Thus, the subspecies of E. minimus are morphologically differentiated at two distinct levels; one level is based on differences in the skull and baculum, while the other is based on differences in color.
Although there is considerable controversy concerning the glacial chronology in the mountains of western North America (Flint 1947:302-303), it is generally agreed that in Wyoming, in Wisconsinan time (the latest glacial age), glaciers covered a large part of the Yellowstone-Teton-Wind River highlands, the Big Horn Mountains, the southern part of the Laramie Range, the Medicine Bow Range, Sierra Madre Range, and the northern foothills of the Uinta Mountains. With this in mind, a possible explanation of the geographic variation in E. minimus of Wyoming, is here attempted.
In Sangamonian time, E. minimus-like chipmunks occurred over most of the region which is now Wyoming, and were divided into two size-groups, much as E. minimus is today.
When permanent snow fields were formed in Wisconsinan time, these chipmunks were restricted in their ranges, not, of course, occurring on the glaciers.
When the glaciers melted at the end of Wisconsinan time, new habitats were thus "uncovered." The chipmunks which moved into these ice-free areas, then, became adapted to the new habitats. This then accounts for the subspeciation of E. m. consobrinus, E. m. confinis, and E. m. operarius.
The Black Hills were not covered by glaciers. In late Pleistocene time these hills were probably of low relief. Subsequent differential erosion produced relief sufficient to provide a different habitat. The chipmunks that continued to occupy this area adapted themselves in color to the new habitat and became E. m. silvaticus.
Cary, M.
| 1917. | Life zone investigations in Wyoming. N. Amer. Fauna, 42:1-96, 15 pls., 17 figs. |
Flint, R. F.