse,
Microtus montanus, in Wyoming
and Colorado

BY

SYDNEY ANDERSON

University of Kansas


Lawrence


1954

University of Kansas Publications, Museum of Natural History

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

Volume 7, No. 7, pp. 489-506, 2 figures in text


Published July 23, 1954

University of Kansas


Lawrence, Kansas

PRINTED BY


FERD VOILAND, JR., STATE PRINTER


TOPEKA, KANSAS


1954

25-3560

Subspeciation in the Meadow Mouse,
Microtus montanus, in Wyoming
and Colorado

BY

SYDNEY ANDERSON

Microtus montanus reaches the eastern limits of its geographic
distribution in Wyoming and Colorado. There the mountains, but
in general not the lowlands, are occupied by this species. A certain
minimum of moisture may be of direct importance to the mouse and
certainly is indirectly important, because certain hydrophytic or
mesophytic grasses used by the mouse for food, for protection from
enemies, and for shelter from the elements are dependent on the
moisture. Areas suitable for Microtus montanus are separated by
deserts that are dominated by sagebrush and other xerophytic plants
or by forests or rocky exposures at higher altitudes. A relatively
small percentage, probably less than ten per cent, of the total area
even in the more favorable parts of the range of the species is suitable
for occupancy. In these mice, as in other microtines (Elton,
1942; Piper, 1909), there are seasonal, and irregularly multiannual
fluctuations in population density, which sometimes are extreme.
Consequently the mice at some times seem to be absent from suitable
habitats, and at some other times occur there in amazingly
large numbers.

Because the species is broken up into partly isolated, or at times
completely isolated, colonies or local populations it may be supposed
that various evolutionary forces such as selection and random genetic
drift operate to foster variation. The degree to which racial distinction
is attained may depend upon these forces and the time available.
In Microtus montanus in the eastern Rocky Mountains the
degree of subspecific distinction is not great.

The study here reported upon is based on 1,187 specimens of
Microtus montanus from Wyoming, Colorado, Idaho and Montana,
and on work in the field. I spent approximately four months in the
field in this area, in the summers of 1950, 1951, and 1952. The specimens
studied were arranged according to localities and the larger
series were compared statistically. Each of two series, totaling 136
specimens, was studied intensively to ascertain the kind and range
of variation within single populations. Twenty-seven measurements,
various proportions based on these measurements, and differences
in color were analyzed. Fifteen characters, judged to be most significant,
were selected for use in comparing all series. In addition,
certain characters that can not be expressed easily by measurements,
such as inflation of the auditory bullae and the curvature of the
zygomatic arch, were observed. The studies by A. B. Howell
(1924) of variation in Microtus montanus yosemite Grinnell in California
and those by O. B. Goin (1943) of Microtus pennsylvanicus
pennsylvanicus (Ord) were useful. The external measurements
are from the collectors' field labels. The measurements of the skull
all were taken with dial calipers reading to a tenth of a millimeter.
The anteroposterior measurements of the skull all were taken along
the shortest line between the points specified below and are not
necessarily along a line parallel to the long axis of the skull. These
measurements were taken on the left side of the skull whenever
possible. Some of the skulls are damaged and therefore some
measurements could not be taken and are omitted in the computations.
Measurements are in millimeters.

The results of these studies were submitted to the Department of
Zoology and the Graduate School of the University of Kansas in
partial fulfillment of the requirements for the degree of Master of
Arts (1952) and are available in manuscript form at the Museum of
Natural History and the library of the University of Kansas.

Explanation of Measurements

Caudal index.—the length of the tail expressed as a percentage of the length
of the head and body. The length of the head and body is the collector's
measurement of total length less the length of the tail.

Cranial Measurements of Length.

Condylobasilar length.—from the exoccipital condyle to the most posterior
point on the border of the alveolus of the upper incisor.

Alveolobasilar length.—from the posterior border of the alveolus of the third
upper molar to the posterior border of the alveolus of the incisor.

Palatilar length.—from the anteriormost part of the posterior border of the
bony shelf of the palate to the posteriormost part of the alveolus of the
incisor.

Alveolar length of upper molar tooth-row.—from the most posterior point of
the alveolus of the third upper molar to the most anterior point of the
alveolus of the first upper molar.

Measurements of Breadth.

Zygomatic breadth.—greatest transverse width.

Interorbital breadth.—the breadth of the interorbital constriction.

Lambdoidal breadth.—between the lateralmost points of the lambdoidal crest.

Prelambdoidal breadth.—between the medialmost margins of the prominent
fenestrae in the posterodorsal parts of the squamosal bones. To these
fenestrae Howell (1924:995) applied the adjective "prelambdoidal," but other
authors have used other names (see Hill, 1935:127).

Depth of braincase.—shortest distance from the ventral side of the cranium at
the suture between the basioccipital and basisphenoid bones to the dorsal
surface of the cranium (usually not perpendicular to the long axis of the
skull).

The history of our knowledge of Microtus montanus in this area
begins with the early work of the United States Bureau of Biological
Survey directed by C. H. Merriam (1891), and participated in by
Vernon Bailey (1900, 1917), Merritt Cary (1911, 1917), and others.
The changes in nomenclature which grew out of increased understanding
of these mice through additional collecting are expressed
in the synonymies under the accounts of subspecies. As a result
of my studies two of the three subspecific names previously proposed
for mice from this area have been retained although changes are
proposed in the ranges assigned to the two subspecies and two additional
heretofore unrecognized subspecies are named and described.
Furthermore the additional specimens and my studies of
variation make modifications in the characterization of these subspecies
necessary. Not all of the samples here assigned to a single
subspecies are identical and I therefore list and discuss some of the
local variants.

