Transcriber's NoteThe cover above is simulated. Thelist of publicationshas been compiled after the article's text.double barUniversity of Kansas PublicationsMuseum of Natural HistorybarVolume 9, No. 1, pp. 1-68, figures 1-18barDecember 10, 1955barSpeciation of the Wandering ShrewBYJAMES S. FINDLEYUniversity of KansasLawrence1955University of Kansas Publications, Museum of Natural HistoryEditors: E. Raymond Hall, Chairman, A. Byron Leonard, Robert W. WilsonVolume 9, No. 1, pp. 1-68, figures 1-18Published December 10, 1955University of KansasLawrence, KansasPRINTED BYFERD VOILAND, JR., STATE PRINTERTOPEKA, KANSAS1955Look for the Union Label25-7903Speciation of the Wandering ShrewBYJAMES S. FINDLEYCONTENTSPAGEIntroduction4Materials Methods and Acknowledgments4Non-geographic Variation7Characters of Taxonomic Worth8Pelage Change9Geographic Distribution and Variation9Pacific Coastal Section9Inland Montane Section11Great Basin and Columbia Plateau Section12Summary of Geographic Variation13Origin of theSorex vagransRassenkreis16Relationships With Other Species26Conclusions60Table of Measurements62Literature Cited66FIGURESFigs. 1-2.—Cranial Measurements5Fig. 3.—Graph Illustrating Wear of Teeth8Fig. 4.—Graph Illustrating Heterogonic Growth of Rostrum10Fig. 5.—Present Geographic Distribution ofSorex vagrans15Fig. 6.—Skulls ofSorex vagrans17Figs. 7-10.—Past Geographic Distribution of Shrews19-20-22-27Figs. 11, 12.—Medial View of Lower Jaws of Two Shrews30Figs. 13, 14.—Second Unicuspid Teeth of Shrews30Fig. 15.—Diagram of Probable Phylogeny of Shrews32Figs. 16-18.—Geographic Distribution of Subspecies33-40-53INTRODUCTIONThe purpose of this report is to make clear the biological relationships between the shrews of theSorex vagrans-obscurus"species group." This group as defined by H. H. T. Jackson (1928:101) included the speciesSorex vagrans,S. obscurus,S. pacificus,S. yaquinae, andS. durangae. The last mentioned species has been shown (Findley, 1955:617) to belong to another species group.Sorex milleri, also assigned to this group by Jackson (1947:131), seems to have its affinities with thecinereusgroup as will be explained beyond. The position of thevagransgroup in relationship to other members of the genus will be discussed.Of this group, the species that was named first wasSorex vagransBaird, 1858. Subsequently many other names were based on members of the group and these names were excellently organized by Jackson in his 1928 revision of the genus. Subsequent students of western mammals, nevertheless, have been puzzled by such problems as the relationship of (1)Sorex vagrans monticolatoSorex obscurus obscurusin the Rocky Mountains, (2)Sorex pacificus,S. yaquinae, andS. obscurusto one another on the Pacific Coast, and (3)S. o. obscurustoS. v. amoenusin California. Few studies have been made of these relationships. Clothier (1950) studiedS. v. monticolaandS. o. obscurusin western Montana and concluded that the two supposed kinds actually were not separable in that area. Durrant (1952:33) was able to separate the two kinds in Utah as was Hall (1946:119, 122) in Nevada. Other mammalogists who worked within the range of thevagrans-obscurusgroups have avoided the problems in one way or another. Recently Rudd (1953) has examined the relationships ofS. vagranstoS. ornatus.MATERIALS METHODS AND ACKNOWLEDGMENTSApproximately 3,465 museum study skins and skulls were studied. Most of these were assembled at the University of Kansas Museum of Natural History, but some were examined in other institutions.Specimens were grouped by geographic origin, age, and sex. Studies of the role of age and sex in variation were made. Because it was discovered that secondary sexual variation was negligible, both males and females, if of like age and pelage, were used in comparisons designed to reveal geographic variation.External measurements used were total length, length of tail, and length of hind foot. After studying a number of cranial dimensions I chose those listed below as the most useful in showing differences in size and proportions of the skull.Figures 1 and 2show the points between which those measurements were taken.Condylobasal length.—From anteriormost projection of the premaxillae to posteriormost projection of the occipital condyles (a to a´).Maxillary tooth-row.—From posteriormost extension of M3 to anteriormost extension of first unicuspid (b to b´).Palatal length.—From anteriormost projection of premaxillae to posteriormost part of bony palate (c to c´).Cranial breadth.—Greatest lateral diameter of braincase (d to d´).Least interorbital breadth.—Distance between medialmost superior edges of orbital fossae, measured between points immediately above and behind posterior openings of infraorbital foramina (e to e´).Maxillary breadth.