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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY

Volume 9, No. 4, pp. 85-104, 2 figs. in text
May 10, 1956



Subspeciation in the Meadow Mouse, Microtus pennsylvanicus,
in Wyoming, Colorado, and Adjacent Areas



BY

SYDNEY ANDERSON

UNIVERSITY OF KANSAS
LAWRENCE
1956



UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

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

Volume 9, No. 4, pp. 85-104, 2 figures in text

Published May 10, 1956

UNIVERSITY OF KANSAS
Lawrence, Kansas

PRINTED BY
FRED VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1956




Subspeciation in the Meadow Mouse, Microtus pennsylvanicus,
in Wyoming, Colorado, and Adjacent Areas

BY

SYDNEY ANDERSON


INTRODUCTION

In the region including Wyoming and Colorado, _Microtus pennsylvanicus_
has been divided into two subspecies: the pale _M. p. insperatus_ (J.
A. Allen) inhabits the Black Hills of the northeasternmost part of
Wyoming; the dark _M. p. modestus_ (Baird) inhabits extensive areas
in both Wyoming and Colorado. Initial examination of _Microtus
pennsylvanicus_ revealed that specimens from the Big Horn Mountains of
north-central Wyoming (within the range of _modestus_ as mapped by Hall
and Cockrum 1952:407), in color at least, resemble the subspecies
_insperatus_ more than they do _modestus_, and that specimens from
southwestern Wyoming are notably dark. Durrant (1952:363) noted that
specimens from Utah are dark, and Davis (1939:315) did the same for
specimens from near Pocatello, Idaho. It seemed, therefore, that dark
color might characterize populations of a wide geographic region and
distinguish them from _modestus_ named from southern Colorado. Also,
there seemed to be a hiatus of at least 180 miles between the ranges of
_modestus_ in northern Colorado and _modestus_ in eastern Wyoming, and
an even greater distance separating populations of _modestus_ in
northern Colorado from those in western Wyoming. _Microtus
pennsylvanicus_ has not been taken in central or southeastern Wyoming
despite extensive collecting there, which yielded numerous records of
other kinds of _Microtus_ (_M. longicaudus_, _M. montanus_, and _M.
ochrogaster_). Subsequent study revealed a pattern of geographic
variation within the range now ascribed to _modestus_ which, in my
opinion, can be described best by the recognition of three new
subspecies.


MATERIALS, METHODS, AND ACKNOWLEDGMENTS

To study geographic variation in color a method was devised as follows:
A single skin (KU 42407, from 1-1/2 miles east of Buckhorn in Weston
County, Wyoming) was selected as a representative of the paler mice and
arbitrarily given the number 2. A single skin (KU 17491, from 3 miles
east of Moran in Teton County, Wyoming) was selected as a
representative of the darker mice from the western part of Wyoming and
arbitrarily given the number 4. These mice were selected so that they
were respectively paler and darker than the estimated average of the
total variation within the populations to be studied, but the two mice
were not at the extremes of paleness and darkness. Comparisons were
based on visual inspection of the dorsal pelage as a whole. Skins were
compared with these two mice and given whole numbers from one to five.
If paler than the standard for 2, the skin was numbered one; if not
distinguishably paler or darker, it was given the number two; if
intermediate in color to the standards for 2 and 4 and not definitely
more nearly referable to one than the other, it was given the number
three; if it resembled the standard for 4, it was numbered four; and if
darker, it was given the number five. In this manner skins from a given
locality could be evaluated one by one and the results plotted,
averaged, and treated statistically. On Figure 1 the average values for
color of 32 series are mapped to show the geographic variation of
color. The following series of adults are the basis for Figure 1
(abbreviations for collections other than at the University of Kansas
are included in parentheses): Each locality is followed by the month
(or months) of capture, the number of specimens, and the average value
for color.

    _Montana_: Glacier County, August, 6, 1.8; Hill and Chouteau
    counties combined (Mich), July, 24, 1.5; Malta, Philips County,
    August, 14, 1.5; Sheridan County, August, 6, 1.5; Fergus County
    (USBS), August, 5, 2.4; Ravalli County (KU and USBS), August, 12,
    2.8; Silver Bow County, August, 7, 3.0; Sweet Grass County (Mich),
    June and July, 7, 2.7; Park County, August, 10, 2.6. _Idaho_:
    Pocatello and vicinity, November and December, 5, 3.4. _Wyoming_:
    Park County, August, 6, 2.8; Sheridan County, September, 9, 1.2;
    Johnson County, August, 12, 1.5; Campbell and Crook counties, July,
    11, 1.4; Weston County, July, 7, 1.6; Teton County, September, 8,
    3.4; Teton County (Mich), June, 17, 3.1; Afton and vicinity,
    Lincoln County, July, 10, 4.2; Sage, Lincoln County, July, 5, 5.0.
    _South Dakota_: Pennington County (Chi), June, 14, 2.1; Pennington
    County (Mich), December and January, 8, 1.1; Walworth County, July,
    4, 3.7; Buffalo County, July, 6, 3.2. _Colorado_: Loveland and
    vicinity, Larimer County (KU and USBS), July, 13, 2.8; Boulder
    County (Chi), September, 34, 2.6; Park County (Denv), March, 8,
    1.9; Colorado Springs (ERW), March, April, and May, 5, 2.8;
    Saguache County (USBS), August, 46, 3.0; Conejos County, June, 4,
    3.0; Wray, Yuma County (USBS), 3, 4.7. _Nebraska_: Dundy County,
    August and November, 14, 4.6. _New Mexico_: Colfax County, June, 8,
    3.2. Variation in color is discussed in the accounts of the
    subspecies concerned.

