Part 5

The grouse genus Dendrogapus is divided into a Great Basin species, D. obscurus, which extends northward into British Columbia, and a Rocky Mountain species, D. fuliginosus, that is found in the Sierra Nevada and northward along the coast and Cascades into British Columbia. Although the two kinds have at times been considered conspecific, they differ in voice, hooting mechanism, and characters of the downy young, and so far no actual intergradation between the two has been shown (Grinnell and Miller, 1944:113). These grouse thus seem to offer additional evidence for a Pleistocene, possibly Wisconsinan, separation of the boreal fauna into a Rocky Mountain and a Pacific coastal segment.

A notable sidelight on these data is the frequency with which species in the Sierra Nevada have their closest relatives in the Rocky Mountains, rather than in the geographically nearer Cascades or coastal areas. This similarity in fauna of the Sierra Nevada and the Rockies was noted long ago by Merriam (1899:86).

RELATIONSHIPS WITH OTHER SPECIES

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. They would therefore have remained specifically distinct when Wisconsinan Sorex vagrans, reoccupied these outlying areas, and may still be found isolated in places peripheral to the range of the ancestral species.

In fact, we do find species closely related to Sorex vagrans in just such places today (fig. 10). Probably Sorex ornatus, including members of the ornatus group such as S. trigonirostris, S. sinuosus, S. willeti, S. tenellus, and S. nanus, and also S. veraepacis, arose by separation from the ancestral vagrans stock in Sangamonian time. Probably the eastern S. longirostris arose in a like manner. The ancestor of S. ornatus may have been isolated in southwestern California during Sangamonian time, spread north and south during the Wisconsinan age, and afterward given rise to S. trigonirostris and the modern S. ornatus complex of California and Baja California. In at least one place reproductive isolation between ornatus and the invading S. vagrans has broken down (Rudd, 1953); the place is a salt marsh along San Pablo Bay, where a hybrid population between S. vagrans and S. sinuosus, an ornatus derivative, has formed. Sorex tenellus may have been isolated in the Sierra Nevada in the Sangamonian interval, moved into the valleys east of the mountains during the Wisconsinan age, and become restricted to its present range since the retreat of the last ice. Sorex nanus may have occurred in the Black Hills and isolated mountains of Arizona and New Mexico during the Sangamonian interval and remained in these general areas during the Wisconsinan age. Its present range is peripheral to the main body of the Rockies and the Colorado Plateau.

The eastern species Sorex longirostris has many similarities with shrews of the ornatus-vagrans stock. S. l. longirostris is close in many ways to S. nanus. Indeed, the differences between the species S. nanus, S. ornatus, and S. longirostris seem to me to be of the same magnitude and indicate a similar period of differentiation from a common ancestor. 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. This entire group of shrews has the third unicuspid smaller than the fourth, a pigmented ridge from the apex to the cingulum of each upper unicuspid, and, in most individuals, lacks a post-mandibular foramen in the lower jaw (Findley, 1953:636-637). The pigment is not always prominent in S. longirostris.

Two other species of North American shrews, Sorex palustris, the water shrew, and Sorex bendiri, the marsh shrew, show these three characters to a greater or lesser degree, and it seems that these two species and the vagrans-ornatus-veraepacis group had a common ancestor, probably before Illinoian time for reasons stated beyond. I judge, however, that far from being subgenerically distinct as they have been considered to be, S. palustris and S. bendiri are actually closely related species of the same subgenus and may have differentiated from one another because of separation into eastern (palustris) and western (bendiri) segments in the Sangamonian interval, much as has been postulated concerning the eastern and western stocks of Sorex vagrans. Indeed, Jackson (1928:192) has noted that in the Pacific northwest the characters of the two kinds approach one another and become differences of degree only.

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 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. Sorex milleri (Coahuila and central western Nuevo Leon) seems to me to resemble S. cinereus more than it does other species of North American Sorex, and I judge that it also belongs to the cinereus group. Sorex cinereus and its close relatives seem more closely related to the species which have thus far been discussed than they do to such other North American species as S. arcticus, S. fumeus, S. trowbridgii, S. merriami, and the members of the S. saussurei group; most of these five species last mentioned possess a post-mandibular foramen, lack pigmented unicuspid ridges, and have the third unicuspid larger than the fourth. Because of the morphological resemblances mentioned above, it seems likely to me that S. cinereus and the vagrans-ornatus-veraepacis-palustris complex had a common ancestor in early Pleistocene time. Sorex cinereus has recently been considered to be conspecific with the Old World S. caecutiens Laxmann (Van den Brink, 1953) which name, being the older, would apply to the circumpolar species.

Hibbard (1944:719) recovered S. cinereus and a species of Neosorex (a name formerly applied to the water shrew) from the Pleistocene (late Kansan) Cudahy Fauna. This indicates that the ancestors of the modern S. cinereus and of the water shrew had diverged from one another before that time. Brown (1908:172) recorded S. cinereus and S. obscurus from the Conard Fissure in Arkansas. These materials were deposited probably at a later time than was the Cudahy Fauna. The S. obscurus from Conard Fissure probably represents the ancestral S. vagrans stock which I think reached eastern United States in Illinoian time and gave rise to S. longirostris. The Conard Fissure material was deposited at a time (Illinoian?) when northern faunas extended farther south than they do today.