—Distance between lateral tips of maxillary processes (f to f´).Figs. 1 and 2.Showing where certain cranial measurements were taken. × 3½. (Based onSorex vagrans obscurus, from Stonehouse Creek, 5½ mi., W junction of Stonehouse Creek and Kelsall River, British Columbia, ♀, 28545 KU.)In descriptions of color, capitalized terms refer to those in Ridgway (1912). In addition the numerical and alphabetical designations of these terms are given since a knowledge of the arrangements of these designations enables one quickly to evaluate differences between stated colors. Color terms which are not capitalized do not refer to any precise standard of color nomenclature.In the accounts of subspecies, descriptions, unless otherwise noted, are of first year animals as herein defined. Descriptions of color are based on fresh pelages.Unless otherwise indicated, specimens are in the University of Kansas Museum of Natural History. Those in other collections are identified by the following abbreviations:AMNHAmerican Museum of Natural HistoryCMCarnegie MuseumChMChicago Museum of Natural HistoryCMNHCleveland Museum of Natural HistoryFCCollection of James S. FindleyHCCollection of Robert HoldenreidSGJCollection of Stanley G. JewettCDSCollection of Charles D. SnowAWCollection of Alex WalkerNMCNational Museum of CanadaOSCOregon State CollegePMBCBritish Columbia Provincial Museum of Natural HistorySDSan Diego Natural History MuseumBSUnited States Biological Surveys CollectionUSNMUnited States National MuseumUMUniversity of Michigan Museum of ZoologyOUUniversity of Oregon Museum of Natural HistoryUUUniversity of Utah Museum of ZoologyWSCWashington State College, Charles R. Conner MuseumIn nature, the subspecies ofSorex vagransform a cline and are distributed geographically in a chain which is bent back upon itself. The subspecies in the following accounts are listed in order from the southwestern end of the chain clockwise back to the zone of overlap.The synonymy of each subspecies includes the earliest available name and other names in chronological order. These include the first usage of the name combination employed by me and other name combinations that have been applied to the subspecies concerned.In the lists of specimens examined, localities are arranged first by state or province. These are listed in tiers from north to south and in any given tier from west to east. Within a given state, localities are grouped by counties, which are listed in the same geographic sequence as were the states and provinces (N to S and W to E). Within a given county, localities are arranged from north to south. If two or more localities are at the same latitude the westernmost is listed first. Marginal localities are listed in a separate paragraph at the end of each account. The northernmost marginal locality is listed first and the rest follow in clockwise order. Those records followed by a citation to an authority are of specimens which I have not personally examined. Marginal records are shown by dots on the range maps. Marginal records which cannot be shown on the maps because of undue crowding are listed in Italic type.To persons in charge of the collections listed above I am deeply indebted. Without their generous cooperation in allowing me to examine specimens in their care this study would not have been possible. Appreciated suggestions in the course of the work have been received from Professors Rollin H. Baker, A. Byron Leonard, R. C. Moore, Robert W. Wilson, and H. B. Tordoff, and many of my fellow students. Mr. Victor Hogg gave helpful suggestions on the preparation of the illustrations. My wife, Muriel Findley, devoted many hoursto secretarial work and typing of manuscript. Finally I am grateful to Professor E. Raymond Hall for guidance in the study and for assistance in preparing the manuscript. During the course of the study I received support from the University of Kansas Endowment Association, from the Office of Naval Research, and from the National Science Foundation.NON-GEOGRAPHIC VARIATIONNon-geographic variation, that is to say, variation within a single population of shrews, consists of variation owing to age and normal individual variation. InSorexI have detected no significant secondary sexual differences between males and females; accordingly the two sexes are here considered together.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. Pruitt has recently (1954) noted these same cranial differences in specimens ofSorex cinereusof different ages.Several students of American shrews, notably Pearson (1945) onBlarina, Hamilton (1940) onSorex fumeus, and Conaway (1952) onSorex palustris, have shown that young are born in spring and summer, usually reach sexual maturity the following spring, and rarely survive through, or even to, a second winter. The result is that collections made, as most of them are, in spring and summer, contain two age classes, first year and second year animals. 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 theSorex vagransgroup. Separation of the two age classes in an August-taken series ofSorex vagransfrom coastal Washington is shown infigure 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. 