Numerous members of summer field parties from the Museum of Natural
History at the University of Kansas collected most of the specimens studied
and wrote field notes that have been helpful. I am grateful to these persons
and to Professor E. R. Hall and Dr. R. H. Baker for their assistance and helpful
suggestions. Specimens in the following museums were made available by
their respective curators: Chicago Natural History Museum by Mr. Colin C.
Sandborn, The Museum of Zoology at the University of Michigan by Dr. E. T.
Hooper, The American Museum of Natural History by Mr. G. G. Goodwin,
The United States National Museum by Dr. David H. Johnson and the Biological
Surveys Collection by Miss Viola S. Schantz. A fellowship from the
National Science Foundation made possible the studies at the museums other
than at the University of Kansas.

Microtus montanus nanus (Merriam)

Arvicola (Mynomes) nanus Merriam, N. Amer. Fauna, 5:62, July 30, 1891.

Microtus montanus nanus, Hall, Proc. Biol. Soc. Washington, 51:131, August
23, 1938.

Microtus nanus, Bailey, N. Amer. Fauna, 17:30, June 6, 1900 (part).

Microtus montanus caryi Bailey, Proc. Biol. Soc. Washington, 30:29, February
21, 1917.

Microtus nanus nanus, Cary, N. Amer. Fauna, 42:43, October 3, 1917 (part).

Type.—Adult male No. 23853/31253, U. S. National Museum, Department of
Agriculture collection, from Pahsimeroi Mountains, Custer County, Idaho; obtained
by C. Hart Merriam and Vernon Bailey, September 16, 1890.

Range.—Idaho; southwestern Montana; most of the southwestern half of
Wyoming; southward to central Colorado. See figure 1.

Comparisons.—Comparisons with subspecies named as new in this paper
will be found in the accounts of those subspecies beyond. From Microtus
montanus fusus Hall, the subspecies to the south, M. m. nanus from Idaho
differs as follows: averages smaller; slightly darker and less reddish and less
yellowish in color; slightly wider braincase (see measurement of prelambdoidal
breadth); larger bullae.

Measurements.—Average (= arithmetical mean) measurements of 34 specimens,
both male and female, from several localities in eastern Idaho are: total
length, 151; length of tail, 39; hind foot, 19.2; condylobasilar length of the
skull, 25.0; zygomatic breadth, 15.0; alveolar length of upper molar tooth-row,
6.4; prelambdoidal breadth, 8.9; and lambdoidal breadth, 11.7.

Average and extreme measurements of six adult males from near Pocatello,
Bannock County, Idaho, and nine adult males from near Afton, Lincoln County,
Wyoming, are, respectively, as follows: total length, 143(135-150), 163(143-179);
length of tail, 35.1(33-38), 42.8(36-49); caudal index, 32.0(28.0-33.1),
35.7(30.6-41.9); hind foot, 18.9(18-20), 18.8(17-20); condylobasilar length
of skull, 24.4(24.0-26.0), 25.6(24.5-26.2); alveolobasilar length, 14.1(13.7-14.5),
14.6(13.8-15.0); palatilar length, 13.2(12.9-13.6), 13.8(13.2-14.5);
alveolar length of upper molar tooth-row, 6.3(6.1-6.5), 6.3(6.0-6.6); depth
of braincase, 7.7(7.5-7.9), 8.0(7.7-8.3); lambdoidal breadth, 11.4(11.0-11.7),
12.0(11.3-12.7); prelambdoidal breadth, 9.1(8.6-9.4), 8.7(8.0-9.4); zygomatic
breadth, 14.3(13.8-14.7), 15.3(14.4-16.3); interorbital breadth, 3.6(3.5-3.7),
3.5(3.3-3.7). The average length of the nasal bones in the series from Pocatello
is 7.1 mm. The averages, which have not been included in Table 1, for
three measurements of the series from Carbon County, Wyoming, are as follows
(Encampment, males; Encampment, females; Savery, males; and Savery, females,
respectively): alveolobasilar length, 14.4, 14.3, 14.5, 14.3; interorbital
breadth, 3.5, 3.4, 3.5, 3.4; depth of braincase, 7.8, 7.6, 7.9, 7.6. Additional
measurements are included in Table 1 for other series.

Discussion.—The name Microtus montanus caryi Bailey is here
placed in synonymy under M. m. nanus (Merriam). Vernon Bailey
(1917) in his description of caryi made four assumptions that have
been found to be entirely or partly invalid. First, he assumed that
this is an "extreme variant which gradually changes in characters
across Nevada and Utah, and reaches its maximum variation in
Wyoming." The differences pointed out in subsequent descriptions
of subspecies found in the above area do not show a gradual change
in any character, or in the number of characters, nor is caryi an
extreme when compared with the other subspecies. Second,
Microtus nanus was not, as Bailey assumed, a different species than
Microtus montanus. Third, he assumed that the characteristics of
adults of nanus were adequately ascertainable from the thirteen
topotypes available to him. Subsequent sampling from Idaho shows
that the series of specimens available to Bailey was made up mostly
of young and subadult animals. Finally, caryi does not occupy as
Bailey stated "the meadows along streams in the arid sagebrush
country of the Bear River, Green River, and Wind River valleys"
exclusively, or characteristically. When the localities from which
the species actually is known are plotted, it seems that the arid
basin serves as a barrier and that the species is more commonly and
abundantly found in montane meadows in the Transition and
Canadian life-zones.

