University of Kansas Publications

Museum of Natural History

Volume 9, No. 12, pp 363-384, 7 figs, in text, 1 table

February 21, 1958

Geographic Variation

in the Pocket Gopher, Thomomys bottae,

in Colorado

BY

PHILLIP M. YOUNGMAN

University of Kansas

Lawrence

1958

University of Kansas Publications,
Museum of Natural History

Editors: E. Raymond Hall, Chairman, Henry S. Fitch,

Robert W. Wilson

Volume 9, No. 12, pp. 363-384, 7 figs. in text, 1 table

Published February 21, 1958

University of Kansas

Lawrence, Kansas

PRINTED IN

THE STATE PRINTING PLANT

TOPEKA, KANSAS

1958

27-1765

Geographic Variation
in the Pocket Gopher, Thomomys bottae,
in Colorado

BY

PHILLIP M. YOUNGMAN

INTRODUCTION

Two species of pocket gophers of the genus Thomomys (Family Geomyidae)
occur in Colorado, Thomomys bottae (see fig. 1) in the low valleys in
the south-central and southwestern parts of the state and Thomomys
talpoides mainly in the mountains and high valleys.

Thomomys bottae occurs primarily in the Piñon-juniper,
Ponderosa Pine, and Short Grass zones of Daubenmire (1943) but in some
localities is found in the Douglas Fir Zone. Thomomys talpoides
occupies primarily the Douglas Fir Zone and Engelmann Spruce-Subalpine
Spruce Zone but is found also in the Piñon-juniper and Short
Grass zones in some localities.

The ranges of the two species do not overlap in the strict sense but
interdigitate in a parapatric type of distribution.

Two other pocket gophers, Geomys bursarius and Cratogeomys
castanops, also occur in Colorado—in the Upper Sonoran Life-Zone.
Geomys bursarius occupies much of the Great Plains, whereas
Cratogeomys castanops is found only on the plains in the southeastern
part of the state.

The objectives of the study, reported on here, were to learn the
geographic distribution of Thomomys bottae in Colorado, to find means
for recognizing the different subspecies, and to describe individual and
geographic variation.

I am indebted to Mr. Sydney Anderson and Professor E. Raymond Hall for
many helpful suggestions and for their critical reading of the
manuscript, to Dr. Richard S. Miller, who made the collection of many of
the specimens possible, and to Dr. Richard M. Hansen for numerous
suggestions. I wish to express my appreciation also to the following for
the loan of specimens in their care: Alfred M. Bailey and A. A. Rogers,
Colorado Museum of Natural History, Denver, Colorado; David H. Johnson,
United States National Museum, Washington, D. C; Robert W. Lechleitner,
Colorado State University, Fort Collins, Colorado; and Robert Z. Brown,
Colorado College, Colorado Springs, Colorado.

METHODS

Adults of approximately equal age were compared in the study of
geographic variation. Three criteria of adulthood are: (a) suture
obliterated between supraoccipital and exoccipital, (b) suture at least
partly obliterated between basisphenoid and basioccipital, (c)
supraorbital crests not widely separated and almost parallel. In males
the crests encroach on the lateral borders of the interparietal; in
females the crests approach the lateral borders of the interparietal but
are more widely separated than in males.

In studying geographic variation, greater emphasis was placed on females
than on males. As noted by Grinnell (1931:4), males vary more than females,
especially in length of rostrum and associated nasal measurements.

Color terms are those of Munsell (1954). Color measurements were standardized
by the use of a single 100 watt General Electric blue daylight bulb in
a 12 inch white reflector suspended 24 inches above the specimen. All other
light was excluded. The individual hairs of Thomomys bottae are either bicolored
or tricolored. The darkness of a specimen often may be attributed to
the presence of dark-tipped hairs. The color given in the description is the
basic reddish or yellowish color of the hairs. The presence of a grizzled effect
or a dark dorsal stripe, or any other pattern resulting from dark hairs, is noted
in the remarks.