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.CHARACTERS OF TAXONOMIC WORTHWithin theSorex vagranscomplex, the only characters of taxonomic significance that I have detected are in size and color. It is true that cranial proportions, such as relative size of rostrum, may change from population to population, but these proportions seem to me to be dependent upon actual size of the individual shrew as I shall elsewhere point out. Of the cranial measurements here employed, palatal length and least interorbital breadth are the most significant and useful. Color in theS. vagransgroup seems to be in Orange and Cadmium Yellow, colors 15 and 17 of Ridgway (1912). No specimens actually possess these pure colors, but most colors in these shrews are seen to be derived from the two mentioned by admixture of black and/or neutral gray. In color designations an increase in neutral gray is indicated by an increased number of prime signs ( ´ ), whereas increase in black is indicated by progressive characters of the Roman alphabet (i,k,m). Thus, 17´´kis grayer than 17´kand 17´´mis blacker than 17´´k. In subspecific diagnoses in this report, color and size, and sometimes relative size, are the characters usually mentioned.Fig. 3.Two measurements (in millimeters) reflecting tooth-wear plotted against one another. First year and second year individuals ofSorex vagrans vagrans, all taken in August at Willapa Bay, Washington, are completely separated. Open circles represent teeth of second year shrews; solid circles represent teeth of first year shrews.PELAGE CHANGEIn 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. This was noted by Dalquest (1944) who assumed that the color change resulted from molt although he was unable to detect actual replacement of hairs. Summer pelage usually comes in first on the back or head and moves posteriorly and laterally. Time of molt depends on latitude and altitude. Summer pelage may appear fairly late in the season and may account for the anomalous midsummer molt noted by Dalquest. Fresh pelages of summer and winter are best seen in first year animals and are less variable than are worn pelages and hence are used as the basis of color descriptions.GEOGRAPHIC DISTRIBUTION AND VARIATIONPacific Coastal SectionThe largest shrews of thevagransgroup (large in all dimensions) occur in the coastal forests of northern California and of Oregon. Those shrews are reddish, large-skulled, large-toothed, and have rostra that are large in proportion to the size of the skull as a whole. The very largest of these shrews live along the coast of northwestern California. To the southward they are somewhat smaller, and at successively more northern localities, to as far as southwestern British Columbia, they are likewise progressively smaller and also somewhat less reddish. The relative size of the rostrum decreases with the decrease in size of the skull; consequently smaller shrews have relatively smaller rostra (seefig. 4). In addition the zygomatic ridge of the squamosal decreases in relative size with decrease in actual size of the skull. Thus, these features change in a clinal fashion as one proceeds from, say, Humboldt County, California, northward to Astoria, Oregon.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. At placesalong 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 ofSorex vagransto 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 thevagransgroup 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 thevagransgroup on the Pacific coast were divided into three species by H. H. T. Jackson in his revision of the North AmericanSorexin 1928. The large reddish shrews of the coast of California and southern Oregon were calledS. pacificus. The somewhat smaller ones from the coast of central Oregon were calledS. yaquinae. Still smaller shrews from northwestern Oregon and from the rest of the Pacific coast north into Alaska were calledS. 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 SectionInland from the coasts of British Columbia and Alaska the size of thevagransshrew 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 thevagransshrew 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 thevagransshrew 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 ofS. vagransfrom 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.As mentioned above, the shrews from east of the continental divide in Montana are smaller than those of the other mountains of the state, and it is upon such small animals that the nameSorex vagranshas been based in this area. It is clear, however, that these smaller animals intergrade with the larger shrews of the more western mountains. The small size might be an adaptation to the lesser precipitation and harsher continental climate east of the continental divide in Montana.Great Basin and Columbia Plateau SectionThe 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. Those from nearest the Rockies are sometimes slightly larger and those from some places in Nevada are slightly paler than the average. This small reddish shrew is found all the way to the Pacific coast of California, Oregon, and Washington. In these coastal areas it is somewhat darker and sometimes a trifle larger than elsewhere. It intergrades with a somewhat larger, grayer shrew in the Sierra Nevada of California. Along the Wasatch front in Utah, this Great Basin shrew intergrades with the larger, grayer shrew of the Rockies. Owing to the abrupt change in elevation, the zone of intergradation is rather narrow horizontally. In the latitude of Salt Lake City, populations of intergrades occur at between 8,700 and 9,000 feet elevation. The lowland shrew occurs in the eastern part of the Snake River Plains, and along the valleys of the Bear and Salt rivers into Wyoming. Along the northern edge of the Snake River Plains and on the western edge of the mountains of central Idaho the transition from lowland to montane habitats is abrupt and in consequence the zone of contact between small and large shrews is narrow. In northern Idaho and northwestern Montana