Certain samples, here assigned to M. m. nanus, that vary from the
average of the subspecies deserve comment. For example, mice
from the area in Wyoming southwest of the Green River (in the
Uinta Mountains) have relatively smaller feet, but are larger in both
total length and size of skull. Specimens from near Afton, Lincoln
County, Wyoming, are relatively large in both total length and size
of skull. This series and specimens from Teton County, Wyoming,
are intermediate between nanus from Idaho and the newly named
subspecies from near Cody, Park County, Wyoming, described
below, in terms of both darkness and the amount of reddish color.
Mice from Laramie County are more richly reddish-brown. The
specimens from near Savery, in Carbon County, Wyoming, are
darker. The alveolobasilar length relative to the condylobasilar
length is smaller in the series from along Deer Creek, 16 mi. S, 11
mi. W Waltman, Natrona County, Wyoming. The series from the
southern tier of counties in Wyoming and some of the specimens
from Colorado have relatively wider zygomatic arches. The specimens
from southern Sweetwater County, Wyoming, are relatively
paler, have a relatively longer tail and longer hindfeet, lesser
condylobasilar length, and wider braincase. Most of these variations
are of questionable significance; they may be chance variations
owing to errors in sampling.

Much of the south-central part of the state is relatively low and
relatively arid. This area includes the arid basin of the Green
River and its major tributaries and the arid Red Desert along the
continental divide in Sweetwater County. This area might have
acted as a barrier to the mice; gene flow might have been prevented
between the populations of the western part of the state and those
farther east in the Medicine Bow Mountains and Laramie Mountains.
Nevertheless geographic variations of subspecific worth have
not taken place. The barrier has either not been of as long duration,
or has not been so complete and effective, as the other barriers
in the state, namely the Absaroka Range, the Big Horn Basin, the
Shoshone Basin, and the valley of the North Platte River. These
four barriers presumably have led to the differentiation of the two
subspecies that are newly named beyond. Each of the two areas
which is set apart by these barriers and in which one of the newly
named subspecies has evolved is small; therefore there is a lesser
amount of suitable habitat available for each of the newly named
mice than there is for M. m. nanus. It is conceivable, therefore,
that in periods of adverse conditions in each of the small areas the
size of the effective breeding population may have been so small
that random genetic drift could have operated effectively, or that
selection was more critical than in a larger, more stable population.
It is difficult to test these possibilities because the selective value of
the taxonomic characters is unknown. The observed pattern of
variation and facts of distribution are, however, not contradictory
to the above possibilities.

Specimens examined.—Total, 993, distributed as follows: All specimens
unless otherwise indicated are in the University of Kansas Museum of Natural
History. Specimens in other museums are labeled as follows: Chicago Natural
History Museum (Chi); University of Michigan, Museum of Zoology (Mich);
American Museum of Natural History (AMNH); United States National Museum
(USNM); Biological Surveys Collection (USBS). Localities that are not
represented in Fig. 1 because overlapping or crowding of the symbols would
result are Italicized. Localities are arranged from north to south by states,
within a state from northwest to southeast by counties, and within a county from
north to south.

