James S. Findley

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 ”

“ 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. ”

“ 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. ”

“ Most of the valleys are in the Transition Life-zone; the forests are rather dry and of pine with more or less isolated hydrosere communities about streams and ponds. These hydrosere situations are the habitat of Sorex vagrans. Shrews from these situations are usually referable to vagrans. The high ridges and mountain ranges are usually in the Canadian Life-zone or higher and most of the shrews referable to obscurus come from such places. Marginal localities with regard to life-zone produce most of the populations which seem to represent intergrades between the two subspecies. ”

“ 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. ”

“ Remarks.—The name yaquinae actually applies to a population of intergrades between pacificus and bairdi. There is much variation over the range of the subspecies, and individuals from the western and southern parts are larger than those from the west slope of the Cascades. Specimens from Vida and McKenzie Bridge are smaller than those from Mapleton, Mercer, and the type locality but still seem closer to yaquinae than to topotypes of bairdi. ”

“ Thus the name S. v. vagrans may apply to shrews in the region under consideration. The subspecies, as thus thought of, embraces several incipient subspecies, namely (1) the populations on the isolated mountain ranges of Nevada, (2) the coastal rain forest population and possibly (3) the population on the Columbian Plateau.
In western British Columbia, Washington, and Oregon no evidences of intergradation between S. v. vagrans and the races setosus, permiliensis, bairdi, yaquinae, or pacificus are seen. In this region S. v. vagrans occurs sympatrically with one or the other of these subspecies. ”

“ Merriam (loc. cit.) further notes that dobsoni is "intermediate in size and cranial characters between S. vagrans and obscurus;" a statement which hits very close to the heart of the matter. I consider the name dobsoni to apply to intergrades. To attempt to apply the name to the highly variable populations of intergrades from British Columbia to southern Idaho seems inadvisable. I have examined the possibility of using the name amoenus for the animals from this region. ”

“ Isolated areas of Canadian Life-zone, even though surrounded with Transition Life-zone, often harbor a population of obscurus, whereas the streams in the nearby dry valleys harbor populations of vagrans. Farther south in the Rocky Mountain chain, obscurus seemingly intergrades regularly with vagrans. This intergradation is seen in populations from several localities in Utah. ”

“ In southern British Columbia some populations are clearly intergrades while at 6 mi. S Yahk intergradation seemingly has not taken place. Where some intergradation has occurred the result often has been increased size of the lowland shrews, although they usually retain the reddish summer pelage rather than acquiring the more grayish pelage of obscurus. The name dobsoni was based upon shrews from a place where lowland and highland forms occur almost together with only a slight amount of intergradation. ”

“ Still later changes in the character of the northern plains owing to desiccation divided the range of the species and isolated S. v. soperi in Manitoba and central Saskatchewan and a population of S. v. obscurus, in the Cypress Hills. A number of semi-isolated stocks in central Montana became differentiated as a recognizable subspecies there.
A number of other boreal mammals have geographic ranges which resemble that of Sorex vagrans, except that the geographic ranges of subspecies do not overlap. ”

“ During the Sangamonian interval, isolated segments of the once widespread ancestral Sorex vagrans quite possibly persisted in such places as the Sierra Nevada, coastal southern California, the mountains of Arizona, New Mexico, and southern Mexico, and in the Black Hills (see fig. 6) . One might expect that by Wisconsinan time these populations would have become reproductively isolated from their parent stock. ”

“ Large reddish shrews of the coast of California and southwestern Oregon become smaller and darker to the north. From southwestern British Columbia they again become larger as one proceeds northward along the coast to Wrangell, Alaska, and north of that they again become smaller. ”

“ The widespread species Sorex cinereus resembles all the foregoing species in the ridges on the unicuspid teeth and in the lack of a post-mandibular foramen, but differs from those other species in [Pg 29] having the third upper unicuspid larger than the fourth. The subspecies S. cinereus ohionensis, however, often has the sizes of these teeth reversed. With S. cinereus I include S. preblei (eastern Oregon) and S. lyelli (Sierra Nevada) , both obviously closely related to cinereus as Jackson (1928:37) recognized when he included them in the cinereus group. ”

“ Variation with age must be considered in order to assemble comparable samples of these shrews. Increased age results in wear on all teeth and in particularly striking changes in the size and shape of the first incisors. Skulls of older shrews develop sagittal and lambdoidal ridges, and further differ from skulls of young animals in being slightly broader and shorter, and in developing thicker bone, particularly on the rostrum which thus seems to be, but is not always in fact, more robust. ”

“ In general, winter pelage is darker than summer pelage in these shrews. Winter pelage comes in first on the rump and spreads caudad and ventrad. The growth line of incoming hair is easily detected on the fur side of the skin. Throughout the winter the color of the pelage changes, often becoming somewhat browner, although no actual molt takes place. ”

“ Within the first year, that is to say from spring to late fall, animals vary but little. Dental characters are best studied in first year shews. For this reason I have used them as the basis for the study of geographic variation, and descriptions are based on first year animals unless otherwise noted. ”

“ Sorex vagrans paludivagus von Bloeker, Proc. Biol. Soc. Washington, 52:93, June 5, 1939.
Type.—Adult male, skin and skull; No. 5053, Los Angeles Museum of History, Science and Art, obtained on November 3, 1938, by Jack C. von Bloeker, Jr., from salt marsh at mouth of Elkhorn Slough, Moss Landing, Monterey Co., California, original no. 9456.
Diagnosis.—Size medium for the species; average and extreme measurements of 6 topotypes are: total length, 115 (113-118) ; tail, 46.5 (42-48) ; hind foot, 14.5 (14-15) (von Bloeker, 1939:94) . In winter nearly black dorsally, deep mouse gray ventrally ”

“ At places [Pg 10] along the coast successively farther north of southwestern British Columbia the shrews become larger again, the largest individuals being those from near Wrangell, Alaska. From that place northwesterly along the coast of Alaska, size decreases again.
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. ”