the transition from lowland to highland is more gradual. Tributaries of the Columbia River system, especially the Clark Fork, provide a path for movement of lowland forms into intermontane basins of western Montana. In addition, the vegetational zones are found at lower elevations, and there are boreal forests in the lowlands rather than only inthe mountains as is the case in Utah and Colorado. In this area, therefore, the zone of intergradation between the smaller lowland shrew and the larger montane shrew is more gradual and gradually intergrading populations are found over a relatively large area. This has been well demonstrated for northwestern Montana by Clothier (1950). In southern British Columbia and northern Washington this shrew in the mountains is large and in the intermontane valleys is small. There is extensive interdigitation of valleys and mountain ranges, and, consequently, of life-zones in this region. In a few places, recognizably distinct populations of the vagrant shrew occur within a few miles of one another, but in other places there are populations of intergrades. West of the Cascades no evidence of intergradation has been found and the two kinds occur almost side by side and maintain their distinctness.These Great Basin shrews dwell in hydrosere communities as do their Rocky Mountain counterparts. In this arid region such a habitat obviously is the only one habitable for a shrew of thevagransgroup. These shrews often maintain their predilection for such habitats when they reach the Pacific coast, and are commonly found in such places as coastal marshes, marshy meadows, and streamsides, while the woodlands are inhabited by other species.These small shrews of the Great Basin and the small vagrant shrews of the Pacific Coast were calledSorex vagransby Jackson.Summary of Geographic VariationLarge 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. Moving inland from the coast the shrews become markedly smaller in Alaska and British Columbia. The smaller inland and montane form occurs south through the Rocky Mountains, becoming slightly smaller in central Montana, slightly larger in southeastern New Mexico, and slightly smaller in western New Mexico and in Arizona. This montane form intergrades with a smaller more reddish Great Basin shrew, the zone of intergradation roughly following the western slope of the Rocky Mountains. The Great Basin shrew occurs westward to the Pacific Coast; there the Great Basin shrew occurs with, although in part it is ecologically separated from, the large reddish coastal shrews.There seems to be an intergrading chain of subspecies of onespecies, the end members of which (the small Great Basin form and the large coastal form) are so different in size and ecological niche that they are able to coexist without interbreeding. In southern British Columbia the morphological differences are not so marked as farther south along the Pacific Coast. 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 oldest name applied to a shrew of the group under consideration isSorex vagransBaird, 1858, the type locality of which is Willapa Bay, Pacific County, Washington. The name applies to the small vagrant shrew of this area, rather than to the larger forest dweller which has been known asSorex obscurus. The nameS. vagrans, in the specific sense, must therefore apply to all the shrews discussed which have heretofore been known by the namesS. pacificus,S. yaquinae,S. obscurus, andS. vagrans.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 ofSorex vagransreally do coexist; specimens of the overlapping subspecies have been taken together at the same localities from California to British Columbia. I have taken a specimen ofS. v. vagransand several ofS. v. setosusin the same woodlot at Fort Lewis, Pierce County, Washington. Two subspecies of deer,Odocoileus hemionus, in the Sierra Nevada of California, occur together over a sizeable area but for only a part of each year that does not include the breeding season (Cowan, 1936:156-157). In the deer mouse,Peromyscus maniculatus, the geographic ranges of several pairs of subspecies meet at certain places without intergradation of the two kinds. In these instances well marked ecological differences exist between the subspecies involved. In western Washington, for example, the geographic range of the lowland subspecies,P. m. austerus, interdigitates to the east and west with the range of the montane and coniferous forest-inhabiting subspecies,P. m. oreas, and the two kinds have not been shown to intergrade.Peromyscus maniculatus artemesiaeandP. m. osgoodicome together without interbreeding in Glacier National Park, Montana.P. m. artemesiaeis almost entirely a forest-dwelling subspecies, whereasosgoodiis an inhabitant of open country. The two kinds do not actually occur together ecologically although they occur together in buildings at the edge of the woods (A. Murie, 1933:4-5).Fig. 5.Probable present geographic distribution ofSorex vagrans. The range ofS. v. vagransand its derivativesS. v. vancouverensis,S. v. halicoetes, andS. v. paludivagus, is shown by lines slanting in a different direction than those which mark the range of all the other subspecies ofS. vagrans. The region in whichS. v. vagransoccurs together with other subspecies ofS. vagransis shown by the superposition of one pattern