[Illustration: FIGURE 1. Geographic variation in color in _Microtus
pennsylvanicus_ in the Rocky Mountains. Paler colors are represented
by smaller numbers. Numbers are derived from the series of specimens
listed in the text by the method described there. The subspecies that
occur in the region studied are as follows:

    _a. M. p. pullatus_
    _b. M. p. insperatus_
    _c. M. p. uligocola_
    _d. M. p. finitus_
    _e. M. p. modestus_
    _f. M. p. aztecus_
    _g. M. p. drummondi_
    _h. M. p. pennsylvanicus_]

    For each of the series listed in Table 1 all adult mice having
    skulls that measured more than 24.0 mm. in condylobasilar length
    were studied. Total length, length of tail, and length of hind foot
    were taken from the collector's field labels. The measurements of
    the skulls listed below were taken by means of dial calipers
    reading to one-tenth of a millimeter, and in the same fashion as
    described previously (Anderson, 1954:492). Measurements of
    specimens in each series were averaged (the arithmetic means were
    computed). If the averages differed noticeably the significance of
    the difference was tested statistically. Averages referred to in
    the text as significantly different differ by as much as, or more
    than, the sum of two times the standard error of each of the two
    averages. Linear measurements are in millimeters; color values are
    in the arbitrary units described in a preceding paragraph.
    Measurements taken of the skulls are: condylobasilar length,
    zygomatic breadth, interorbital breadth, lambdoidal breadth,
    prelambdoidal breadth, depth of braincase, and alveolar length of
    upper molar tooth-row.

    Secondary sexual variation was not detected in the material
    studied. Variation with age is important to the taxonomist even
    among specimens designated as "adults", because growth and changes
    in various proportions continue throughout the life of the mice.
    The possibility that differences detected in the statistical
    treatment or observed directly could be the result of differences
    in average age within the samples of "adults" was considered in
    each case.

In order to study certain variations, the following "method of pairs"
was used. Skulls of two series to be compared were matched in pairs so
that they corresponded in size and ontogenetic stage of development.
Then the two skulls of each pair were examined for differences in each
of the following features: size of circle inscribed by the upper
incisor teeth, width of nasal bones relative to their length, curvature
of the zygomatic arch, elongation of the braincase relative to its
width when viewed from the dorsal aspect, degree of indentation in the
anterior edge of the zygomatic arch near the rostrum, degree of
depression of the nasal bones when viewed from the side, width in the
vertical plane of the zygomatic arch at the suture between the
maxillary and jugal bones, length relative to width of the prominent
fenestra in the posterodorsal part of the squamosal bone, size of the
meatus of the auditory canal, distance between the internal margin of
an occipital condyle at its posteriormost point and the tip of the
paraoccipital process of the same side of the skull, size of the
foramen magnum, vertical height of the supraoccipital bone from the
dorsalmost point on the margin of the foramen magnum to the midpoint of
the lambdoidal crest, constriction posteriorly or narrowness of the
incisive foramen relative to its length, distance across the
premaxillary bone from the anteriormost point of the incisive foramen
to the posteriormost point of the margin of the alveolus of the upper
incisor, area of the maxillary septum (Howell 1926:112, or "zygomatic
plate" of Ellerman 1941:1), acuminateness of the anterior border of the
palatine opening (internal nares), size of auditory bullae, size of
foramen ovale, acuteness of the angle between the basioccipital and
basisphenoidal bones at the suture between them (degree to which the
area of the suture is raised between the bullae when viewed from the
ventral aspect), width of first upper molar tooth, least distance
between alveoli of first upper molars. Any differential feature present
in more than 75 per cent of the pairs of animals is reported in the
discussion of the subspecies concerned. The significance of each
difference reported was calculated by the Chi-square test and the
confidence limit is given in each case. The probability used in the
Chi-square formula is one-half of the percentage of all pairs compared
in which the skulls were different in regard to the character being
considered. For example, in 68 per cent of the total number of pairs of
skulls compared in this study a difference in the size of the auditory
bullae was noted. Therefore the probability that a specified skull of a
pair will have larger bullae than the other skull was taken as 34 per
cent. A different probability for each feature compared was derived in
like manner.

    This study is concerned primarily with mice from Wyoming and
    Colorado; I realize, however, that the physiographic and ecological
    conditions important to the distribution and subspeciation of
    _Microtus pennsylvanicus_ do not correspond to political
    boundaries. Geographic variation within these two states can be
    seen in proper perspective only when related to the neighboring
    areas and to previous studies. I have attempted to do this in the
    accounts of the subspecies.

    Approximately five months in the field in Wyoming and Colorado in
    the summers of 1950, 1951, 1952, and 1953 gave me a familiarity
    with the region that has helped to clarify the pattern of
    distribution. My study was based, in addition, on 762 specimens
    that are listed under "specimens examined" in the accounts of
    subspecies, and on comparative material from other states. Most of
    these specimens are skins with skulls but some are skins only and
    others are skulls only. Some localities are represented by too few
    adult individuals to permit significant comparisons. Owing to
    damaged skulls, certain measurements of some specimens were omitted
    from the calculations. If it seemed that the damaged skull was
    exceptionally large or small or a deviant in any other regard it
    was not used, in order not to bias the computed averages, which
    might be used in comparing proportions of the skulls. In the lists
    of specimens examined, localities that are omitted from Figure 2
    because overlapping or undue crowding of the symbols would have
    resulted are _italicized_.

[Illustration: FIGURE 2. Distribution of the subspecies of _Microtus
pennsylvanicus_ in Wyoming and Colorado. Solid dots represent
localities from which specimens have been examined, and triangles
represent localities reported in the literature from which I have not
examined specimens. The question mark in southern Colorado denotes a
questionable record discussed in the text.

    A. _M. p. pullatus_
    B. _M. p. insperatus_
    C. _M. p. uligocola_
    D. _M. p. finitus_
    E. _M. p. modestus_
    F. _M. p. aztecus_]

    I am grateful to Professor E. Raymond Hall for critical reading of
    the manuscript and helpful suggestions, to Dr. Rollin H. Baker and
    various of my fellow students at the Museum of Natural History for
    stimulating comments pertinent to the problems involved in this
    study, to my wife, Justine Anderson, for assistance in the
    preparation of the manuscript, to numerous members of field parties
    from the Museum of Natural History, who collected much of the
    material studied, and to the curators and other persons, at the
    museums listed below, who courteously made specimens available for
    study. The field work of the Museum of Natural History was assisted
    by the Kansas University Endowment Association. A National Science
    Foundation Fellowship made it possible for me to visit the museums
    listed below. An honorarium awarded by the American Society of
    Mammalogists enabled me to present this paper at the 34th Annual
    Meeting of the Society, in June of 1954. Unless otherwise
    indicated, specimens are in the University of Kansas Museum of
    Natural History. Specimens in other museums are designated beyond
    as follows: American Museum of Natural History (AMNH); Museum of
    Zoology, University of Michigan (Mich); Chicago Natural History
    Museum (Chi); United States National Museum (USNM); Biological
    Surveys Collection (USBS); Museum of Vertebrate Zoology, Berkeley,
    California (MVZ); Colorado Museum of Natural History, Denver
    (Denv); E. R. Warren collection, Colorado College, Colorado Springs
    (ERW); University of Colorado Museum, Boulder (UC).


