Geographic Variation

in Red-backed Mice (Genus Clethrionomys)

of the Southern Rocky Mountain Region

BY

E. LENDELL COCKRUM and KENNETH L. FITCH

University of Kansas Publications

Museum of Natural History

Volume 5, No. 22, pp. 281-292, 1 figure in text

November 15, 1952

UNIVERSITY OF KANSAS

LAWRENCE

1952

University of Kansas Publications, Museum of Natural History

Editors: E. Raymond Hall, Chairman, A. Byron Leonard,

Edward H. Taylor, Robert W. Wilson

Volume 5, No. 22, pp. 281-292, 1 figure in text

November 15, 1952

 

 

 

University of Kansas

Lawrence, Kansas

 

 

 

PRINTED BY

FERD VOILAND, JR., STATE PRINTER

TOPEKA, KANSAS

1952



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 [283]

In the course of the preparation of a synopsis of the North
American terrestrial microtines by one of us (Cockrum), and the
completion of a Master's thesis on the geographical variation of
the red-backed mice of Wyoming by the other (Fitch) we had
occasion to study the red-backed mice of the southern Rocky
Mountain region (see figure 1). Results of these studies are the
recognition of two heretofore unnamed subspecies of the red-backed
mouse in the southern Rocky Mountain region, and a clarification
of the taxonomic status of two additional kinds.

Remarks.--C. g. galei, with the largest geographic range of any
of the Rocky Mountain subspecies, is also the most variable. Three
principal areas of geographic variation were found. These areas
are: The mountains of north-central Colorado and southern Wyoming
(this area includes the type locality); the Big Horn area
probably northwest into Montana (no adult specimens from Montana
or Alberta examined); and the Teton area which includes the
mountains east and southeast of Yellowstone National Park. Specimens
from these areas have noticeable differences in pelage, but
no constant cranial differentiation could be detected. Specimens
from the Medicine Bow Mountains of southern Wyoming have a
more reddish dorsal stripe, and more buff and less gray on the
sides than either of the northern geographic variants. The dorsal
stripe continues farther anteriorly and is better defined through its
entire length. There are fewer differences between the two northern[284]
geographic variants than between either one of them and the
southern variant. Specimens from the Teton Mountains, however,
have grayer sides, and the outer margin of the ear is tipped with
chestnut (little or no chestnut shows on the ears of the specimens
from the Big Horn Mountains); the dorsal stripe is less distinct
(with slightly more gray throughout) than in either of the other
geographic variants of the one subspecies.

Three specimens (two adults) are available from the Little Medicine
Range in Converse County (22 miles south and 24.5 miles
west of Douglas, 7600 feet), Wyoming. Although red-backed mice
probably are found in the mountains of Natrona and Albany counties,
the population, in the Little Medicine Range is somewhat isolated.
In coloration these mice are lighter than any of the three
geographic variants described above; the dorsal stripe is narrower;
the sides are more buffy; the dorsal stripe does not project anteriorly
beyond the ears as it does in the specimens from the Medicine Bow
Mountains; and the face is grayer. These specimens resemble the
population in the Big Horn Mountains to the north more than the
population in the Medicine Bow Mountains.


The specimens from the Little Medicine Range, the Big Horn
Range, and the Tetons are possibly subspecifically distinct from
the southern specimens. Examination of specimens now allocated
to galei from Montana and Alberta should aid in revealing whether
the northern animals are an unnamed subspecies.

Click on map to see larger sized image.

Fig. 1.--Geographic ranges of the subspecies of Clethrionomys gapperi in the southern Rocky Mountains.
1. C. g. galei	3. C. g. uintaensis	5. C. g. limitis
2. C. g. brevicaudus	4. C. g. gauti	6. C. g. arizonensis

Remarks.--Merriam (N. Amer. Fauna, 5:119, July 30, 1891)
named this subspecies on the basis of two specimens collected in
the Black Hills of South Dakota in July, 1888, and assigned it to
the species Evotomys [= Clethrionomys] gapperi. He reported the
diagnostic characteristics as: "Similar to E. gapperi, but with larger
ears and shorter tail. The hazel of the dorsal area is not so bright
as in gapperi; the sides are the same golden brown." Of the cranial
and dental characteristics he wrote: "Much as in E. gapperi."