upon the other.Cases of sympatric existence of two subspecies of one species are known in birds and in reptiles. Notable examples are in the gull,Larus argentatus(Mayr, 1940), in the Old World warbler,Phylloscopus trochiloides(Ticehurst, 1938), and in the great titmouse,Parus major(Rensch, 1933), of the Old World. In the first species the two end-members, the herring gull and the lesser black-backed gull, occur together over an extensive region from northern Europe and the British Isles throughout Fennoscandia. Fitch (1940) described a rassenkreis with overlapping subspecies in the garter snakeThamnophis ordinoides.The geographic distribution of the speciesSorex vagransis shown in figure 5. The geographic range of the Great Basin subspecies is shown by a different pattern of lines than the other subspecies ofS. vagrans. In the region in which the geographic range of the Great Basin subspecies overlaps those of the subspecies of the Pacific Coast, the pattern of shading for the Great Basin subspecies is superimposed on the patterns for the other subspecies.ORIGIN OF THESOREX VAGRANSRASSENKREISThe distribution of the speciesSorex vagransand that of its immediate ancestors obviously has not always been the same; during glacial ages much of the present range of the species in Canada and in some of the higher mountains of the United States was covered with ice and not available to the shrew. Furthermore, large areas that are now too hot and dry to permit the existence ofS. vagranswere at one time habitable. If we are to speculate on the manner in which theSorex vagransrassenkreis originated we must inquire into the nature and extent of these climatic changes.The most recent epoch of geological time, the Pleistocene, is known to have been divided into a series of alternating glacial and interglacial ages. During the glacial ages continental and montane glaciers are judged to have covered much of Canada and the northern United States. Concurrently the major storm tracks of the west probably were shifted southward; in any event much of the now arid intermontane west was much better watered than it is today.The increased precipitation, and probably glacial meltwater, formed large lakes in the closed basins of the Great Basin. There were boreal forests at lower elevations than there are today in comparable latitudes and continuous boreal habitat probably connected many of the isolated mountain ranges of the southwest. That probability is supported by the presence of boreal animals and plants on manyof these isolated ranges today. A boreal tree squirrel, such asTamiasciurus, could hardly be suspected of crossing a treeless, intermontane desert valley, miles wide.Figs. 6a-6f. Fig. 6a.Sorex vagranspacificus, 1 mi. N Trinidad, Humboldt Co., California, FC 1442. Fig. 6b.S. v. yaquinae, Newport, Lincoln Co., Oregon, AW 707. Fig. 6c.S. v. yaquinae(nearbairdi), McKenzie Bridge, Lane Co., Oregon, AW 82. Fig. 6d.S. v. setosus, Reflection Lake, Jefferson Co., Washington, CMNH 4275. Fig. 6e.S. v. obscurus, 10 mi. SSW Leadore, Lemhi Co., Idaho, FC 1499. Fig. 6f.S. v. vagrans, Baker Creek, White Pine Co., Nevada, 88042 (after Hall, 1946:113).Interglacial ages were characterized by warmth and aridity as compared to the glacial ages. Glaciers retreated or disappeared, boreal forests became montane in much of the United States, and the lakes in the Great Basin were reduced or disappeared. One can envision that during such times boreal mammals were isolated, their geographic ranges were restricted, and Sonoran mammals expanded their ranges.Evidence is more extensive concerning the number and extent of glacial ages in the eastern than in the western part of North America. This evidence suggests a division of the Pleistocene into four glacial ages and four interglacial ages, the fourth interglacial age corresponding to the present time. More information is available about the Wisconsinan, or last, glacial age, than about the earlier ones, because the last glaciation in many montane areas destroyed evidence of earlier glaciations. The names of currently recognized glacial and interglacial ages of the Pleistocene are listed below. The names of interglacial ages are in Italic type.WisconsinanSangamonianIllinoianYarmouthianKansanAftonianNebraskanWe may think of these ages as an alternating series of cool moist and warm dry periods during which boreal mammals, and other organisms, alternately moved southward (disappearing in the glaciated regions) and northward into previously glaciated areas (while disappearing from southern areas except on isolated mountain ranges).Sorex vagransprobably followed this pattern of movement and now is restricted to forested or well-watered places.One possible series of events culminating in the formation of theSorex vagransrassenkreis may be thought of as having begun during the Illinoian age. With much of Canada, and perhaps also many areas in the Rockies, Cascades, and the Sierra Nevada covered with glacial ice, the shrew-stock ancestral toSorex vagransmay well have occupied a more or less continuous range over the Colorado Plateau, the Columbian Plateau, the Great Basin, and in the forests of the Pacific Coast (as well as over part of eastern United States, as willbe explained beyond; seefig. 7). At that time the species probably was a continuously interbreeding unit.
