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.
[Illustration: 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 1/2 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.