ACCOUNTS OF SUBSPECIES

Microtus pennsylvanicus modestus (Baird)

        _Arvicola modesta_ Baird, Repts. Expl. and Surv...., pt. 1,
        Mammals, p. 535, July 14, 1858.

        _Microtus pennsylvanicus modestus_, Bailey, N. Amer. Fauna,
        17:20, June 6, 1900.

    _Type._--Immature specimen (sex not specified), skin and skull,
    number of skin 594, number of skull 1717, deposited in the
    collections of the Smithsonian Institution, obtained by F.
    Kreutzfeldt, Sawatch Pass, Rocky Mountains [=Saguache Pass or
    Cochetopa Pass, Saguache County, Colorado], exact date unknown. I
    have not examined the type specimen.

    _Range._--Northern New Mexico, and southern Colorado (see list of
    specimens and Fig. 2).

    _Comparisons._--For comparison with the subspecies newly described
    below from northern Colorado see the account of that subspecies.
    The subspecies _M. p. aztecus_ has been compared with _M. p.
    modestus_ by Hall and Cockrum (1952:308) who reduced _aztecus_ to
    subspecific rank. Although _aztecus_ is separated by approximately
    100 miles from _modestus_, and although no proof of intergradation
    is available, my studies of variation in this species lead me to
    agree with Hall and Cockrum that "the morphological differences
    between the two kinds of animals are of the degree and kind that
    separate subspecies, rather than species." A more adequate series
    of adults of _aztecus_ is needed to clarify even the subspecific
    differences between _aztecus_ and _modestus_.

    _Measurements._--Averages, extremes, and standard deviations of a
    number of series are included in Table 1 in order to facilitate
    comparisons between different subspecies.

_Remarks._--The dividing line between _M. p. modestus_ and the
subspecies to the north on Figure 2 is drawn somewhat arbitrarily
because few specimens are available from this area. Actual
intergradation, in the form of a geographically intermediate population
also morphologically intermediate between these two subspecies, is
lacking. However, in most populations of both subspecies some
individuals are intermediate between the two subspecies or even more
like the other subspecies than the one to which they are referred.
Warren (1942:226) states that _modestus_ has been recorded from Lake
County, although no reference to a specimen is given. Bailey (1900:21)
cites Twin Lakes, in Lake County. That county is near the dividing line
as I have drawn it, and therefore specimens from Lake County would be
of special interest. An isolated colony of _modestus_ occurs at San
Rafael in Valencia County, New Mexico (Bailey, 1932:201). A hiatus of
approximately 150 miles separates that colony from the southernmost
locality shown in Figure 2. A single specimen, the skin of an immature
_Microtus_ without skull, from Trinchera, Colorado, taken by L. R.
Hersey in 1912, is in the Colorado Museum of Natural History. No
species of Microtine has been recorded from within 50 miles of this
locality. The specimen is seemingly more like _M. pennsylvanicus_ than
any other species of _Microtus_. This locality is represented in Figure
2 by a question mark. Ecologically _M. montanus fusus_ Hall and _M.
longicaudus mordax_ (Merriam) in the same region occupied by _modestus_
seem to be more montane than _modestus_. It favors lush grass on the
wet floors of alluvial valleys, and also irrigated areas such as that
at Manassa.

    _Specimens examined._--Total 130. _Colorado_: CHAFFEE CO.: Salida,
    3 (ERW). SAGUACHE CO.: Monshower Meadows, 27 mi. NW Saguache [=3
    mi. E Cochetopa Pass], 8 (USBS); _Tevebaugh's Ranch, 20 mi. W
    Saguache_, 46 (USBS, there are additional specimens not examined in
    detail by me); _Cochetopa Pass, 33 mi. W Saguache_, 1; 3 mi. N, 16
    mi. W Saguache, 8500 ft., 5; 5 mi. NW Hooper, 1 (AMNH); Medano
    Ranch, 15 mi. NE Mosca, 2 (1 ERW, 1 USBS). CUSTER CO.: Westcliffe,
    7800 ft., 1 (ERW). ALAMOSA CO.: _Hooper_, 10 (2 AMNH, 8 Denv);
    _Mosca_, 3 (ERW). CONEJOS CO.: 1-1/2 mi. E Manassa, 11. COSTILLA
    CO.: Alamosa, 3 (Mich); 2 mi. S Blanca, 7800 ft., 6 (MVZ). _New
    Mexico_: TAOS CO.: Arroyo Hondo, 7600 ft., 6 (USBS); Taos, Pueblo,
    1 (USNM). COLFAX CO.: 1 mi. S, 2 mi. E Eagle Nest, 8100 ft., 21;
    Taos Mountains, east slope, 8800 ft., 1 (USBS); _Coyote Creek_, 1
    (USBS).


Microtus pennsylvanicus uligocola new subspecies

    _Type._--Adult male, skin and skull, number 26898, University of
    Kansas, Museum of Natural History, obtained by James O. Lounquist,
    original number 349, 6 miles west and 1/2 mile south of Loveland,
    5200 ft., Larimer Co., Colorado, on July 26, 1948.

    _Range._--Northern Colorado. See Figure 2 and list of specimens
    examined.

    _Diagnosis._--Entire animal and skull large; color average for the
    species, neither extremely pale or dark in summer pelage; molar
    tooth-row long; nasals narrow; maxillary septum large; first upper
    molar wide; anterior margin of zygomatic arch above infraorbital
    foramen not deeply indented; fenestrae in posterodorsal parts of
    squamosal bones relatively long; braincase not elongate; auditory
    bullae and meatus large.