Bailey (Proc. Biol. Soc. Washington, 11:129, May 13, 1897), in
his "Revision of the American voles of the genus Evotomys," with
one additional specimen available, raised the Black Hills population
to specific status, re-emphasizing the shortness of the tail, and
pointing out a few slight cranial differences ("zygomatic arches
low and flaring out, so that the inner instead of the outer side shows
in top view; auditory bullae as large as in gapperi, but less
rounded").

Bailey (loc. cit.) remarked that: "though based on so scanty
material, the characters distinguishing the species are fairly pronounced.
Its range is isolated and widely separated from that of
any other members of the genus by open prairie country and a wide
belt of the Transition zone. There seems to be no valid reason for
considering it a subspecies."

Additional specimens have been taken in recent years from the
Black Hills of South Dakota and northeastern Wyoming. This
material has shed light on the relationships and morphological
characteristics of the red-backed mice of this region. Bole and
Moulthrop (Sci. Publ. Cleveland Mus. Nat. Hist., 5:153, September
11, 1942) listed, as comparative material, eight specimens from
Bull Springs, Custer County, South Dakota, under the name Clethrionomys
gapperi brevicaudus (Merriam). They gave no reason
for arranging brevicaudus as a subspecies of C. gapperi.

Twenty adults (11 skins and skulls, 9 skulls only) from Pennington
County, South Dakota (specimens in the University of Michigan
[287]
Museum of Zoology), have the following measurements (averages
of external measurements based on 11 specimens only): Total
length, 142 (123-155); tail, 35 (30-39); hind foot, 19.5 (18.6-21.0);
basal length, 23.3 (21.7-24.5); condylobasilar length, 23.3 (21.9-24.5);
zygomatic breadth, 13.7 (12.9-14.7); lambdoidal breadth,
11.7 (11.3-12.9); alveolar length upper cheek-teeth, 5.5 (5.2-5.8);
interorbital breadth, 3.9 (3.6-4.1); length of nasals, 7.7 (7.1-8.5);
breadth of rostrum, 3.2 (2.9-3.6); and length of incisive foramina,
5.0 (4.6-5.3).

Measurements of the type and one "more fully adult topotype"
(as given by Bailey, op. cit.) are: Total length, 125, 130; tail length,
31, 32; hind foot, 19, 19; basal length, 21.2, 21.8; length of nasals,
6.6, 7.0; zygomatic breadth, 12.5, 12.8; mastoid breadth, 11.3, 11.0;
alveolar length of upper molar series, 5.4, 5.3. In every measurement
the figures for Bailey's specimens are smaller than the average
of the same measurement in the 20 adults from Pennington County,
and, in most measurements, are even lower than the minimum of
the latter series. Therefore, we conclude that the material available
to Merriam (op. cit.) and Bailey (op. cit.) consisted of only subadults.

In comparison with a series of 23 adult Clethrionomys gapperi
galei from 28 mi. E Lovell, Big Horn County, Wyoming, C. g. brevicaudus
has a slightly shorter tail, longer hind foot, greater basal
and condylobasilar lengths, greater zygomatic and lambdoidal
breadths and conspicuously longer nasals.

In comparison with three adult C. g. loringi from Elk River, Sherburne
County, Minnesota, C. g. brevicaudus has a greater total
length, longer hind foot, greater basal length, conspicuously greater
zygomatic and lambdoidal breadths, much longer nasals, and a narrower
rostrum.

Clethrionomys gapperi brevicaudus, although isolated geographically
and although morphologically more distinct than many of the
currently recognized subspecies of C. gapperi, is probably best arranged
as a subspecies of C. gapperi rather than as a full species.
In certain characters, such as interorbital breadth and breadth of
rostrum, it is intermediate between C. g. galei and C. g. loringi, but
it resembles C. g. galei more than it does any other named kind.

Remarks.--From the description given by Doutt (Proc. Biol.
Soc. Washington, 54:161, 1941) in his original description of this
subspecies, it appears that he had available for comparisons, only
subadult specimens of C. g. galei. As judged from the material of
C. g. uintaensis available to us (1 topotype, KU 38081, and 7 specimens
from Uinta County, Wyoming, listed below) and from Doutt's
(op. cit.) description and measurements, the subspecies C. g. uintaensis
is but weakly differentiated from C. g. galei. No marked
cranial differences are evident between the two subspecies; the
differences in pelage noted by Doutt (op. cit.:161), however ("Similar
to Clethrionomys gapperi galei from Ward, Colorado, but head
and cheeks grayer; sides and back paler; belly whiter."), do seem
to be valid.