Wyoming: Yellowstone Park: Canyon Camp, 1 (USBS); Lower Geyser
Basin, 1 (USBS); Upper Yellowstone River, 2 (AMNH); North end of Lake,
Yellowstone National Park, 2 (AMNH). Teton Co.: Pacific Creek, 1 (USBS);
Big Game Ridge, 3 (USBS 1, Mich 2); Whetstone Creek, 7 (Mich); Moran
and environs (6 localities within a 5 mile radius), 28 (USBS 2, Mich 5);
S fork Buffalo River, 7 (AMNH); 2 mi. W pass, Black Rock Creek, 1 (USBS);
Jenny Lake, 5 (Mich); Bar BC Ranch, 2½ mi. NE Moose, 6500 ft., 2; Teton
Pass above Fish Creek, 1 (USBS); Jackson and environs, 142 (Mich 141);
Sheep Creek, 2 (Mich). Lincoln Co.: 13 mi. N, 2 mi. W Afton, 2; 10 mi. N,
2 mi. W Afton, 4; 9½ mi. N, 2 mi. W Afton, 3; 9 mi. N, 2 mi. W Afton, 9;
7 mi. N, 1 mi. W Afton, 12; Afton, 1 (USBS); Labarge Creek, 1 (USBS);
Border, 6 (USBS); Cokeville, 2 (USBS); 6 mi. N, 2 mi. E Sage, 1; Cumberland,
5 (USBS). Sublette Co.: 34 mi. N, 4 mi. W Pinedale, 1; 33 mi. N, 2
mi. W Pinedale, 6; 32 mi. N, 1 mi. W Pinedale, 1; 31 mi. N Pinedale, 4; Dell
Creek, on Ferris Ranch, 7 (Mich); Horse Creek, 7800 ft., Merna, 4 (USBS);
Big Piney, 1 (USBS). Fremont Co.: 17½ mi. W, 2½ mi. N Lander, 9500 ft.,
3; 17 mi. W, 2 mi. N Lander, 9300 ft., 4; Milford and environs (5 localities
within a 1 mile radius), 23 (USBS 4); 15½ mi. S, 7½ mi. W Lander, 9200 ft.,
1; South Pass City, 8000 ft, 8 (USBS); 23½ mi. S, 5 mi. W Lander, 8600 ft.,
7. Natrona Co.: Deer Creek, 16 mi. S, 11 mi. W Waltman, 6950 ft., 44;
6 mi. S, 2 mi. W Casper, 5900 ft., 4; 6-4/5 mi. S, 2 mi. W Casper, 6100 ft., 1;
7 mi. S, 2 mi. W Casper, 6370 ft., 3; 10 mi. S Casper, 7750 ft., 33; Sun, 2
(USBS); 5 mi. W Independence Rock, 6000 ft., 4; 5 mi. W, 1 mi. S Independence
Rock, 2. Converse Co.: Beaver, 1 (USBS). Uinta Co.: 1½ mi. W,
½ mi. S Cumberland, 6; 16 mi. S, 2 mi. W Kemmerer, 6700 ft., 3; 10 mi. SW
Granger, 3 (Mich); Fort Bridger, 6650 ft., 25 (USNM 6); 9 mi. S Robertson,
8000 ft., 9; 9½ mi. S, ½ mi. W Robertson, 8600 ft., 1; 10 mi. S, 1 mi. W Robertson,
8700 ft., 25; 14 mi. S, 2 mi. E Robertson, 9000 ft., 5; 4 mi. S Lonetree, 1
(USBS). Sweetwater Co.: Farson, 3; Bitter Creek, 3 (AMNH); Kinney
Ranch, 21 mi. S Bitter Creek, 6800 ft., 9 (USNM 1, AMNH 2); 32 mi. S, 22
mi. E Rock Springs, 7025 ft., on Vermillion Creek, 15. Carbon Co.: 18 mi.
NNE Sinclair, 6500 ft., 10; Bridgers Pass, 18 mi. SW Rawlins, 7500 ft., 7;
Saratoga, 1 (USBS); 6 mi. S, 13 mi. E Saratoga, 8500 ft., 5; 6 mi. S, 14 mi.
E Saratoga, 8800 ft., 1; Lake Marie, 10,440 ft., 2; 1 mi. S Lake Marie, 2; ½
mi. S, 2 mi. E Medicine Bow Peak, 10,800 ft., 1; Encampment (12 localities
from 10 mi. N, 14 mi. E to 9 mi. N, 3 mi. E Encampment and from 6500 to
8400 ft.), 63; 1/4 mi. N Riverside, 7380 ft., 2; S base Bridger Peak, 8800 ft.,
Sierra Madre Mountains, 1; 2 mi. S Bridger Peak, 9300 ft., 2; Savery (10
localities from 8 mi. N, 21 mi. E to 4 mi. N, 8 mi. E Savery and from 7300 to
8800 ft.), 80. Albany Co.: 30 mi. N, 10 mi. E Laramie, 6760 ft., 6; 29¾ mi.
N, 9½ mi. E Laramie, 6350 ft., 1; 26 mi. N, 4½ mi. E Laramie, 6960 ft., 8;
26¾ mi. N, 6½ mi. E Laramie, 6700 ft., 3; 3 mi. N, 13 mi. E Laramie, 7500
ft., 1; 7 mi. N, 2 mi. E Laramie, 1 (Chi); 5 mi. N Laramie, 7400 ft., 15;
Laramie, 4 (AMNH); 1 mi. E Laramie, 7160 ft., 4; 7-7/10 mi. SSW Laramie,
7200 ft., 4; 6½ mi. S, 8¾ mi. E Laramie, 8200 ft., 1; Headquarters Park, 10,200
ft., Medicine Bow Mountains, 3 (USBS); Centennial, 8120 ft., 1; 2-1/4 mi. ESE
Brown's Peak, 10,300 ft., 3; 3 mi. ESE Brown's Peak, 10,000 ft., 12; 2 mi. S
Brown's Peak, 10,600 ft., 1; Pole Mountain, 15 mi. SE Laramie, 4 (USBS 3);
1 mi. SSE Pole Mountain, 8350 ft., 4; 2 mi. SW Pole Mountain, 8300 ft., 13;
3 mi. S Pole Mountain, 8100 ft., 1; Sherman, 2 (AMNH). Laramie Co.: 5
mi. N, 1 mi. W Horse Creek P. O., 7200 ft., 1; Meadow, 2 (USBS); 11 mi.
N, 5½ mi. E Cheyenne, 5450 ft., 7; 7 mi. W Cheyenne, 6500 ft., 10; Cheyenne,
3 (USNM).

Colorado: Moffat Co.: Lay, 6160 ft., 1 (AMNH). Routt Co.: Wright's
Ranch, Yampa, 7700 ft., 2; Gore Range, 8 mi. E Toponas, 8000 ft., 2 (USBS).
Larimer Co.: 12½ mi. W, 1½ mi. S Rustic, 1; 11 mi. W, 1 mi. S Rustic, 1;
Cache La Poudre River, 1 (Chi); Estes Park, 3 (USBS 1, AMNH 2); 19½ mi.
W, 2½ mi. S Loveland, 7280 ft., 6; 16 mi. W Loveland, 6840 ft., 1; 6 mi. W,
½ mi. S Loveland, 5200 ft., 1. Rio Blanco Co.: Meeker, 1 (USBS); 9½ mi.
SW Pagoda Peak, 7700 ft., 3; 5 mi. S Pagoda Peak, 9100 ft., 2. Eagle Co.:
Eagle, 1 (USBS); Pando, 2 (USBS). Grand Co.: Mt. Whiteley, 2 (USBS);
Arapahoe Pass, Rabbit Ear Mountains, 2 (USBS); Coulter (near Granby), 5
(USBS); Arrowhead (near Dale), 1 (USBS). Boulder Co.: ¾ mi. N, 2 mi.
W Allenspark, 8400 ft., 4; 3 mi. S Ward, 9000 ft., 3; Nederland, 16 (Chi).
Clear Creek Co.: Mt. McLellan, 2 (USBS); Berthoud Pass, 4. Park Co.:
Trout Creek Ranch, 2 mi. N Garo, 1 (USBS).