    _Comparisons._--From _M. p. modestus_, _M. p. uligocola_ differs as
    follows: averages paler; prelambdoidal breadth and alveolar length
    of molar tooth-row significantly greater. Six pairs of skulls were
    compared. Of the features listed above under the "method of pairs"
    only two features differed in more than 75 per cent of the pairs;
    in 5 of 6 pairs _uligocola_ had a less distinctly indented anterior
    margin of the zygomatic arch (Confidence Limit .95) and a more
    elongate posterodorsal squamosal fenestra (C. L. .85). Seven pairs
    of skulls from Boulder, Colorado, representing _uligocola_ and from
    Colfax County, New Mexico, representing _modestus_ differed in more
    than 75 per cent of the pairs in three features. Only one of these
    differences, the elongation of the posterodorsal squamosal
    fenestra, was the same as a difference noted above between
    topotypes of _uligocola_ and _modestus_. A comparison of ten pairs
    of skulls of _uligocola_ from Boulder, Colorado, and topotypes of
    _uligocola_ revealed no significant differences. These observations
    are indicative of 1) the differences between samples and
    populations which may be assigned to a single subspecies, and 2)
    the fact that in general these local differences are less than the
    differences between subspecies. From _insperatus_, the subspecies
    to the north, _uligocola_ differs as follows: darker in both summer
    and winter pelage; averaging larger in most measurements of the
    skull; significantly longer molar tooth-row; hind foot averaging
    longer. For comparisons with the subspecies to the east and the
    northwest see the accounts of those below.

_Remarks._--_M. p. uligocola_ is more closely restricted to wet
situations than _M. ochrogaster haydeni_ (Baird) whose general range
lies to the eastward. The numerous lakes, the continuous supply of
water from the mountains, and the irrigation systems at lower altitudes
along the eastern base of the mountains provide the conditions to which
_uligocola_ is suited. It is named for its predilection for water.

The variability in color is relatively greater in topotypes of both
_uligocola_ and of _modestus_ than in many of the other series studied.
Specimens from Denver and Colorado Springs taken in late autumn and
winter (October to February) are paler, more reddish and less blackish,
than specimens taken in June and July at Loveland. This reddishness
results from longer, and more intensely reddish tips of the hair. The
entire hairs also are longer. The average weight of 16 adults (12 males
and 4 non-pregnant females) from near Loveland is 49.7 gms. The average
length of the ear is 13.4.

    _Specimens examined._--Total 228. _Colorado_: LARIMER CO.: 6 mi. W,
    1/2 mi. S Loveland, 5200 ft., 18; 3 _mi. N Loveland_, 3;
    _Loveland_, 4 (USBS). MORGAN CO.: 4 mi. W Orchard, 4 (Mich);
    _Orchard_, 1 (Mich). BOULDER CO.: Boulder, 91 (USNM 19, UC 12, Chi
    60 examined, additional specimens in the collection); _Valmont_, 4
    (UC). CLEAR CREEK CO.: Clear Creek, N side of Idaho Springs, 1.
    JEFFERSON CO.: Olivet, 10 (Denv). ADAMS CO.: Crook's Lake, 8
    (Denv); Barr, 16 (Denv 15, ERW 1). ARAPAHOE CO.: Denver, 21 (USBS
    3, AMNH 8, Denv 10). PARK CO.: William's Ranch, near Tarryall, 11
    (Denv). TELLER CO.: Divide, 2 (ERW). EL PASO CO.: Colorado Springs,
    31 (ERW 22, AMNH 7, MVZ 2); 12 mi. S Colorado Springs, 3 (MVZ).


Microtus pennsylvanicus finitus new subspecies

    _Type._--Adult female, skin and skull, number 50204, University of
    Kansas, Museum of Natural History, obtained by J. K. Jones, Jr.,
    original number 906, 5 miles north and 2 miles west of Parks in
    Dundy County, Nebraska, on August 16, 1952.

    _Range._--The valley of the north fork of the Republican River in
    eastern Colorado and southwestern Nebraska.

    _Diagnosis._--Entire animal and skull large; color dark for the
    species; zygomatic breadth large; upper molars large and upper
    molar tooth-row relatively long; braincase elongate; auditory
    meatus relatively small; bullae large; incisors relatively
    procumbent.

    _Comparisons._--From _M. p. uligocola_, the subspecies to the west,
    _M. p. finitus_ differs in darker color. These subspecies resemble
    each other in large size and large molar teeth. A comparison of
    nine pairs of skulls by the "method of pairs" shows three features
    in which _finitus_ (from at, or near, the type locality) differs
    from _uligocola_ (from at, or near, the type locality); in 7 of 9
    pairs _finitus_ had a relatively more elongate braincase
    (Confidence Limit .60); in 9 of 9 pairs _uligocola_ had larger
    auditory meatuses (C. L. .99); in 7 of 9 pairs _uligocola_ had
    relatively larger bullae (C. L. .80). From _M. p. pennsylvanicus_
    (Ord) from eastern Nebraska and eastern South Dakota _finitus_
    differs in larger size; darker color; larger molar teeth and longer
    upper molar tooth-row. Five pairs of skulls were compared; in all 5
    pairs, _finitus_ had more procumbent incisor teeth (C. L. .97) and
    wider first upper molars (C. L. .97); and in 4 of 5 pairs _finitus_
    had a relatively more elongate braincase (C. L. .60). From _M. p.
    insperatus_, the subspecies to the north, _finitus_ differs as
    follows: color darker, size larger, molar teeth relatively larger
    and alveolar length of the upper molar tooth-row greater. Five
    pairs of skulls were compared and in all 5 pairs _finitus_ had more
    procumbent upper incisors (C. L. .97) than _insperatus_; in 4 of 5
    pairs _insperatus_ had a relatively more elongate braincase (C. L.
    .60), narrower first upper molariform tooth (C. L. .75), and
    shorter distance between the alveoli of the first upper molars (C.
    L. .88).

_Remarks._--The species, _Microtus pennsylvanicus_, in Pleistocene time
ranged onto the plains of Kansas as far southward as Meade County,
Kansas (Hibbard, 1940:421). This occurrence indicates a cooler more
humid climate then than now in southwestern Kansas. _M. p. finitus_ is
more closely associated with water than _Microtus ochrogaster_, the
only other species of _Microtus_ now occupying the same region,
although both species have been captured at certain places in the same
runways. In Nebraska, a marginal part of the range of the species, _M.
pennsylvanicus_ has been taken at scattered localities. This scattered
and localized distribution of suitable habitat undoubtedly limits
gene-flow between these relict populations. Presumably as a result of
this isolation _finitus_ has accumulated and maintained its distinctive
characteristics. The subspecies is so named because of its limited
range. The average weight of eight specimens (4 males and 4
non-pregnant females) from Dundy County is 57.2 grams.