Transcriber's NoteThe cover above is simulated. Thelist of publicationshas been compiled after the article's text.
Transcriber's Note
The cover above is simulated. Thelist of publicationshas been compiled after the article's text.
double barUniversity of Kansas PublicationsMuseum of Natural HistorybarVolume 9, No. 1, pp. 1-68, figures 1-18barDecember 10, 1955barSpeciation of the Wandering ShrewBYJAMES S. FINDLEYUniversity of KansasLawrence1955
University of Kansas Publications
Museum of Natural History
Volume 9, No. 1, pp. 1-68, figures 1-18
Speciation of the Wandering Shrew
BY
JAMES S. FINDLEY
University of KansasLawrence1955
University of Kansas Publications, Museum of Natural HistoryEditors: E. Raymond Hall, Chairman, A. Byron Leonard, Robert W. WilsonVolume 9, No. 1, pp. 1-68, figures 1-18Published December 10, 1955University of KansasLawrence, KansasPRINTED BYFERD VOILAND, JR., STATE PRINTERTOPEKA, KANSAS1955Look for the Union Label25-7903
University of Kansas Publications, Museum of Natural HistoryEditors: E. Raymond Hall, Chairman, A. Byron Leonard, Robert W. WilsonVolume 9, No. 1, pp. 1-68, figures 1-18Published December 10, 1955University of KansasLawrence, Kansas
PRINTED BYFERD VOILAND, JR., STATE PRINTERTOPEKA, KANSAS1955Look for the Union Label25-7903
Speciation of the Wandering Shrew
BY
JAMES S. FINDLEY
CONTENTS
Introduction
Materials Methods and Acknowledgments
Non-geographic Variation
Characters of Taxonomic Worth
Pelage Change
Geographic Distribution and Variation
Relationships With Other Species
Conclusions
Table of Measurements
Literature Cited
FIGURES
Figs. 1-2.—Cranial Measurements
Fig. 3.—Graph Illustrating Wear of Teeth
Fig. 4.—Graph Illustrating Heterogonic Growth of Rostrum
Figs. 7-10.—Past Geographic Distribution of Shrews
Figs. 11, 12.—Medial View of Lower Jaws of Two Shrews
Figs. 13, 14.—Second Unicuspid Teeth of Shrews
Fig. 15.—Diagram of Probable Phylogeny of Shrews
Figs. 16-18.—Geographic Distribution of Subspecies
INTRODUCTION
The purpose of this report is to make clear the biological relationships between the shrews of theSorex vagrans-obscurus"species group." This group as defined by H. H. T. Jackson (1928:101) included the speciesSorex vagrans,S. obscurus,S. pacificus,S. yaquinae, andS. durangae. The last mentioned species has been shown (Findley, 1955:617) to belong to another species group.Sorex milleri, also assigned to this group by Jackson (1947:131), seems to have its affinities with thecinereusgroup as will be explained beyond. The position of thevagransgroup in relationship to other members of the genus will be discussed.
