still in old winter
pelage and does not appear to have been actively molting; the entire
venter is in summer pelage. KU 50146, obtained on August 22, 1952, 3 mi.
E Chadron, Dawes Co., Nebraska, has small patches or tufts of winter
pelage remaining on the rump and likewise does not appear to have been
actively molting. KU 72085, obtained on October 13, 1956, 4 mi. E
Barada, Richardson Co., Nebraska, is in the process of molting from
summer to winter pelage, but has tufts of old winter pelage on the rump.

Geographic Variation

Geographic variation, both in color of pelage and in external and
cranial dimensions, is less in _R. megalotis_ in the region studied than
in most other cricetine species that occur there. Nevertheless,
meaningful variation is present. The assumption that variation in _R.
megalotis_ paralleled in degree that of other species, _Peromyscus
maniculatus_ for example, led to untenable taxonomic conclusions by some
previous workers.

_Color of Pelage_

Color of pelage is remarkably uniform, considering the geographic extent
of the area involved, over most of the northern part of the central
grasslands. Perhaps this uniformity results partly from the predilection
of the western harvest mouse for grassy habitats, for in most areas on
the Great Plains the species is restricted to riparian communities,
principally along river systems, where soils, cover, and other
conditions approximate those of corresponding habitats farther to the
east to a much greater degree than do conditions in upland habitats.
Differential selective pressure, therefore, theoretically would be less
between eastern and western populations of _R. megalotis_ than in an
upland-inhabiting species. In any event, specimens from western
Nebraska, Wyoming, northern Colorado, and adjacent areas average only
slightly paler dorsally than specimens in corresponding pelages from the
eastern parts of Nebraska and Kansas, and many individuals from the two
areas can be matched almost exactly.

To the southwest, on the other hand, a trend toward paler (pale
brownish, less blackish) upper parts is apparent. Specimens from
southwestern Kansas and adjacent parts of Colorado and Oklahoma average
slightly paler in comparable pelages than specimens from northeastern
Kansas and eastern Nebraska, but most specimens from farther southwest,
in northern New Mexico and southwestern Colorado, are discernibly,
although not markedly, paler than mice from nor