    _Specimens examined._--Total 26. _Colorado_: YUMA CO.: Wray, 3
    (USBS); _1 mi. W Laird_, 2. _Nebraska_: DUNDY CO.: 5 mi. N, 2 mi. W
    Parks (Rock Creek State Fish Hatchery), 19; Haigler, 2 (USBS).


Microtus pennsylvanicus pullatus new subspecies

    _Type._--Adult male, skin and skull, number 37873, University of
    Kansas, Museum of Natural History, obtained by Rollin H. Baker,
    original number 1343, 12 miles north and 2 miles east of Sage, 6100
    ft., in Lincoln County, Wyoming, on July 19, 1950.

    _Range._--North-central Utah, eastern Idaho, western Wyoming, and
    southwestern Montana. See Figures 1 and 2.

    _Diagnosis._--Size average; color dark, especially in southern part
    of range; tail relatively long; molar teeth small; nasals
    relatively broad; maxillary septum relatively small.

    _Comparisons._--From _M. p. uligocola_, the subspecies to the
    southeast, _M. p. pullatus_ differs as follows: relatively darker
    in southern part of its range (see Figure 1); smaller, tail
    relatively longer. In 6 of 7 pairs of skulls compared of _pullatus_
    (from Lincoln Co., Wyoming) and _uligocola_ (from Larimer Co.,
    Colorado), _pullatus_ had relatively broader nasals (Confidence
    Limit .85); _uligocola_ had larger maxillary septa (C. L. .97) and
    larger molar teeth (C. L. .90). From _insperatus_, the subspecies
    to the east, _pullatus_ differs as follows: both summer and winter
    pelage darker; tail longer both actually and relatively; upper
    molar tooth-row shorter. Ten pairs of skulls of specimens from near
    Afton, Wyoming, representing _pullatus_, and from northeastern
    Wyoming, representing _insperatus_, revealed no significant
    differences in the features observed by the "method of pairs".
    Although not compared in detail with the subspecies to the north,
    _M. p. drummondi_ (Audubon and Bachman), examination of specimens
    from western Montana and the accounts of other authors indicate
    that topotypes of _pullatus_ are darker, longer-tailed, slightly
    larger-skulled and perhaps longer over all.

_Remarks._--In this subspecies there is a cline in color from dark in
extreme southwestern Wyoming to pale in north-central Wyoming and
Montana as the range of _M. p. insperatus_ is approached. There is thus
a broad zone of intergradation in color and the line separating the
subspecies must be drawn somewhat arbitrarily. In Wyoming the most
distinct break in this cline is in the Big Horn Basin and if a detailed
study of the species were made in Montana the break would probably be
found where the mountains meet the plains, roughly as shown in Figure
1. There is a similar cline in western Montana in color. The mice are
paler farther north as one approaches the Canadian border although they
do not become so pale as _insperatus_. Darkness is a characteristic of
several non-adjacent subspecies of _Microtus pennsylvanicus_, for
example _M. p._ _kincaidi_ Dalquest in central Washington (Dalquest,
1948:347), _M. p. finitus_, and _M. p. nigrans_ Rhoads in eastern
Virginia, but these subspecies presumably differ in other characters.

Some morphological features of the same kind and degree that
differentiate subspecies in one place may not vary geographically in
another place. Furthermore the geographic variation in one feature may
be only partly correlated with the variation in another feature. The
variation in _M. p. pullatus_ is an example: Specimens from near
Pocatello, Idaho, are darker than topotypes of _modestus_ but specimens
from Fremont County, Idaho, are indistinguishable from topotypes of
_modestus_ (Davis, 1939:315). I have examined a number of mice from the
Bitterroot Valley in western Montana and the color value for 12 adults
is 2.7. They are slightly but not significantly paler than topotypes of
_modestus_. This is a result of the cline mentioned above and does not
indicate relationship with _modestus_. Some average measurements of 10
skulls from this series are as follows: condylobasilar length, 24.9;
zygomatic breadth, 14.4; interorbital breadth, 3.4; lambdoidal breadth,
11.5; prelambdoidal breadth, 9.1; alveolar length of upper molar teeth,
6.5; and depth of braincase, 7.6. Average external measurements of 9
specimens are as follows: total length, 157; length of tail, 36; length
of hind feet, 19.4. Mice from the Bitterroot Valley were compared with
topotypes of _modestus_ by the "method of pairs," and _modestus_ had a
larger foramen magnum in 6 of 6 pairs (Confidence Limit .97) and larger
first upper molar teeth in 5 of 6 pairs (C. L. .75).

A comparison of topotypes of _pullatus_ with _modestus_ shows a similar
difference in the teeth, _modestus_ being larger, but in the size of
the foramen magnum there is no difference. A comparison of the
measurements of _pullatus_ (Bitterroot Valley), _pullatus_ (near
topotypes), and _modestus_ (topotypes) shows that the two series of
topotypes differ significantly in condylobasilar length (of borderline
significance), zygomatic breadth and lambdoidal breadth (both of which
vary greatly with age), and length of molar series; the specimens from
the Bitterroot Valley agree with _pullatus_ rather than _modestus_ in
all of these characters. The specimens from the Bitterroot Valley are
smaller than _pullatus_ (topotypes) in total length; they more closely
resemble _modestus_ in length of tail, and the hind foot is shorter
than in either _modestus_ or _pullatus_ (which do not differ
significantly). Specimens from western Montana resemble _modestus_ in
certain respects but in most respects resemble topotypes of _pullatus_
and are referred to _pullatus_. Specimens from Coeur d'Alene, Idaho,
and from Blackfoot, Montana (marginal records of _modestus_, Hall and
Cockrum, 1953:410) may be referred to _drummondi_. Marginal records of
_pullatus_ in Montana to my knowledge are: Florence, Ravalli Co.;
Highwood Mtns., Chouteau Co.; 7 mi. NE Hilger, Fergus Co.; 10 mi. NW
Park City, Stillwater Co. (all represented by specimens in the USBS).
The line separating _pullatus_ and _drummondi_ is tentatively drawn as
shown in Figure 1.

