James S. Findley

“ The vagrant shrews of the Great Basin and adjoining Columbia Plateau and Snake River Plains are smaller than their relatives in the Rocky Mountains and, by virtue of less gray in their pelage, are reddish in summer and blackish rather than grayish in winter. There is little significant geographic variation in shrews throughout this region, although owing to their restriction to the vicinity of water, the populations of shrews are more or less isolated from one another and each is somewhat different from the next. ”

“ I judge S. milleri to be a relict population of S. cinereus, isolated in the mountains of northeastern Mexico, probably in the late Pleistocene. Sorex cinereus reported from Pleistocene deposits in San Josecito Cave, Nuevo Leon (Findley, 1953:635) , probably represents a population ancestral to the modern S. milleri. Sorex milleri should retain specific status because of constant cranial differences from S. cinereus, particularly relatively broader rostrum.
LITERATURE CITED
Baker, R. H.
1953. ”

“ A situation such as the one here described where well differentiated end members of a chain of subspecies overlap over an extensive geographic range throughout the year without interbreeding—thus reacting toward one another as do full species—so far as I know has not previously been found to exist in mammals. The overlapping end-members of the chain of subspecies of Sorex vagrans really do coexist; specimens of the overlapping subspecies have been taken together at the same localities from California to British Columbia. ”

“ Turning our attention now farther inland to the Cascade Mountains of northern Oregon, the shrews there also are smaller and less reddish (more brownish) than in northwestern California, and the trend to smaller and darker shrews culminates in the northern Cascades of Washington. Shrews from there, and from the southwestern coast of British Columbia, compared with those from northwestern California, are much smaller and have so great a suffusion of black that they appear brown rather than red. ”

“ There, in British Columbia, reproductive isolation is not complete and occasional populations of intergrades occur. In Montana extensive intergradation occurs in a broad zone of transitional habitat. Along the western edge of the Rockies from Idaho south to Utah the zone of transition from montane to basin habitat is sharp and the zone of intergradation, although present, is fairly narrow, perhaps because there is little intermediate habitat which logically might be expected to be most suitable for intergrading populations. ”

“ The post-mandibular foramen is large and the opening of the mandibular canal is within the posterointernal ramal fossa. In addition the coronoid does not incline anteriorly. Specimen "A" is intermediate between No. 3943 and "B" in the characters mentioned and differs from both in that the post-mandibular foramen is widely separated from the mandibular foramen.
Comparison of the Pleistocene specimens with specimens of Recent species of North American Sorex reveals that the presence or [Pg 637] absence of the post-mandibular foramen is almost constant in any one species. ”

“ C. parva and C. micrura may intergrade but a distance of 140 miles separates the geographic ranges as now known of the two kinds and every specimen examined by me is clearly referable to one or the other of the two named kinds and shows no evidence of intergradation. ”

“ The four species first mentioned and the fossils seem to fall into one group. The remaining species fall into another group characterized by a smaller occlusal area of the talonid on all three molar teeth with respect to the trigonid, and [Pg 638] especially by the smaller and weaker talonid of m3 which possesses only one bladelike cusp, the hypoconid. In the first four species the talonids are larger than in the other species when compared to the trigonids, and the talonid of m3 possesses a well developed hypoconid and entoconid with a distinct basin between them. ”

“ Specimens from the western foot-hills of the Rocky Mountains show an amazing series of relationships with the montane S. v. obscurus. In Utah, as previously pointed out, complete intergradation occurs. At 1 mi. N Heath, Washington Co., Idaho, the lowland and the highland forms approach each other within a short distance and still maintain a degree of distinctness, especially in size. In northwestern Montana intergradation is extensive (Clothier, 1950) . In northeastern Washington distinctly separable populations occur within a few miles of one another. ”