Of this group, the species that was named first wasSorex vagransBaird, 1858. Subsequently many other names were based on members of the group and these names were excellently organized by Jackson in his 1928 revision of the genus. Subsequent students of western mammals, nevertheless, have been puzzled by such problems as the relationship of (1)Sorex vagrans monticolatoSorex obscurus obscurusin the Rocky Mountains, (2)Sorex pacificus,S. yaquinae, andS. obscurusto one another on the Pacific Coast, and (3)S. o. obscurustoS. v. amoenusin California. Few studies have been made of these relationships. Clothier (1950) studiedS. v. monticolaandS. o. obscurusin western Montana and concluded that the two supposed kinds actually were not separable in that area. Durrant (1952:33) was able to separate the two kinds in Utah as was Hall (1946:119, 122) in Nevada. Other mammalogists who worked within the range of thevagrans-obscurusgroups have avoided the problems in one way or another. Recently Rudd (1953) has examined the relationships ofS. vagranstoS. ornatus.
MATERIALS METHODS AND ACKNOWLEDGMENTS
Approximately 3,465 museum study skins and skulls were studied. Most of these were assembled at the University of Kansas Museum of Natural History, but some were examined in other institutions.
Specimens were grouped by geographic origin, age, and sex. Studies of the role of age and sex in variation were made. Because it was discovered that secondary sexual variation was negligible, both males and females, if of like age and pelage, were used in comparisons designed to reveal geographic variation.
External measurements used were total length, length of tail, and length of hind foot. After studying a number of cranial dimensions I chose those listed below as the most useful in showing differences in size and proportions of the skull.Figures 1 and 2show the points between which those measurements were taken.
Condylobasal length.—From anteriormost projection of the premaxillae to posteriormost projection of the occipital condyles (a to a´).
Maxillary tooth-row.—From posteriormost extension of M3 to anteriormost extension of first unicuspid (b to b´).
Palatal length.—From anteriormost projection of premaxillae to posteriormost part of bony palate (c to c´).
Cranial breadth.—Greatest lateral diameter of braincase (d to d´).
Least interorbital breadth.—Distance between medialmost superior edges of orbital fossae, measured between points immediately above and behind posterior openings of infraorbital foramina (e to e´).
Maxillary breadth.—Distance between lateral tips of maxillary processes (f to f´).
Figs. 1 and 2.Showing where certain cranial measurements were taken. × 3½. (Based onSorex vagrans obscurus, from Stonehouse Creek, 5½ mi., W junction of Stonehouse Creek and Kelsall River, British Columbia, ♀, 28545 KU.)
Figs. 1 and 2.Showing where certain cranial measurements were taken. × 3½. (Based onSorex vagrans obscurus, from Stonehouse Creek, 5½ mi., W junction of Stonehouse Creek and Kelsall River, British Columbia, ♀, 28545 KU.)
In descriptions of color, capitalized terms refer to those in Ridgway (1912). In addition the numerical and alphabetical designations of these terms are given since a knowledge of the arrangements of these designations enables one quickly to evaluate differences between stated colors. Color terms which are not capitalized do not refer to any precise standard of color nomenclature.
In the accounts of subspecies, descriptions, unless otherwise noted, are of first year animals as herein defined. Descriptions of color are based on fresh pelages.
Unless otherwise indicated, specimens are in the University of Kansas Museum of Natural History. Those in other collections are identified by the following abbreviations:
In nature, the subspecies ofSorex vagransform a cline and are distributed geographically in a chain which is bent back upon itself. The subspecies in the following accounts are listed in order from the southwestern end of the chain clockwise back to the zone of overlap.
The synonymy of each subspecies includes the earliest available name and other names in chronological order. These include the first usage of the name combination employed by me and other name combinations that have been applied to the subspecies concerned.
In the lists of specimens examined, localities are arranged first by state or province. These are listed in tiers from north to south and in any given tier from west to east. Within a given state, localities are grouped by counties, which are listed in the same geographic sequence as were the states and provinces (N to S and W to E). Within a given county, localities are arranged from north to south. If two or more localities are at the same latitude the westernmost is listed first. Marginal localities are listed in a separate paragraph at the end of each account. The northernmost marginal locality is listed first and the rest follow in clockwise order. Those records followed by a citation to an authority are of specimens which I have not personally examined. Marginal records are shown by dots on the range maps. Marginal records which cannot be shown on the maps because of undue crowding are listed in Italic type.
To persons in charge of the collections listed above I am deeply indebted. Without their generous cooperation in allowing me to examine specimens in their care this study would not have been possible. Appreciated suggestions in the course of the work have been received from Professors Rollin H. Baker, A. Byron Leonard, R. C. Moore, Robert W. Wilson, and H. B. Tordoff, and many of my fellow students. Mr. Victor Hogg gave helpful suggestions on the preparation of the illustrations. My wife, Muriel Findley, devoted many hoursto secretarial work and typing of manuscript. Finally I am grateful to Professor E. Raymond Hall for guidance in the study and for assistance in preparing the manuscript. During the course of the study I received support from the University of Kansas Endowment Association, from the Office of Naval Research, and from the National Science Foundation.