    _Specimens examined._--Total 256. _Wyoming_: YELLOWSTONE NATIONAL
    PARK: Mammoth Hot Springs, 17 (USNM). PARK CO.: 3-1/5 mi. E, 3/5
    mi. S Cody, 5020 ft., 15. TETON CO.: _Whetstone Creek_, 4 (Mich); 5
    mi. N Moran, 13 (Mich); _Moran and environs_ (_4 localities within
    4 miles of Moran_), 6200 ft., 54; Teton Park, Trappers Lake, 3
    (Mich); _Teton Park, Jenny Lake_, 1 (Mich); _Teton Park, String
    Lake_, 1 (Mich); Sheep Creek, 1 (Mich); _Jackson and environs_, 115
    (Mich 113, USBS 1). SUBLETTE CO.: 34 mi. N, 4 mi. W Pinedale, 7950
    ft., 2; Kendall, 5 (Mich). LINCOLN CO.: 9-1/2 mi. N, 2 mi. W Afton,
    6100 ft., 1; _9 mi. N, 2 mi. W Afton_, 3; _7 mi. N, 1 mi. W Afton_,
    11; _15 mi. N, 3 mi. E Sage_, 6100 ft., 1; _12 mi. N, 2 mi. E
    Sage_, 4; 6 mi. N, 2 mi. E Sage, 2.


Microtus pennsylvanicus insperatus (Allen)

        _Arvicola insperatus_ Allen, Bull. Amer. Mus. Nat. Hist.,
        6:347, December 7, 1894.

        _Microtus pennsylvanicus insperatus_, Anderson, Canadian
        Field-Nat., 57:92, October 17, 1943.

        _Microtus pennsylvanicus wahema_ Bailey, Jour. Mamm., 1:72,
        March 2, 1920.

    _Type._--Adult male, skin and skull, number 8105/6731 American
    Museum of Natural History, obtained by W. W. Granger, at Custer,
    Black Hills, South Dakota, August 9, 1894.

    _Range._--Western South Dakota, southwestern North Dakota, eastern
    Montana, southern Alberta and Saskatchewan, eastern Montana, and
    northeastern Wyoming.

    _Comparisons._--_Microtus pennsylvanicus insperatus_ is paler than
    any adjacent subspecies. It has been compared with _pullatus_ in
    the preceding account. Bailey's studies of _M. p. wahema_
    [=_insperatus_] and his comparisons with _M. p. pennsylvanicus_ and
    _M. p. drummondi_ to the east and north, in North Dakota (1920,
    1927), are the basis for the northern and eastern boundaries of the
    range of _insperatus_ in Figure 1. Comparison of _insperatus_ with
    _finitus_ to the south is made in the account of the latter.


TABLE 1. AVERAGE MEASUREMENTS (IN MILLIMETERS) OF ADULT MICROTUS
PENNSYLVANICUS

========================+======+========+=======+=======+=========+=======
                        |      |        |       |       |         |
                        |No. of|        | Length| Length| Condylo-| Zygo-
      Locality          |adults|  Total |   of  |   of  | basilar | matic
      (or area)         | ave- | length |  tail |  hind | length, |breadth
                        |raged |        |       |  feet |  skull  |
                        |      |        |       |       |         |
------------------------+------+--------+-------+-------+---------+-------
                        |
                        |        _M. p. insperatus_, Wyoming
                        |
Crook and Campbell Cos. |  12  |  166.5 |  41.4 |  20.7 |  25.85  | 14.80
Sheridan County         |  20  |  169.5 |  46.7 |  21.2 |  26.20  | 15.25
Johnson County          |  20  |  167.8 |  47.9 |  20.7 |  25.67  | 15.29
Weston County, mean     |  15  |  161.0 |  39.8 |  20.7 |  25.55  | 14.91
 " Co., stand. dev.     |  ... |   11.3 |   3.0 |   0.6 |    .82  |   .43
 " Co., minimum         |  ... |  150.  |  35.  |  20.  |  24.2   | 14.2
 " Co., maximum         |  ... |  168.  |  45.  |  22.  |  26.8   | 15.6
------------------------+------+--------+-------+-------+---------+-------
                        |
                        |         _M. p. pullatus_, Wyoming
                        |
Park County             |  10  |  164.8 |  44.5 |  20.3 |  25.96  | 15.92
Teton County            |  20  |  161.9 |  43.6 |  19.8 |  25.59  | 14.68
Sage, Lincoln Co.       |   6  |  165.0 |  47.2 |  21.2 |  25.87  | 14.93
Afton, "   " , mean     |  14  |  163.3 |  48.8 |  20.8 |  25.59  | 14.70
  "  , stand. dev.      |  ... |    7.4 |  10.4 |   1.4 |   1.00  |   .61
  "  , minimum          |  ... |  142.  |  39.  |  19.  |  24.5   | 13.8
  "  , maximum          |  ... |  181.  |  57.  |  23.  |  27.5   | 15.9
------------------------+------+--------+-------+-------+---------+-------
                        |
                        |        _M. p. uligocola_, Colorado
                        |
Park Co. (Denv)         |   7  |        |       |       |  26.00  | 15.40
Boulder Co. (Chi)       |  30  |  171.2 |  43.0 |  22.2 |  26.53  | 15.28
Denver                  |   8  |  156.9 |  40.0 |  21.3 |  26.77  | 15.31
Colorado Springs        |  16  |  158.4 |  41.4 |  21.1 |  26.40  | 15.43
Loveland, mean          |  16  |  166.0 |  46.6 |  21.5 |  26.54  | 15.60
   "  , stand. dev.     |  ... |   15.9 |   5.9 |   0.8 |   1.42  |  1.23
   "  , minimum         |  ... |  142.  |  38.  |  20.  |  24.6   | 13.9
   "  , maximum         |  ... |  192.  |  56.  |  23.  |  29.4   | 17.8
------------------------+------+--------+-------+-------+---------+-------
                        |
                        |             _M. p. finitus_
                        |
Wray, Colorado          |   3  |  169.0 |  39.3 |  22.3 |  27.30  | 16.20
Dundy Co., Nebr., mean  |  12  |  165.8 |  42.6 |  21.8 |  27.65  | 16.10
   "  , stand. dev.     |  ... |   16.5 |   5.2 |   0.7 |   2.57  |   .92
   "  , minimum         |  ... |  147.  |  36.  |  21.  |  25.0   | 14.7
   "  , maximum         |  ... |  202.  |  55.  |  23.  |  29.7   | 17.6
------------------------+------+--------+-------+-------+---------+-------
                        |
                        |        _M. p. modestus_, Colorado
                        |
Alamosa, Colorado       |   3  |  160.7 |  46.3 | 22.0  |  25.57  | 15.00
Cochetopa Pass, Colo.   | 25[1]|  172.7 |  44.7 |  21.2 |  26.42  | 15.43
   "  , stand. dev.     |  ... |    8.8 |   3.1 |   0.8 |    .69  |   .52
   "  , minimum         |  ... |  160.  |  38.  |  20.  |  25.2   | 14.6
   "  , maximum         |  ... |  191.  |  51.  |  23.  |  28.5   | 16.8
------------------------+------+--------+-------+-------+---------+-------