[pg 367]
Specimens examined are listed by counties in the following order:

• Mesa


• Montrose


• San Miguel


• Dolores


• Montezuma


• La Plata


• Archuleta


• Conejos


• Chaffee


• Fremont


• El Paso


• Pueblo


• Custer


• Huerfano


• Alamosa


• Las Animas

Localities are listed from north to south within a county. If two localities
lie on the same line of latitude, the western precedes the eastern. Localities
omitted on the map in order to prevent overlapping of symbols are in Italics.
Unless otherwise indicated, specimens are in the University of Kansas, Museum
of Natural History. The following initials are used to designate specimens in
other collections:

The following measurements of the skull are listed in the tables:

Condylobasal length.—The shortest distance between the anteriormost projections
of the premaxillaries and a line touching the posterior surfaces of the
exoccipital condyles.

Length of nasals.—The distance from the most anterior projection of the
nasal bones to the most posterior projection of a nasal bone.

Zygomatic breadth.—The greatest distance across the zygomatic arches, at
right angles to the long axis of the skull.

Squamosal breadth.—The greatest distance between the mastoidal processes
of the squamosal.

Length of rostrum.—The shortest distance from the shallow notch that lies
lateral to the hamulus of the lacrymal bone, to the tip of the nasal on the same
side of the skull.

Breadth of rostrum.—The greatest width of the rostrum, anterior to the
maxillae, transverse to the long axis of the skull.

Alveolar length of upper maxillary tooth-row.—Distance between the anterior
margin of the alveolus of the first cheek-tooth and the posterior margin
of the alveolus of the last upper cheek-tooth, on one side of the skull.

Least interorbital breadth.—The least distance across the frontal bones at the
interorbital constriction as seen in dorsal view.

PHYSIOGRAPHY

Thomomys bottae occurs in the Colorado Plateau Province (terminology
of Fenneman, 1931), the Southern Rocky Mountain Province
and a small part of the Great Plains Province.

The Colorado Plateau Province, in the southwestern part of the
state, is mostly above 5000 feet and is characterized by the great
number of canyons cut by rivers and streams in the nearly horizontal
strata. Prominent features of the landscape are cuestas, such as
Mesa Verde, and laccoliths, such as Ute Peak.

The Southern Rocky Mountain Province consists mainly of high
granitic mountains running north and south, many of which extend
[pg 368]
to more than 14,000 feet above sea level. Included in this region
are several large basins, such as North Park and South Park and the
San Luis Valley. The San Juan Mountains, which separate the
Colorado Plateau Province from the San Luis Valley, and the Sangre
De Cristo and Wet mountains, which intervene between the San
Luis Valley and the Great Plains, importantly influence the distribution
of Thomomys bottae.

The Great Plains Province is a broad highland that slopes gradually
eastward from the Rocky Mountains. Of importance to the
present study are two subdivisions of the Great Plains, the Colorado
Piedmont and the Raton Section.

The Colorado Piedmont is a much dissected fluviatile plain,
roughly extending from the vicinity of the Arkansas River to the
northern boundary of the state. In general the topography of the
Colorado Piedmont is broadly rolling with greater relief than the
high plains to the east; however, buttes and steep bluffs occur locally.

The Raton Section imperceptibly blends into the southern boundary
of the Colorado Piedmont and extends south into New Mexico
and Texas. A trenched peniplane of greater relief and altitude
than the Colorado Piedmont, it is characterized by high mesas, extensive
dissected lava-capped plateaus, deep canyons, and mountains
of volcanic origin.

GEOGRAPHIC VARIATION

Six subspecies of Thomomys bottae occur in Colorado. T. b.
aureus and T. b. howelli occupy the Colorado Plateau Province (see
fig. 1) and are characterized by a yellowish color; nasals posteriorly
truncate or rounded; posterior extensions of premaxillae long; basioccipital
wide; and interpterygoid space U-shaped with a median
spicule.

