S Wood's Landing, 1.

Utah: Uintah Co.: PR Springs, 7,950 ft., 43 mi. S Ouray, Uintah-Grand county line, 1 UU.

Colorado: Jackson Co.: Mount Zirkel, 10,000 ft., on trail, 2 WC; Buffalo Pass, 10,380 ft., 1 WC; Buffalo Pass road, 10,130 ft., 1 WC. Larimer Co.: 2 mi. E Log Cabin, 7,450 ft., 1 WC; Estes Park, 7,600 ft., 1; 1½ mi. SW Estes Park, 1; 2½ mi. SW Estes Park, 2; 3½ mi. SW Estes Park, 1; 12 mi. SW Estes Park, 1. Rio Blanco Co.: 1 mi. NW Pagoda Peak, 10,400 ft., 1. Boulder Co.: Long's Peak, 7 BS; 1 mi. NE Ward, 10,000 ft., 1; 3 mi. S Ward, 9,000 ft., 5; ½ mi. E and 3 mi. S Ward, 9,400 ft., 1; 1 mi. S Gold Hill, 8,200 ft., 1. Clear Creek Co.: Davidson Mine, 3 mi. SW Idaho Springs, 1; Georgetown, 1 CM. Jefferson Co.: Silver Plume, 1 CM. Park Co.: Tarryall Creek Camp, 8,700 ft., 1 WC. Gunnison Co.: S side Crested Butte Mountain, 9,500 ft., 1 WC; mouth of Virginia Basin, Gothic, 1 FC.

Discussion

The chipmunks that heretofore have been assigned to the species Eutamias quadrivittatus are here assigned to two species, E. quadrivittatus and E. umbrinus, for the following reasons:

1. The baculum of E. quadrivittatus differs from that of E. umbrinus in having a narrow base (see figs. 3, 4). This difference permits any specimen which has an associated baculum to be readily identified to species.

2. The cranial breadth in the subspecies of E. quadrivittatus is significantly larger than in the subspecies of E. umbrinus.

3. Specimens of E. umbrinus are darker than any specimen of E. quadrivittatus.

4. Where the geographic ranges of E. quadrivittatus and E. umbrinus come close to one another (probably they meet at some places), E. umbrinus occupies a higher position in terms of life-zones. Wherever either of these two species, but not the other, occurs on a mountain the species occupies both the higher and lower life-zones.

Figs. 3 and 4. Baculum of Eutamias quadrivittatus quadrivittatus, No. 35648/47919 BS; from Canon City, Fremont Co., Colorado. Figure 3, lateral view of right side; figure 4, dorsal view.

Fig. 5. Baculum of Eutamias umbrinus umbrinus, No. 38062; from Paradise Park, 21 mi. W and 15 mi. N Vernal, 10,050 ft., Uintah Co., Utah. Lateral view of right side.

Fig. 6. Baculum of Eutamias umbrinus montanus, No. 20105; from ½ mi. E and 3 mi. S Ward, 9,400 ft., Boulder Co., Colorado. Dorsal view.

The differences between E. quadrivittatus and E. umbrinus are as great as, or greater than, between many species of chipmunks, such as between E. minimus and E. amoenus, and between E. quadrivittatus and E. cinereicollis.

Although I know of no ecological differences between E. umbrinus and E. ruficaudus, the morphological differences, as for example, differences in the structure of the baculum, and differences in color pattern, lead me to maintain E. ruficaudus and E. umbrinus as separate species.

The present distribution of these two species is attributable to the uplift of the Rocky Mountains in the Pleistocene. That the uplift of the Rocky Mountains and the erosion which produced the present-day relief took place in Pleistocene times is supported by the evidence found by several geologists such as Hunt and Sokoloff (1950:109-123).

The present geographic distribution of E. umbrinus and E. quadrivittatus conceivably came about as follows: E. umbrinus-like chipmunks were present, before the uplift of the major chains of mountains, on isolated, low mountain ranges that were not covered with glaciers (such as the laccolithic mountains that occur in Utah) in Pleistocene time, while E. quadrivittatus-like chipmunks were present in the central parts of New Mexico, Colorado, and southern Wyoming. With the advent of uplift, the habitats in the central parts of these states were changed from a plains-like habitat to a habitat that resembled the forest habitats that exist today. E. umbrinus-like chipmunks then invaded this newly formed habitat and displaced any E. quadrivittatus-like chipmunks that were less well adapted to live there. The Colorado River probably served as a barrier that kept the E. umbrinus-like chipmunks and E. quadrivittatus-like chipmunks separated up to this time. Invasion of the new forest-niche by E. umbrinus-like chipmunks may have taken place through the Uinta Mountains of northeastern Utah, after the glaciers disappeared from these mountains, since the Colorado River probably prevented any eastward migration farther south.

Table 1

Average and Extreme Measurements in Millimeters of Adult
Eutamias quadrivittatus and E. umbrinus