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

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.

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.