T. b. internatus, T. b. cultellus, and a new subspecies from the
vicinity of Cañon City described on page 376, inhabit the Sangre De
Cristo and Wet mountains in the Southern Rocky Mountain Province
and adjacent parts of the Colorado Piedmont and Raton Section
of the Great Plains Province (see fig. 1). This group of closely
related subspecies is characterized by reddish color; posterior margins
of nasals forming a V; posterior extensions of premaxillae short;
basioccipital narrow; and interpterygoid space V-shaped, lacking
a median spicule.

T. b. pervagus occupies part of the San Luis Valley to the west
of the Rio Grande (see fig. 1). In Colorado T. b. pervagus is iso
[pg 369]lated
from T. b. internatus and T. b. cultellus by the Sangre De
Cristo and Culebra ranges and is separated from T. b. aureus by the
San Juan Mountains. T. b. pervagus occupies an area geographically
intermediate between T. b. aureus to the west and T. b. internatus
and T. b. cultellus to the east and has some characters in common
with these subspecies. T. b. pervagus resembles T. b. aureus
in having long posterior extensions of the premaxillae and in sometimes
having rounded posterior margins of the nasals. T. b. pervagus
resembles T. b. internatus and T. b. cultellus in color, the
presence of a V-shaped interpterygoid space, and a narrow basioccipital.
Kelson (1951:69) has pointed out that in New Mexico the
separation of the ranges of T. b. pervagus and T. b. cultellus is
probably complete, but probably incomplete between T. b. pervagus
and T. b. aureus. Nevertheless, the similarities between T. b. pervagus
and T. b. cultellus and T. b. internatus suggest that T. b.
pervagus was originally derived from the more eastern stock.

T. b. aureus is a variable subspecies which, according to Durrant
(1952:211), intergrades with T. b. howelli in Utah. Specimens of
T. b. aureus showing the greatest amount of geographic variation
cranially are from the ecotone between the Piñon-juniper and Douglas
Fir zones at the edge of the range of the subspecies.

T. b. howelli is a markedly distinct subspecies that shows certain
similarities to T. b. aureus, but the degree of cranial difference from
T. b. aureus suggests an isolation of long duration, or a rapid evolution
from the parent stock.

T. b. internatus and T. b. cultellus probably intergrade east of the
Sangre De Cristo Range in the vicinity of the Colorado-New Mexico
boundary. The amount of intergradation is obscured by the great
amount of geographic variation occurring in T. bottae at the edge of
the plains and by the lack of specimens from this area.

T. b. internatus is a widespread subspecies showing its greatest
variation at the edge of the plains. This area is an ecotone between
the coniferous forest and the grassland and is by nature an area of
change owing to the alternation of wet and dry periods such as the
pluvial, interpluvial, and postpluvial periods. This seems to support
Durrant's observation (1952:496) that "the greatest range of
morphological variation is in animals from the least stable environments."

Specimens from a small area north of the Arkansas River in the
vicinity of Cañon City (see fig. 1) differ sufficiently from T. b. internatus
to be given nominal recognition. High mountains and the
Arkansas River isolate the new subspecies found at Cañon City from
[pg 370]
populations of T. b. internatus to the west and south; however there
are no apparent geographic barriers between the newly named subspecies
and populations of T. b. internatus twelve miles to the
north or from the vicinity of Pueblo to the east. This new subspecies
is the most extreme of the variants occurring in the unstable
environment at the edge of the plains.

Thomomys bottae aureus Allen

Remarks.—T. b. aureus is a variable subspecies, which differs considerably
from T. b. internatus, T. b. cultellus, and T. b. rubidus
and includes several microgeographic races distinguishable to a
taxonomist specializing in the group. These slightly varying populations
are here not considered sufficiently distinct for nominal
recognition.

Characters such as color of the pelage and conformation of the
bullae and zygomatic arches vary with the locality, and to some
extent vary among specimens from a single locality.

The name Thomomys bottae optabilis, given to specimens from
[pg 371]
Coventry by Goldman (1936:116), is here placed in synonymy
under T. b. aureus Allen. The characters originally used to describe
T. b. optabilis are of the type that vary between populations only
a few miles apart, or often vary within a population. The skulls of
specimens from Coventry are not lighter in structure than those of
T. b. aureus. The premaxillae are not narrower, nor is the frontal
region narrower or more constricted than in T. b. aureus.

