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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; ¼ 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; _2¼ 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.

[Illustration: 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.

[Illustration: 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).

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

=========================================================================
       Locality   |   A  |  B  |  C  |  D  |  E  |  F  |  G  |  H  |  I
------------------+------+-----+-----+-----+-----+-----+-----+-----+-----
                  |               _M. m. codiensis_, all
                  |
Average           | 34   |165.3|44.2 |19.6 |25.47| 6.56|15.55|12.05|8.76
2 × stand. error  |      | 3.56|1.84 |.395 | .308| .067| .198| .144|.129
Coeff. variab     |      |  6.0|11.6 | 5.6 |  3.5|  3.0| 3.65| 1.20|1.47
------------------+------+-----+-----+-----+-----+-----+-----+-----+-----
                  |            _M. m. nanus_, Eastern Idaho
                  |
Average           | 21[1]|151.1|39.4 |19.2 |25.00| 6.44|14.99|11.74|8.94
2 × stand. error  |      | 3.20|2.89 |.293 | .286| 1.15| .295| .210|.182
Coeff. variab.    |      |  6.1|21.1 |4.38 | 2.49| 3.99| 4.10| 3.79|4.31
------------------+------+-----+-----+-----+-----+-----+-----+-----+-----
                  |            _M. m. nanus_, Wyoming
                  |
Teton Co.         | 35   |160.5|40.7 |18.6 |25.16| 6.51|15.17|11.86|8.77
Fremont Co.       | 26   |157.0|41.4 |19.6 |25.23| 6.25|15.05|11.88|8.91
Lincoln Co.       | 24   |159.9|41.8 |18.9 |25.08| 6.26|15.10|11.82|8.75
Uinta Co.         | 26   |162.4|41.3 |19.0 |25.33| 6.42|15.31|12.16|8.89
Sweetwater Co.    | 12   |159.8|43.7 |20.1 |24.98| 6.31|15.00|11.84|9.02
Natrona Co.       | 40   |159.6|41.0 |19.6 |25.04| 6.40|15.00|11.84|8.93
Carbon Co.        |108   |158.7|40.0 |19.1 |24.96| 6.27|15.05|11.83|8.72
 Encampment ♂     | 27   |161  |41.7 |18.9 |25.1 | 6.16|15.2 |11.9 |8.7
 Encampment ♀     | 11   |159  |41.1 |19.4 |24.9 | 6.18|14.9 |11.6 |8.6
 Savery ♂         | 23   |159  |41.0 |19.2 |25.3 | 6.32|15.2 |12.2 |8.8
 Savery ♀         | 25   |155  |37.1 |18.8 |24.7 | 6.33|14.8 |11.6 |8.6
------------------+------+-----+-----+-----+-----+-----+-----+-----+-----
                  |                     _M. m. nanus_
                  |
Northern Colo.    |  8   |163.1|42.4 |19.6 |25.20| 6.44|14.86|11.70|8.56
------------------+------+-----+-----+-----+-----+-----+-----+-----+-----
                  |                     _M. m. fusus_
                  |
Southern Colo.    | 17[2]|159.8|42.4 |19.2 |24.97| 6.43|14.98|11.73|8.69
------------------+------+-----+-----+-----+-----+-----+-----+-----+-----

[Footnote 1: For external parts, 34 individuals were used.]

[Footnote 2: For external parts, 29 individuals were used.]

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.


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

Italic typeface in the original is indicated by by _underscores_.

Bold typeface in the original is indicated by =equals=.

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