Specimens examined of M. m. nanus from eastern Idaho and Montana are
as follows: Idaho: Custer Co.: Challis, 7 (USBS); Mill Creek, Challis Nat.
Forest, 1 (USBS); Pahsimeroi Mts., 12 (USBS); Lost River Mts., 1 (USBS).
Fremont Co.: N fork Snake River, 10 mi. SW Island Park, 6200 ft., 2 (AMNH);
Black Springs Creek, 4 mi. W Ashton, 5200 ft., 1 (AMNH); 5 mi. W St.
Anthony, 5000 ft., 1 (AMNH). Camas Co.: Camas Prairie, Corral, 5100 ft.,
2 (USBS). Blaine Co.: Alturas Lake, 3 (USBS); Sawtooth Lake, 2 (USBS);
Craters of the Moon, Laidlow Park, 2 (Mich); Ticura, 10 mi. S Picabo, 1
(USBS); 19 mi. NE Carey (Lava Lake), 8 (Mich). Butte Co.: 26 mi. SW
Arco, 12 (Mich). Bingham Co.: Shelley, 6 (USBS). Bonneville Co.: 10
mi. SE Irwin, 4 (USBS). Owyhee Co.: Three Creeks, 3 (USBS). Twin
Falls Co.: Castleford Fenced Plot, 11 mi. W, 9 mi. S Twin Falls, 1. Minidoka
Co.: Heyburn, 2 (USBS). Cassia Co.: 2 mi. S, 2 mi. W Burley, 5. Bannock
Co.: Pocatello, 23 (USBS 4); Swan Lake, 1 (USBS). Bear Lake Co.: Montpelier
Creek, 6700 ft., 3 (USBS). Montana: Gallatin Co.: W. Fork of W.
Fork, Gallatin River, 1 (USBS). Park Co.: Lamar River, 7000 ft., 1 (USBS);
Gardiner, 1 (USBS). Sweet Grass Co.: 14 mi. S Big Timber, 1 (USBS);
McLeod, 1 (USBS); West Boulder Creek, 18 mi. SE Livingston, 2 (USBS).

Microtus montanus codiensis, new subspecies

Type.—Female, adult, skin and skull; No. 27578, Museum of Natural History,
University of Kansas, from 3⅕ mi. E and ⅗ mi. S Cody, 5020 ft., Park
Co., Wyoming; obtained on August 11, 1948, by James W. Bee, original number
18-8-11-48.

Range.—In northwestern Wyoming eastward from the Absaroka and Wind
River ranges into the western part of the Big Horn Basin.

Diagnosis.—A relatively large Microtus montanus; tail actually and relatively
long; hind foot actually but not relatively large; skull large; zygomatic expanse
actually and relatively large; alveolobasilar length relatively large; upper molar
tooth-row relatively long; color relatively light, not reddish.

Fig. 1. Geographic range of Microtus montanus in Wyoming, Colorado,
and adjacent areas. The solid circles represent localities from which specimens
have been examined; the hollow circles represent type localities. The ranges
of subspecies in Utah are after Durrant, 1952.

Guide to subspecies

1. M. m. nanus


2. M. m. codiensis


3. M. m. zygomaticus


4. M. m. fusus


5. M. m. micropus


6. M. m. nexus


7. M. m. amosus


8. M. m. rivularis

Comparisons.—As compared with the specimens of M. m. nanus from Idaho,
the size is larger (see diagnosis and measurements). Certain proportions
which differ from those of nanus and which are not in close agreement with
the observed differences with age in specimens of nanus of a size comparable
to codiensis are relatively large alveolobasilar length, relatively long alveolar
length of upper molar tooth-row, relatively wide-spreading zygomatic arches,
and relatively long tail. The color in codiensis is lighter than in nanus. As
compared to the new subspecies named below from the Big Horn Mountains
to the east, codiensis is of similar size in head-body length, but has a relatively
as well as actually longer tail; the hind foot averages longer; the upper molar
tooth-row is relatively longer; the color is slightly paler and less grizzled; the
bullae are larger and less flattened; the angle formed at the suture between
the basioccipital and basisphenoid bones is less acute; and the region of the
suture is less prominently elevated between the bullae when viewed from the
ventral aspect. The pterygoid plates mesial and posterodorsal to the posterior
end of the last upper molar are less fenestrated, and the incisive foramina are
less constricted posteriorly.

Fig. 2. Map showing the major barriers to Microtus montanus in Wyoming
and Colorado; the barriers are the low areas named on the map (the name
"Black Hills" is on the map for another reason; these hills are not a barrier).
The major mountainous areas higher than approximately 8000 feet in elevation
in Wyoming, Colorado and Utah are stippled. These mountainous areas
include the habitat that is most suitable for the montane meadow mouse. The
Black Hills are unoccupied by this species but these hills seem to be ecologically
suitable for the species.