“ All three nominal species are relatively small, short-tailed shrews. The skulls of the three kinds resemble one another in relatively short rostrum and in dental details. Slight differences in cranial proportions differentiate the three and they should, until more specimens of each are obtained and studied, retain subspecific rank. The specific name, Sorex oreopolus Merriam 1892, should apply to the three kinds since it antedates the names ventralis and emarginatus. The two names last given, therefore, should stand as Sorex oreopolus ventralis Merriam and Sorex oreopolus emarginatus Jackson. ”

“ Between Marshfield and Umpqua on the one hand, and the Columbia River and the Cascade Mountains on the other, the size of Sorex vagrans decreases quite rapidly from the large pacificus to the smaller permiliensis. Size decreases less rapidly northward along the coast than it does eastward toward the mountains; consequently, at any given latitude, coastal shrews are larger than mountain shrews. In this area of rapid change in size it is difficult to draw subspecific boundaries between pacificus, yaquinae, and bairdi, and this must be done somewhat arbitrarily. ”

“ Specimens of Sorex vagrans west [Pg 54] of the Cascade Mountains have long been referred to the nominate subspecies which has its type locality at Willapa Bay, Pacific Co., Washington. Over so wide an area it is only to be expected that some geographic variation is to be found. Thus specimens from central Nevada average slightly paler in summer pelage than those from the Pacific Coast or from the foothills of the Rocky Mountains. In addition there are slight average differences in size from place to place. ”

“ 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. ”

“ In the Rocky Mountains of Wyoming and Colorado the subspecies S. v. obscurus ranges almost uninterruptedly over relatively large areas, but southward in New Mexico and southwestward into Utah and Arizona, suitable boreal habitat becomes insular in nature and obscurus there is confined to the higher mountains. With one exception, once the shrew populations become 'insular' in this region they become smaller and show intergradation with Sorex vagrans monticola. ”

“ The ancestor of S. longirostris may have gained access to the eastern United States in the Illinoian Age via the northern Great Plains south of the glacial boundary (fig. 7) . The ancestor of Sorex veraepacis of southern Mexico probably reached that area in Illinoian time as part of the ancestral vagrans stock and probably attained its differentiation during the Sangamonian interval.
All the kinds of shrews so far discussed, including the S. vagrans complex, might thus be thought of as having had a common ancestor in the Illinoian Age. ”

“ The species palustris is found throughout the Rocky Mountains, north into Alaska, across the Great Basin into the Sierra Nevada, and west to the Pacific coast in Washington. The species bendiri is found from northwestern California north along the Pacific coast to southwestern British Columbia and east to the Cascades. Where the ranges of the two species overlap in western Washington they do not interbreed so far as is known, and are somewhat different in their ecology, bendiri being a lowland, and palustris being a montane, species. ”

“ These two age classes are readily separable on the basis of differences in the skull as well as on the decreased pubescence of the tail and the increased weight of second year animals. My own examination of hundreds of museum specimens confirms this for the Sorex vagrans group. Separation of the two age classes in an August-taken series of Sorex vagrans from coastal Washington is shown in figure 3, in which two tooth-measurements that are dependent upon wear are plotted against one another.
First year animals are more abundant in collections than are second year animals. ”

“ Western Canada and Alaska were populated by shrews which originated in the habitable parts of the Rocky Mountains and Colorado Plateau during Wisconsinan time (as opposed to shrews originating, as subspecies, in the Great Basin or on the Pacific Coast) . These shrews differentiated into the currently recognized subspecies of the west coast and coastal islands of British Columbia and Alaska in response to the different environments in these places, many of which were isolated ”

“ Possible distribution in Illinoian (inset) and Sangamonian times of the ancestor of the Sorex vagrans-ornatus-longirostris-veraepacis complex. Approximate southern boundary of Illinoian glaciation marked by heavy line.
In the ensuing Sangamonian interglacial age all glaciers retreated or disappeared thereby opening up extensive areas in the north and in the higher mountains which were occupied by a boreal fauna, including S. vagrans. Concurrently the Great Basin, and probably also much of the Columbian Plateau, became dry, and desert conditions developed, perhaps much as they are today. ”

“ 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. ”