Fig. 4. Condylobasal length (in millimeters) plotted against palatal index (palatal length/condylobasal length × 100) in several subspecies of Sorex vagrans to show relative increase in size of rostrum with actual increase in size of skull.
The shrews so far discussed inhabit forests in a region of high rainfall and a minimum of seasonal fluctuation in temperature. Such a habitat seems to be the optimum for shrews of the vagrans group since the largest individuals are found there. In addition, shrews seem to be as common, or commoner, in this coastal belt, than they are in other places.
The large shrews of the vagrans group on the Pacific coast were divided into three species by H. H. T. Jackson in his revision of the North American Sorex in 1928. The large reddish shrews of the coast of California and southern Oregon were called S. pacificus. The somewhat smaller ones from the coast of central Oregon were called S. yaquinae. Still smaller shrews from northwestern Oregon and from the rest of the Pacific coast north into Alaska were called S. obscurus. I find these kinds to intergrade continuously one with the next in the manner described and conclude that all are of a single species.
Inland Montane Section
Inland from the coasts of British Columbia and Alaska the size of the vagrans shrew decreases rapidly. Specimens from western Alaska, central Alaska, and the interior of British Columbia are uniformly smaller than coastal specimens. In addition the red of the hair is masked more by neutral gray than by black with the result that the pelage is grayish rather than brownish or reddish. Shrews of this general appearance are found southward through the Rocky Mountain chain to Colorado and New Mexico. On the more or less isolated mountain ranges of Montana east of the continental divide the vagrans shrew is somewhat smaller still. On the Sacramento Mountains of southeastern New Mexico the shrew is somewhat larger and slightly darker. Southwestward from the Colorado Rockies this shrew becomes smaller and slightly more reddish (less grayish).
All of these montane populations of the vagrans shrew are commonest in hydrosere communities, that is to say, streamsides and marshy areas where the predominant vegetation is grass, sedges, willows, and alders. Since these animals are less common within the montane forests, hydrosere communities, rather than the actual forest, seem to be the positive feature important for the shrews.
The shrews of the montane region just described were regarded by Jackson as belonging to two species: Sorex obscurus, occupying all the Rocky Mountains south to, and including, the Sacramento Mountains; S. vagrans, made up of small individuals from various places in Wyoming, Montana, and Colorado, and all the shrews of western New Mexico and all of Arizona. My study of these animals has led me to conclude that the smaller shrews of Arizona and New Mexico intergrade in a clinal fashion with the shrews of Colorado and in fact represent but one species. Since some individuals from Colorado are as small as larger individuals from this southwestern population of small animals, I conclude that such specimens are the basis for reports of S. vagrans from Colorado. The shrews of the Sacramento Mountains resemble those of the Colorado Rockies more than they do the smaller shrews of western New Mexico and Arizona, possibly because the climate is similar in the Sacramento Mountains and the higher Colorado Rockies. There is less precipitation in the more western mountain ranges in New Mexico and in Arizona in April, May, and June than in the Colorado Rockies. These months are critical for the reproduction and growth of shrews.