Measurements.—The average and the extremes for some measurements of
34 males and females, 27 from the type locality and 7 from other localities in
the range assigned to this subspecies, are as follows: total length, 165 (146-186);
length of tail, 44.2 (35-55); hind foot, 19.6 (17-21); condylobasilar length of
the skull, 25.5 (24.0-27.5); zygomatic breadth, 15.6 (14.7-16.6); alveolar length
of upper molar tooth-row, 6.6 (6.2-7.0); prelambdoidal breadth, 8.8 (8.1-9.5);
lambdoidal breadth, 12.0 (11.2-12.8). As an indication of variability and for
comparison with other series the coefficient of variability and two times the
standard error of the mean for each measurement in this series are included
in Table 1. The averages for some measurements of 27 topotypes are as follows:
total length, 162; length of tail, 45.5; hind foot, 19.9; condylobasilar
length, 25.6; palatilar length, 14.0; molar series, 6.6; alveolobasilar length,
14.9; zygomatic breadth, 15.6; interorbital breadth, 3.5; lambdoidal breadth,
12.1; prelambdoidal breadth, 8.9; depth of braincase, 7.8.

Discussion.—Three species of Microtus were collected by James
W. Bee at the type locality. Microtus montanus codiensis, Microtus
longicaudus mordax, and Microtus pennsylvanicus modestus were
taken in the same runways in the same meadow, at the same time.
Microtus ochrogaster haydeni, although not taken at this locality,
occurs in the Big Horn Basin. These four species differ in their
geographic ranges, being largely allopatric, except M. montanus and
M. longicaudus which are sympatric. Although the different species
have ecological preferences and habits which differ, several species
of Microtus may occur together in local areas such as the above.
Certain of the characteristics of M. m. codiensis are intermediate
between those of the species M. montanus on one hand and those
of the other three species on the other hand. Could interspecific
hybridization between "good species" of Microtus take place in
nature and possibly alter the characteristics of a local population?

Specimens examined.—Total, 50, distributed as follows (abbreviations for
collections are given in the account of M. m. nanus; localities that are not
represented in Fig. 1 because overlapping or crowding of the symbols would
result are Italicized):

Montana: Carbon Co.: Beartooth Mountains, 2 (USBS); Beartooth Lake,
1 (USBS).

Wyoming: Park Co.: Black Mountain, head of Pat O'Hara Creek, 3 (USBS);
13 mi. N, 1 mi. E Cody, 5200 ft., 1; SW slope Whirlwind Peak, 9000 ft., 1;
5 mi. N Cody, 6300 ft., 1 (USBS); 3⅕ mi. E, ⅗ mi. S Cody, 31; Ishawooa
Creek, 6300 ft., 2 (USBS); Valley, 1 (USBS); Needle Mountain, 10,500 ft.,
4 (USBS). Hot Springs Co.: 3 mi. N, 10 mi. W Thermopolis, 4950 ft., 3.

Microtus montanus zygomaticus, new subspecies

Type.—Male, adult, skin and skull, No. 32761, Museum of Natural History,
University of Kansas, from Medicine Wheel Ranch, 9000 ft., 28 mi. E Lovell,
Big Horn County, Wyoming; obtained by R. Freiburg, original number 105.

Range.—The Big Horn Mountains of north-central Wyoming.

Diagnosis.—A large Microtus montanus with a relatively short tail; short
molar series; broad zygomatic arches well rounded in lateral outline when
viewed from above; small and flattened bullae; raised basioccipito-basisphenoid
suture.

Comparisons.—For comparison with M. m. codiensis from the west, on the
other side of the Big Horn Basin, see the account of that subspecies. In comparison
with nanus this subspecies is slightly paler, in this respect showing
more resemblance to codiensis although not so pale, and more grizzled or
unevenly colored. This difference in color between zygomaticus and codiensis
may not be of taxonomic significance. From both the topotypes of nanus, and
the series of it from Wyoming, zygomaticus differs on the average in having a
relatively shorter tail, a relatively shorter upper molar tooth-row, relatively
more rounded and relatively more wide-spread zygomatic arches, and smaller
more flattened bullae.

Measurements.—Average and extreme measurements of 24 adult males and
females from several localities here referred to M. m. zygomaticus are as follows:
total length, 159(150-175); length of tail, 37.6(31-46); hind foot,
18.6(17-20); condylobasilar length of the skull, 25.3(24.2-26.7); zygomatic
breadth, 15.3(14.1-16.7); alveolar length of upper molar tooth-row, 6.2
(5.7-6.8); prelambdoidal breadth, 8.7(8.3-9.4); lambdoidal breadth, 11.9(11.0-12.5).
Average and extreme measurements of a series of 12 adult male
topotypes are as follows: total length, 159(144-174); length of tail, 36.4
(30-41); hind foot, 18.2(16-20); condylobasilar length of skull, 25.8(24.7-26.7);
alveolobasilar length, 14.8(13.8-15.3); palatilar length, 13.8 (12.7-14.2); alveolar
length of upper molar tooth-row, 6.4(5.9-6.6); zygomatic breadth, 15.9
(15.0-16.7); interorbital breadth, 3.6(3.4-3.7); lambdoidal breadth, 12.1
(11.5-12.5); prelambdoidal breadth, 8.6(8.3-8.9); depth of braincase, 8.0
(7.6-8.3).

Discussion.—This subspecies is separated from M. m. codiensis to
the west by the Big Horn Basin. A series from along Buffalo Creek,
27 mi. N, 1 mi. E Powder River, 6075 ft., in Natrona County, Wyoming,
is intermediate between the topotypes of zygomaticus and
nanus in the characters cited above as distinguishing the two, but
shows greater resemblance to zygomaticus in the shape of the
zygomatic arch, in color which is paler than in topotypes of
zygomaticus, and in the short hind foot. On these and on geographic
grounds this population is referred to zygomaticus. Unfortunately
we cannot be certain in many cases that an intermediate condition
in a certain character indicates a genetically intermediate population
and therefore true intergradation between the two subspecies
to which the population is geographically intermediate. The topotypes
of this subspecies are the most distinct of all the series which
I have studied from the eastern Rocky Mountains, in terms of the
degree of morphological departure from the norm for the species.
After zygomaticus the following populations are arranged according
to their degree of deviation from this norm (codiensis deviates
most): topotypes of codiensis, fusus and a population from southern
Sweetwater County, Wyoming, and lastly the nanus-caryi complex.
Within the latter group, as I have mentioned, there are a number
of local variants most of which do not differ significantly and do
not conform to any geographic pattern.