On the basis of these differences in pelage and the geographic
isolation of the range, we judge that uintaensis should be retained
as a subspecies of C. gapperi. It is clear, however, that C. g. uintaensis
is less distinct from C. g. galei than are the other adjacent
subspecies.

Remarks.--Two specimens from a locality 21 mi. W and 3 mi. N
Saguache, Saguache County, Colorado, although referred to this
subspecies on the basis of paler pelage, inflation of auditory bullae,
and heavier teeth, show characters of C. g. galei in the narrowness
across the zygomata and lambdoidal crest. Four specimens from
Silverton (1 adult and 3 young adults) are referable to this subspecies
on the basis of color of pelage and cranial proportions but
are smaller than either C. g. gauti or C. g. galei.

The specimen from Pecos Baldy, Pecos Mountain, San Miguel
County, New Mexico, referred by Bailey (N. Amer. Fauna, 52:192)
to Clethrionomys gapperi galei, is here referred to C. g. gauti on
geographical grounds.

The name gauti is proposed in honor of the collector of the
type specimen, James H. Gaut.

Remarks.--Bailey (Proc. Biol. Soc. Washington, 26:133, May 21,
1913) described this animal as a species and gave as general characteristics:
"Size slightly larger than E. [= Clethrionomys gapperi]
galei; colors duller, grayer and less buffy; skull and dentition
heavier." He further characterized the skull as: "Larger, heavier
and conspicuously more ridged than in galei; bullae large and especially
deep; dentition heavy throughout." The type of C. limitis,
as judged from the measurements given by Bailey (loc. cit.), is an
exceptionally old male.

Our comparison of six adult topotypes with a series of C. g. galei
from Wyoming (18 adults from 3 mi. SSE Browns Peak, 10,000
ft., Albany County, in Univ. Kansas Mus. Nat. Hist.) and with
three near-topotypes of C. g. galei (3 mi. S Ward, 9000 ft., Boulder
County, Colorado, in Univ. Kansas Mus. Nat. Hist.) revealed that
most of the differences noted by Bailey (loc. cit.) are not evident
when individuals of comparable ages are examined. Some specimens
of C. g. galei exceed C. limitis in ridging of the skull and size
of the teeth although conspicuous ridges and large teeth are supposedly
distinctive of C. limitis. The bullae, although averaging
larger in C. limitis, can be matched in size by those of specimens
of C. g. galei from Wyoming.

The differences evident between C. limitis and C. g. galei are of
the kind and degree that serve to separate subspecies in the species
Clethrionomys gapperi and, although actual evidence of intergradation
is lacking, we think that the relationships of limitis are better
expressed by arranging it as a subspecies of C. gapperi than by
retaining it as a full species.

Remarks.--Hall and Davis (Proc. Biol. Soc. Washington, 47:55,
February 9, 1934) reported 12 specimens of red-backed mice from
Hannagan Meadow, 9500 to 9600 ft., and ten from Hannagan
Creek, 8600 ft., all in Greenlee County, Arizona. Although they
pointed out most of the cranial differences here described as diagnostic
of C. g. arizonensis, they did not name the animals as new
since they had no seasonally comparable materials; thus they were
unable to evaluate the differences noted in pelage. We have not
examined the material referred to by Hall and Davis (loc. cit.),
but, on the basis of their description, here refer it to C. g. arizonensis.

Grateful acknowledgment is made of the opportunity to study the specimens
from New Mexico and Arizona in the Biological Surveys Collection of
the United States National Museum and the material from South Dakota in
the University of Michigan Museum of Zoology, as well as for the financial
support afforded one of us (Cockrum) by the University of Kansas from its
Research appropriation. Cockrum's work was part of a larger investigation of
the geographic distribution of all North American native mammals, aided by
a contract between the Office of Naval Research, Department of the Navy,
and the University of Kansas (NR. 161-791). Also, assistance with some of
field work was given by the Kansas University Endowment Association.

Transmitted June 21, 1952.

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