      [1] 29 specimens were used in taking measurements of the skull.

========================+======+========+=======+=======+=========+=======
                        |No.   | Lamb-  |Pre-   | Molar | Inter-  |Depth |
Locality                |of    | doidal |lamb-  | length| orbital |of    |
(or area)               |adults| breadth|doidal | (al-  | breadth |brain-|
                        |aver- |        |breadth|veolar)|         |case  |
                        |aged  |        |       |       |         |      |
------------------------+------+--------+-------+-------+---------+-------
                        |
                        |    _M. p. insperatus_, Wyoming
                        |
Crook and Campbell Cos. |  12  |  11.70 |  9.05 |  6.90 |   3.52  | 7.92 |
Sheridan County         |  20  |  12.20 |  9.33 |  6.90 |   3.55  | 7.95 |
Johnson County          |  20  |  12.04 |  9.02 |  7.14 |   3.52  | 7.84 |
Weston Co., mean        |  15  |  12.03 |  9.25 |  6.84 |   3.60  | 8.02 |
  "    "    stand. dev. |  ... |    .40 |   .37 |   .16 |    .13  |  .23 |
  "    "    minimum     |  ... |  11.4  |  8.7  |  6.6  |   3.4   | 7.7  |
  "    "    maximum     |  ... |  12.7  |  9.9  |  7.2  |   3.8   | 8.5  |
------------------------+------+--------+-------+-------+---------+-------
                        |
                        |         _M. p. pullatus_, Wyoming
                        |
Park County             |  10  |  12.34 |  9.52 |  6.72 |   3.50  | 8.30 |
Teton County            |  20  |  11.58 |  8.98 |  6.77 |   3.54  | 7.66 |
Sage, Lincoln Co.       |   6  |  11.87 |  9.08 |  6.77 |   3.58  | 7.88 |
Afton,  "     " mean    |  14  |  11.74 |  9.06 |  6.53 |   3.55  | 7.90 |
  "  , stand. dev.      |  ... |    .50 |   .15 |   .23 |    .12  |  .43 |
  "  , minimum          |  ... |  11.0  |  8.9  |  6.1  |   3.2   | 7.1  |
  "  , maximum          |  ... |  12.5  |  9.4  |  7.0  |   3.6   | 8.2  |
------------------------+------+--------+-------+-------+---------+-------
                        |
                        |          _M. p. uligocola_, Colorado
                        |
Park Co. (Denv)         |   7  |  12.20 |  9.40 |  7.07 |   3.60  | 7.91 |
Boulder Co. (Chi)       |  30  |  12.14 |  9.24 |  7.07 |   3.36  | 7.85 |
Denver                  |   8  |  12.23 |  9.35 |  7.27 |   3.55  | 7.99 |
Colorado Springs        |  16  |  12.18 |  9.22 |  7.20 |   3.43  | 7.89 |
Loveland, mean          |  16  |  12.31 |  9.46 |  7.14 |   3.59  | 8.14 |
   "    , stand. dev.   |  ... |    .65 |   .41 |   .32 |    .16  |  .42 |
   "    , minimum       |  ... |  11.4  |  9.0  |  6.8  |   3.3   | 7.5  |
   "    , maximum       |  ... |  13.8  | 10.6  |  7.6  |   3.8   | 9.4  |
------------------------+------+--------+-------+-------+---------+-------
                        |
                        |            _M. p. finitus_
                        |
Wray, Colorado          |   3  |  12.30 |  9.43 |  7.40 |   3.53  | 8.23 |
Dundy Co., Nebr., mean  |  12  |  12.64 |  9.39 |  7.52 |   3.66  | 8.22 |
 "    "  , stand. dev.  |  ... |    .61 |   .40 |   .31 |    .26  |  .43 |
 "    "  , minimum      |  ... |  11.6  |  8.5  |  7.0  |   3.2   | 7.6  |
 "    "  , maximum      |  ... |  13.5  |  9.8  |  7.9  |   3.9   | 8.7  |
------------------------+------+--------+-------+-------+---------+-------
                        |
                        |          _M. p. modestus_, Colorado
                        |
Alamosa, Colorado       |  3   |  11.97 |  9.27 |  6.83 |   3.60  | 8.01 |
Cochetopa Pass, Colo.   | 25[1]|  12.16 |  9.04 |  6.81 |   3.54  | 8.18 |
     "   , stand. dev.  |  ... |    .37 |   .31 |   .21 |    .14  |  .23 |
     "   , minimum      |  ... |  11.4  |  8.6  |  6.5  |   3.3   | 7.7  |
     "   , maximum      |  ... |  13.1  | 10.0  |  7.5  |   3.9   | 8.8  |
------------------------+------+--------+-------+-------+---------+-------

      [1] 29 specimens were used in taking measurements of the skull.

_Remarks._--Bailey (1900:20) had only 7 specimens from northeastern
Wyoming and western South Dakota, of _M. pennsylvanicus_ and thought
that _Arvicola insperatus_ Allen (1894:347) was not subspecifically
distinct from _modestus_. Subsequently Bailey (1920:72) had adequate
numbers of specimens and described _Microtus pennsylvanicus wahema_
from eastern Montana and western North Dakota. Anderson (1943:92)
concluded that _wahema_ was not distinct from _insperatus_ and
therefore the name _M. p. insperatus_ (Allen) is applicable to this
subspecies. On the basis of specimens that I have examined from
Montana, South Dakota, and Wyoming I concur with Anderson. Bailey's
characterization of _wahema_ is applicable to _insperatus_ as I
interpret it. In the Black Hills, _Microtus longicaudus longicaudus_
(Merriam) occurs together with _insperatus_.