The name Thomomys bottae apache, given to specimens from
Lake La Jara, New Mexico, by Bailey (1910:79), and later applied
to specimens from Colorado by Bailey (1915:75), is here also placed
in synonymy under T. b. aureus. Specimens from Lake La Jara,
New Mexico, and nearby localities in Colorado may be separated
from topotypes of T. b. aureus on the basis of color only. The topotypes
of T. b. aureus are mostly pale; some, however, are dark. The
number of pale specimens in any given series decreases gradually
in a clinal pattern from west to east. Since there is no noticeable
step in the cline and since all specimens show close cranial similarity,
it is felt that nominal recognition of the darker specimens
does not present a realistic picture of the relationships of the relatively
unisolated populations in the Colorado Plateau Province.

Since Thomomys bottae in the Colorado Plateau Province is especially
plastic, varying from locality to locality, emphasis is here
placed on similarities that unite specimens from different localities.
The individual and microgeographic variations are outlined below.

Specimens from Bedrock have zygomatic arches that are heavy
anteriorly. Specimens from Coventry are dorsally almost uniformly
Strong Brown (7.5YR 5/6) and lack a strong dorsal stripe. The
venters are Reddish Yellow (7.5YR 8/6). Specimens from 15 miles
west of Cortez are the palest specimens of T. b. aureus from Colorado,
and closely resemble topotypes. The basic color varies from
Reddish Yellow (7.5YR 7/6 and 6/6) to Strong Brown (7.5YR
5/6). Specimens are marked with a narrow dark dorsal stripe.
The venters are white. Specimens from Ute Peak and Cortez have
Reddish Yellow (7.5YR 6/6) flanks and are slightly darker dorsally.
Many specimens from Mesa Verde are indistinguishable from specimens
from Coventry and from Cortez. Others have dark diffuse
dorsal stripes. The venters are Pink (7.5YR 7/4) or Pinkish White
(7.5YR 8/2). Some specimens from the Mancos River have wide
dorsal stripes. Specimens from three miles west of Durango have
especially wide-spreading zygomatic arches posteriorly and have
wide black dorsal stripes. The venters are Pink (7.5YR 7/4). One
specimen from Florida is dark and grizzled and has a dark dorsal
[pg 372]
stripe. Another specimen is pale and has only a small dorsal stripe.
Specimens from 12 miles west of Pagosa Springs have thin rostra
and diffuse dorsal stripes. Specimens from Bondad have a V-shaped
interpterygoid space and in it a small median spicule. One specimen
is uniformly grizzled and lacks a dorsal stripe, giving an overall
effect of Dark Yellowish Brown (10YR 3/3). Another specimen
has Strong Brown (7.5YR 5/6) flanks and is only slightly darker
dorsally.

Thomomys bottae howelli Goldman

Remarks.—T. b. howelli most closely resembles T. b. aureus; however,
since only one adult specimen of T. b. howelli is known, it is
impossible to appraise adequately its characters. Durrant (1952:211)
records intergradation between T. b. howelli and T. b. osgoodi,
and between T. b. howelli and T. b. aureus in Utah.

[pg 373]
An attempt to collect specimens of T. b. howelli, in March, 1957,
by Richard S. Miller and the writer was unsuccessful.

Thomomys bottae pervagus Merriam

Remarks.—T. b. pervagus is a well-defined subspecies. There is
little variation between the topotypes and specimens from Colorado.

Thomomys bottae internatus Goldman

Remarks.—The dividing line between T. b. internatus and T. b.
cultellus is drawn arbitrarily since only one specimen has been collected
between La Veta Pass and the border of New Mexico.

When Goldman (1936:115) named T. b. internatus he included
specimens from Union and Colfax counties, New Mexico, and specimens
from Gardner, Colorado (not Garfield as stated by Kelson,
1951:66). The specimens from New Mexico and a specimen from
Fishers Peak, Colorado, were subsequently assigned to T. b. cultellus
by Kelson (loc. cit.).