All of the species mentioned as having structural characters in common with S. vagrans seem to have arisen from a common ancestor which had already differentiated from the ancestor of such species as S. arcticus, S. saussurei, and others. Consequently all are here included in a single subgenus. The oldest generic name applied to a shrew of this group, other than the name Sorex, is Otisorex DeKay, 1842, type species Otisorex platyrhinus DeKay, a synonym of Sorex cinereus. The subgenus can be characterized as follows.

Subgenus =Otisorex= DeKay

1842. Otisorex DeKay, Zoology of New York, pt. 1, Mammalia, p. 22, and pl. 5, fig. 1. Type, Otisorex platyrhinus DeKay (= Sorex cinereus Kerr).

Third unicuspid usually smaller than fourth; upper unicuspids usually with pigmented ridge extending from apices medially to cingula, uninterrupted by antero-posterior groove; post-mandibular foramen usually absent. Includes the species S. cinereus, S. longirostris, S. vagrans, S. ornatus, S. tenellus, S. trigonirostris, S. nanus, S. juncensis, S. willeti, S. sinuosus, S. veraepacis, S. palustris, S. bendiri, S. alaskanus, and S. pribilofensis.

FIG. 11. Medial view of right ramus of Sorex (Otisorex) vagrans. x 14.

FIG. 12. Medial view of right ramus of Sorex (Sorex) arcticus. x 14.

FIG. 13. Anterior view of left second upper unicuspid of Sorex (Otisorex) vagrans. x 45.

FIG. 14. Anterior view of left second upper unicuspid of Sorex (Sorex) arcticus. x 45.]

Other species of Sorex now occurring in North America differ from Otisorex in having the 3rd unicuspid usually larger than 4th, in lacking a pigmented ridge from the apices to the cingula of the upper unicuspids, and in usually possessing a well-developed post-mandibular foramen. Exceptions to the last mentioned character are S. fumeus and S. dispar. The subgenus Sorex in North America should include only the following species: S. jacksoni, S. tundrensis, S. arcticus, S. gaspensis, S. dispar, S. fumeus, S. trowbridgii, S. merriami, and all the members of the Mexican S. saussurei group.

The subgenera Otisorex and Sorex probably separated in early Pleistocene or late Pliocene. Sorex is unknown in North America earlier than the late Pliocene (Simpson, 1945:51).

In the genus Microsorex the characters of the subgenus Otisorex are carried to an extreme; the unicuspid ridges are prominent and end in distinct cusplets, and the 3rd unicuspid is not merely smaller than the 4th, but is reduced almost to the vanishing point. In addition, the post-mandibular foramen is absent. Although it is closer structurally to Otisorex than to Sorex, the recognition of Microsorex as a distinct genus seems warranted.

Figure 15 is intended to represent graphically some of the relationships discussed above. It must be re-emphasized that much of it is purely speculative, especially as regards actual time when various separations took place. It will be noted that I have indicated most separations as having taken place in interglacial ages. They are generally regarded as periods of warmth and aridity and, therefore, probably are times of segmentation of the ranges of boreal mammals and hence times exceptionally favorable to the process of speciation. Glacial ages, characterized by extensive and continuous areas of boreal habitat, probably were times of relatively unrestricted gene flow between many populations of boreal mammals and hence not favorable to rapid speciation.

=Sorex vagrans=

Wandering Shrew

The size of the wandering shrew varies from small in the subspecies monticola and vagrans to large in the subspecies pacificus. The tail makes up from a little more than a third to almost half of the total length. The color pattern ranges from tricolored through bicolored to almost monocolored. Color ranges from reddish (Sayal or Snuff Brown) to grayish in summer pelage and from black to light gray in winter. Diagnostic dental characters include: 3rd upper unicuspid smaller than 4th, and unicuspids, except 5th, with a pigmented ridge extending from near apex of each tooth medially to cingulum and sometimes ending as internal cusplet. S. vagrans differs from members of the ornatus group in less flattened skull, and in more ventrally situated foramen magnum that encroaches more on the basioccipital and less on the supraoccipital. The wandering shrew differs from S. trowbridgii and S. saussurei in the dental characters mentioned above. These dental characters also serve to distinguish S. vagrans readily from S. cinereus, S. merriami, and S. arcticus which may occur with vagrans. The large marsh shrew and water shrew, S. palustris and S. bendiri, can be distinguished at a glance from S. vagrans by larger size and darker color.

In the following treatment of the 29 subspecies of Sorex vagrans, the subspecies are arranged in geographic sequence, beginning with the southernmost large subspecies on the California coast and proceeding clockwise, north, east, south, and then west back to the starting point.

=Sorex vagrans sonomae= Jackson

Sorex pacificus sonomae Jackson, Jour. Mamm., 2:162, August 19, 1921.