    _Specimens examined._--Total 123. _Wyoming_: SHERIDAN CO.: 3 mi.
    WNW Monarch (=Kleeburn), 3800 ft., 4; 4 mi. NNE Banner, 4100 ft.,
    26; 5 mi. NE Clearmont, 3900 ft., 3. JOHNSON CO.: 5-1/2 mi. W, 1
    mi. S Buffalo, 5520 ft., 1; _5-1/2 mi. W, 1-1/2 mi. S Buffalo_, 1;
    _1 mi. W, 4/5 mi. S Buffalo_, 4800 ft., 36; 1/4 mi. E Klondike,
    5160 ft., 1. CAMPBELL CO.: Belle Fourche River, 45 mi. S, 13 mi. W
    Gillette, 5350 ft., 2. CROOK CO.: 3 mi. S, 2 mi. E Rocky Point,
    3800 ft., 6; Bear Lodge Mts., 6-1/2 mi. SSE Alva, 1 (Mich); _15 mi.
    N Sundance_, 5500 ft., 3; 15 mi. ENE Sundance, 3825 ft., 6; _3 mi.
    NW Sundance_, 5900 ft., 1; _1-1/3 mi. NW Sundance_, 5000 ft., 4;
    Sundance, 1 (USBS). WESTON CO.: 1-1/2 mi. E Buckhorn, 6150 ft., 26;
    Newcastle, 1 (USBS).


GENERAL REMARKS

The region considered in this paper differs in several regards from the
state of Pennsylvania, where variation in the skulls of this species
has been studied in detail by Snyder (1954) who referred all
populations there to a single subspecies. In some characteristics of
the skulls, populations within Pennsylvania differed as much or more
than the subspecies from Wyoming and Colorado. In other characteristics
of the skulls and of the skins differences are greater between
populations in Wyoming and Colorado. The region discussed here is
approximately five times as large as the state of Pennsylvania.
Populations of _M. pennsylvanicus_ are less continuously distributed
than in Pennsylvania owing to major physiographic and climatic barriers
and also owing to competition with one or more of the five other
species of _Microtus_ occurring in this region. The distribution of
three of these species has been discussed by Findley (1945:419). Large
areas of relatively greater aridity, such as the region occupied by the
subspecies _insperatus_, occur in Wyoming and Colorado. I have pointed
out that the populations which I have designated as subspecies are not
absolutely uniform. Also the different subspecies are not of exactly
equal degrees of difference. However, there is considerable uniformity
of populations occupying conveniently mapped geographic areas. In my
opinion, the use of subspecific nomenclature is justified in this case,
although not completely unambiguous.


LITERATURE CITED

ALLEN, J. A.

    1894. Descriptions of five new North American mammals. Bull. Amer.
    Mus. Nat. Hist., 6:347-350, December 7.

ANDERSON, R. M.

    1943. A prior name for the bean mouse revived. Canadian Field-Nat.,
    57:92, October 17.

ANDERSON, S.

    1954. Subspeciation in the montane meadow mouse, Microtus montanus,
    in Wyoming and Colorado. Univ. Kansas Publ., Mus. Nat. Hist.,
    7(7):489-506, 2 figs. in text, July 23.

BAILEY, V.

    1900. Revision of American voles of the genus Microtus. N. Amer.
    Fauna, 17:1-88, 5 pls., 17 figs., June 6.

    1920. Identity of the bean mouse of Lewis and Clark. Jour. Mamm.
    1:70-72, March 1.

    1927. A biological survey of North Dakota. N. Amer. Fauna, 49:vi+
    226, 21 pls., 8 figs. in text, January 8.

    1932. Mammals of New Mexico. N. Amer. Fauna, 53:1-412, 22 pls., 57
    figs. in text, March 1.

BAIRD, S. F.

    1858. Explorations and surveys for a railroad route from the
    Mississippi River to the Pacific Ocean. War Department. Mammals,
    Part I, xxxii + 757, pls. 17-60, 35 figs. in text, July 14.

DALQUEST, W. W.

    1948. Mammals of Washington. Univ. Kansas Publ., Mus. Nat. Hist.,
    2:1-444, 140 figs. in text, April 9.

DAVIS, W. B.

    1939. The Recent mammals of Idaho. The Caxton Printers, Caldwell,
    Idaho, 400 pp., 2 full page half tones, 33 figs. in text, April 5.

DURRANT, S. D.

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

ELLERMAN, J. R.

    1941. The families and genera of living rodents. Volume II. Family
    Muridae. The British Museum. xii + 690 pp., 50 figs. in text, March
    21.

FINDLEY, J. S.

    1954. Competition as a possible limiting factor in the distribution
    of _Microtus_. Ecology, 35:418-420, July.

HALL, E. R., and E. L. COCKRUM

    1952. Comments on the taxonomy and geographic distribution of North
    American microtines. Univ. Kansas Publ., Mus. Nat. Hist.,
    5:293-312, November 17.

    1953. A synopsis of the North American microtine rodents. Univ.
    Kansas Publ., Mus. Nat. Hist., 5:373-498, 149 figs. in text,
    January 15.

HIBBARD, C. W.

    1940. A new Pleistocene fauna from Meade County, Kansas. Trans.
    Kansas Acad. Sci., 43:417-425, December 23.

HOWELL, A. B.

    1926. Anatomy of the wood rat. Monogr. Amer. Soc. Mamm. No. 1, x +
    225 pp., 35 figs., Williams and Wilkins Co., Baltimore.

SNYDER, D. P.

    1954. Skull variation in the meadow vole (_Microtus p. pennsylvanicus_)
    in Pennsylvania. Annals of the Carnegie Museum, 33:201-234,
    September 21.

WARREN, E. R.

    1942. The mammals of Colorado. Univ. Oklahoma Press, xviii + 330
    pp., 50 pls.


_Transmitted June 30, 1955._