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.

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.