Specimens examined.—Total, 55, distributed as follows (abbreviations for
collections are given in the account of M. m. nanus; localities that are not
represented in Fig. 1 because overlapping or crowding of the symbols would
result are Italicized): Wyoming: Big Horn Co.: Medicine Wheel Ranch,
9000 ft., 28 mi. E Lovell, 30; W slope, head of Trappers Creek, 9500 ft., 2
(USBS). Washakie Co.: 9 mi. E, 5 mi. N Tensleep, 7400 ft., 1. Johnson
Co.: 7½ mi. W, 1 mi. S Buffalo, 6500 ft., 3; Big Horn Mountains, 3 (USBS).
Natrona Co.: Buffalo Creek, 27 mi. N, 1 mi. E Powder River, 6075 ft., 16.

Microtus montanus fusus Hall

Microtus nanus, Bailey, N. Amer. Fauna, 17:30, June 6, 1900 (part); Cary,
N. Amer. Fauna 33:123, August 17, 1911.

Microtus montanus fusus Hall, Proc. Biol. Soc. Washington, 51:131-134,
August 23, 1938; Warren, The Mammals of Colorado, Univ. of Okla.
Press, p. 229, 1942.

Type.—Male, adult, skin and skull; No. 61281, Museum of Vertebrate
Zoology; 2½ miles east of summit of Cochetopa Pass, Saguache County, Colorado;
Sept. 21, 1933; collected by Annie M. Alexander; original number 2568.
Type not seen by me.

Range.—Southern Colorado and northern New Mexico.

Comparisons.—For comparison with M. m. nanus, the subspecies to the
northward, see the preceding account of that subspecies. For comparison
with M. m. amosus the subspecies to the west see Hall (1938) and Durrant
(1952). I have not examined specimens of amosus.

Measurements.—Average and extreme measurements for 17 adults including
both males and females from several localities in southern Colorado are as
follows: total length, 160 (136-179); length of tail, 42 (35-55); hind foot,
19.2 (17-23); condylobasilar length of the skull, 25.2 (24.0-26.0); zygomatic
breadth, 15.0 (14.1-15.5); alveolar length of upper molar tooth-row, 6.4
(6.0-6.7); prelambdoidal breadth, 8.7 (8.3-9.2); lambdoidal breadth, 11.7 (11.1-12.6).

Average and extreme measurements of 4 adults (2 males and 2 females)
from the type locality and 11 adults (4 males and 7 females) from other
localities in southern Colorado are as follows: total length, 162 (157-168),
157 (137-169); length of tail (means only), 44.5, 40.5; hind foot, 18.8 (18-19),
18.6 (18-23); condylobasilar length of skull, 24.5 (24.0-24.7), 25.2 (24.3-26.1);
alveolobasilar length, 14.2 (13.9-14.5), 14.6 (14.1-15.1); palatilar length, 13.2
(13.0-13.4), 13.5 (13.1-14.2); alveolar length of upper molar tooth-row, 6.3
(6.0-6.6), 6.4 (6.3-6.7); zygomatic breadth, 15.0 (14.3-15.5), 14.9 (14.1-15.5);
interorbital breadth, 3.5 (3.3-3.6), 3.5 (3.3-3.7); lambdoidal breadth, 11.8 (11.1-12.6),
11.7 (11.2-12.3); prelambdoidal breadth, 8.6 (8.3-9.2), 8.8 (8.3-9.0);
depth of braincase, 7.5 (7.2-7.8), 7.6 (7.1-7.9).

Discussion.—There is no sharp boundary between M. m. fusus of
southern Colorado and the subspecies to the north, M. m. nanus.
Although the line separating these two subspecies is drawn somewhat
arbitrarily, on the whole the samples from north of this line
more closely resemble nanus. All of the means for total length
given above are larger than the maximum given in Hall's description
of fusus. The caudal index (38 and 35% in two series) is
slightly larger than that cited by Hall (33.3%) and is not significantly
different from that in nanus (35.2%). The color in both young and
old mice is variable, but in general is more yellowish, and less
grayish, than in any other series studied.

There is a large area in western Colorado and eastern Utah,
between the known ranges of M. m. fusus and M. m. amosus from
which there are no specimens. Probably the species occurs only at
certain places in this arid region which seems to be a partial barrier
to the species.

Specimens of M. montanus from northern New Mexico have been
referred previously to M. m. arizonensis. When he named M. m.
fusus, Hall mentioned its resemblance to arizonensis in reddish
coloration, but pointed out that fusus is less reddish. Of six specimens
from Valle Santa Rosa, Jemez Mountains (USBS), 8500 ft.,
Rio Arriba County, New Mexico, three are immature, and the skulls
of the remaining specimens are damaged. In reddish color and
relatively large size these few specimens resemble arizonensis more
than fusus although the locality of occurrence is closer to the geographic
range of the northern fusus than to that of arizonensis. The
identification of these specimens as arizonensis is provisional; additional
specimens are needed from the area, 200 miles wide, which
separates the ranges as now known of arizonensis in Arizona from
the occurrence in New Mexico. There is a single specimen from
this area, the damaged skull of which prevents conclusive identification.
The specimen is either M. montanus or M. mexicanus, and
is from Nutria, on the southern edge of the Zuni Mountains (USBS).
Detailed comparison of fusus and arizonensis is not attempted here
although it may be stated that in several characters fusus is intermediate
between arizonensis to the south and nanus to the north.