The specimen from Fishers Peak shows some characters that
might be interpreted as intermediate between internatus and cultellus,
but shows also some unique characters that can be understood
only by further collecting in the regions north and northeast of the
type locality of T. b. cultellus.

Variation is slight in the large series of topotypes of T. b. internatus.
Specimens from other localities in the western part of the
range differ little from the topotypes. Specimens from one mile
west of Coaldale have slightly more inflated bullae that are more
flattened ventrally. Specimens from five miles south of Cotopaxi
also have the bullae more flattened ventrally.

Specimens from localities bordering the plains differ from the
topotypes and near topotypes, and in general show greater variation
from locality to locality. Specimens from 12 miles north of
Cañon City are dark, resembling T. b. rubidus, but cranially agree
with specimens from near Colorado Springs in being indistinguishable
from specimens from Salida. Specimens from St. Charles Mesa
and Bear Creek near Walsenburg differ from the topotypes in having
wider rostra. The specimens from St. Charles Mesa have more
inflated bullae.

Thomomys bottae cultellus Kelson

Remarks.—Kelson (1951:64) named T. b. cultellus on the basis
of six dark specimens (Dark Reddish Brown 5YR 3/4 and 2/2).
Nowhere else within the range of this subspecies, as defined by
Kelson, do any specimens resemble the topotypes in color.

After comparing topotypes of T. b. cultellus with topotypes of
T. b. internatus of approximately equal age, I disagree with Kelson
(loc. cit.) on some of the characters which he used to separate
cultellus from internatus. My findings indicate that T. b. cultellus
is not smaller, that its skull is not smaller and not less angular, and
that the tympanic bullae are not less pointed ventrally. Further
collecting is needed better to limit and diagnose this subspecies.

Figs. 2-7.
Dorsal views of skulls of
Thomomys bottae. × 1.

Fig. 2.
Thomomys b. aureus, 3 mi. W Durango, La Plata Co.,
Colorado. No. 72967, .

Fig. 3.
Thomomys b. howelli, holotype, Grand Junction, 4600 ft.,
Mesa Co., Colorado. No. 75684 USNM, .

Fig. 4.
Thomomys b. pervagus, Espanola, 5000 ft., Rio Arriba Co.,
New Mexico. No. 133614 USNM, .

Fig. 5.
Thomomys b. cultellus, Fishers Peak, 8000 ft., Las Animas
Co., Colorado. No. 129285 USNM, .

Fig. 6.
Thomomys b. internatus, Salida, 7050 ft., Chaffee Co.,
Colorado. No. 2757 ERW, .

Fig. 7.
Thomomys b. rubidus, holotype, 2-9/10 mi. E Cañon City,
Fremont Co., Colorado. No. 72954, .

Thomomys bottae rubidus new subspecies

Remarks.—The range of T. b. rubidus is surrounded by the range
of T. b. internatus; nevertheless, intergradation has not been found.
For a discussion of the geographic relation of T. b. rubidus to T. b.
internatus see page 374.

SUMMARY

A study of 249 specimens of Thomomys bottae from Colorado reveals
six subspecies in the state. T. b. aureus and T. b. howelli
occupy the Colorado Plateau Region in the western and southwestern
parts of the state. T. b. internatus, T. b. cultellus, T. b. pervagus,
and the newly named T. b. rubidus occupy part of the Southern
Rocky Mountain Region and a narrow strip of the Great Plains.

The greatest amount of geographic variation, in Thomomys bottae
in Colorado, occurs in the ecotone between the grassland and coniferous
forest at the edge of the Great Plains, and in the ecotone
between the Piñon, juniper, and sage of the Colorado Plateau and
the Coniferous forest of the southern Rocky mountains.

Table 1. Measurements,
in Millimeters, of Thomomys bottae

Unless otherwise noted, specimens are adults
from Colorado

LITERATURE CITED

Bailey, V.

Daubenmire, R. F.

Durrant, S. D.

Fenneman, N. M.

Goldman, E. A.

Grinnell, J.

Kelson, K. R.

Munsell, A. H.

Transmitted November 14, 1957.

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