Specimens examined.—Total, 89, distributed as follows (abbreviations for
collections are given in the account of M. m. nanus; localities that are not
represented in Fig. 1 because overlapping or crowding of the symbols would
result are Italicized):

Colorado: Pitkin Co.: 5 mi. W Independence Pass, 11,000 ft., 1 (Chi).
Lake Co.: Independence Pass, 12,095 ft., 2 (Chi). Gunnison Co.: Gothic, 2
(USBS); Decker's Ranch, Crested Butte, 2 (AMNH); Almont, 3 (USBS).
Montrose Co.: Coventry, 5 (USBS 4, AMNH 1). Saguache Co.: Cochetopa
Pass and environs, 44 (USBS 22). Hinsdale Co.: Ruby Lake, 1 (USBS).
Mineral Co.: 3 mi. E Creede, 1; 23 mi. S, 11 mi. E Creede, 9300 ft., 7. La
Plata Co.: Florida, 6800 ft., 1. Conejos Co.: 1 mi. S, 19 mi. W Antonito,
10,200 ft., 3; 4 mi. S, 23 mi. W Antonito, 1; 5 mi. S, 24 mi. W Antonito, 9600
ft., 9.

New Mexico: Rio Arriba Co.: 6 mi. W Hopewell, 9900 ft., 6 (USBS);
Tusas River, 8700 ft., 1 (USBS).

Some measurements not given above are included in Table 1,
together with the number of specimens and the sex if restricted to
one sex. So that the variability can be evaluated more adequately,
the coefficient of variability and 2 times the standard error of the
mean are included for the measurements in two series. The series
consist of all the adult specimens (with a condylobasilar length of
24.0 mm. or more) of both sexes from the areas specified. Various
barriers are shown in Fig. 2 for comparison with the distributions
of the subspecies and the localities of known occurrence shown in
Fig. 1. Microtus montanus has not been taken in the Black Hills
area of extreme northeastern Wyoming. Suitable montane habitat
is present and both Microtus pennsylvanicus insperatus and Microtus
longicaudus longicaudus occur there. The arid basin of the Powder
River presumably is a barrier that has prevented M. montanus from
reaching this area.

Table 1. Average Measurements, in Millimeters, of Adults of
Microtus montanus.

Key to column headings:



A: No. of individuals averaged

B: Total length

C: Length of tail

D: Length of hind foot

E: Condylobasilar length

F: Alveolar length of upper molar tooth-row

G: Zygomatic breadth

H: Lambdoidal breadth

I: Prelambdoidal breadth

REFERENCES CITED

Bailey, V.

1900. Revision of American voles of the genus Microtus. N. Amer. Fauna,
17:1-88, June 6.

1917. A new subspecies of meadow mouse from Wyoming. Proc. Biol.
Soc. Washington, 30:29-30, February 21.

Cary, M.

1911. A biological survey of Colorado. N. Amer. Fauna, 33:1-256,
August 17.

1917. Life zone investigations in Wyoming. N. Amer. Fauna, 42:1-95,
October 3.

Davis, W. B.

1939. The Recent mammals of Idaho. 400 p., front., illus., maps, diagrs.,
Caldwell, Id., The Caxton Printers Ltd., April 5.

Durrant, S. D.

1952. Mammals of Utah, taxonomy and distribution. Univ. Kans. Publ.,
Mus. Nat. Hist., 6:1-549, 91 figs, in text, 30 tables, August 10.

Elton, C.

1942. Voles, mice and lemmings; problems in population dynamics. 496
p., illus. (maps), tables, Oxford, The Clarendon Press, London.

Goin, O. B.

1943. A study of individual variation in Microtus pennsylvanicus pennsylvanicus.
Jour. Mamm., 24:212-224, June 7.

Hall, E. R.

1938. Notes on the meadow mice Microtus montanus and Microtus nanus
with descriptions of a new subspecies from Colorado. Proc. Biol.
Soc. Washington, 51:131-134, August 23.

Hill, E. A.

1935. Cranial foramina in rodents. Jour. Mamm. 16(2):121-129.

Howell, A. B.

1924. Individual and age variation in Microtus montanus yosemite. Jour.
Agr. Res., 28(10):977-1015, June 7.

Kellogg, R.

1922. A study of the California forms of the Microtus montanus group of
meadow mice. Univ. California Publ. Zool., 21:245-274, April 18.

Merriam, C. H.

1891. Results of a biological reconnaissance of south-central Idaho. N.
Amer. Fauna, 5:1-113, July 30.

Piper, S. E.

1909. The Nevada mouse plague of 1907-8. U. S. Dept. Agr., Farmers'
Bul. 352, pp. 1-23, 9 figs.

Warren, E. R.

1942. The mammals of Colorado, their habits and distribution. Second
(revised) edition, Univ. Oklahoma Press, Norman, xviii-330 p.,
front., 50 plates.

Transmitted March 22, 1954.

25-3560

Transcriber's Notes

One typographical error was corrected:
"Castlford" was corrected to "Castleford" in "Twin Falls Co.: Castleford Fenced Plot".