NON-GEOGRAPHIC VARIATION
Non-geographic variation, that is to say, variation within a single population of shrews, consists of variation owing to age and normal individual variation. InSorexI have detected no significant secondary sexual differences between males and females; accordingly the two sexes are here considered together.
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. Pruitt has recently (1954) noted these same cranial differences in specimens ofSorex cinereusof different ages.
Several students of American shrews, notably Pearson (1945) onBlarina, Hamilton (1940) onSorex fumeus, and Conaway (1952) onSorex palustris, have shown that young are born in spring and summer, usually reach sexual maturity the following spring, and rarely survive through, or even to, a second winter. The result is that collections made, as most of them are, in spring and summer, contain two age classes, first year and second year animals. 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 theSorex vagransgroup. Separation of the two age classes in an August-taken series ofSorex vagransfrom coastal Washington is shown infigure 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. 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.
CHARACTERS OF TAXONOMIC WORTH
Within theSorex vagranscomplex, the only characters of taxonomic significance that I have detected are in size and color. It is true that cranial proportions, such as relative size of rostrum, may change from population to population, but these proportions seem to me to be dependent upon actual size of the individual shrew as I shall elsewhere point out. Of the cranial measurements here employed, palatal length and least interorbital breadth are the most significant and useful. Color in theS. vagransgroup seems to be in Orange and Cadmium Yellow, colors 15 and 17 of Ridgway (1912). No specimens actually possess these pure colors, but most colors in these shrews are seen to be derived from the two mentioned by admixture of black and/or neutral gray. In color designations an increase in neutral gray is indicated by an increased number of prime signs ( ´ ), whereas increase in black is indicated by progressive characters of the Roman alphabet (i,k,m). Thus, 17´´kis grayer than 17´kand 17´´mis blacker than 17´´k. In subspecific diagnoses in this report, color and size, and sometimes relative size, are the characters usually mentioned.
Fig. 3.Two measurements (in millimeters) reflecting tooth-wear plotted against one another. First year and second year individuals ofSorex vagrans vagrans, all taken in August at Willapa Bay, Washington, are completely separated. Open circles represent teeth of second year shrews; solid circles represent teeth of first year shrews.
Fig. 3.Two measurements (in millimeters) reflecting tooth-wear plotted against one another. First year and second year individuals ofSorex vagrans vagrans, all taken in August at Willapa Bay, Washington, are completely separated. Open circles represent teeth of second year shrews; solid circles represent teeth of first year shrews.
PELAGE CHANGE
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. This was noted by Dalquest (1944) who assumed that the color change resulted from molt although he was unable to detect actual replacement of hairs. Summer pelage usually comes in first on the back or head and moves posteriorly and laterally. Time of molt depends on latitude and altitude. Summer pelage may appear fairly late in the season and may account for the anomalous midsummer molt noted by Dalquest. Fresh pelages of summer and winter are best seen in first year animals and are less variable than are worn pelages and hence are used as the basis of color descriptions.
GEOGRAPHIC DISTRIBUTION AND VARIATION
Pacific Coastal Section
The largest shrews of thevagransgroup (large in all dimensions) occur in the coastal forests of northern California and of Oregon. Those shrews are reddish, large-skulled, large-toothed, and have rostra that are large in proportion to the size of the skull as a whole. The very largest of these shrews live along the coast of northwestern California. To the southward they are somewhat smaller, and at successively more northern localities, to as far as southwestern British Columbia, they are likewise progressively smaller and also somewhat less reddish. The relative size of the rostrum decreases with the decrease in size of the skull; consequently smaller shrews have relatively smaller rostra (seefig. 4). In addition the zygomatic ridge of the squamosal decreases in relative size with decrease in actual size of the skull. Thus, these features change in a clinal fashion as one proceeds from, say, Humboldt County, California, northward to Astoria, Oregon.
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. At placesalong 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 ofSorex vagransto show relative increase in size of rostrum with actual increase in size of skull.
Fig. 4.Condylobasal length (in millimeters) plotted against palatal index (palatal length/condylobasal length × 100) in several subspecies ofSorex vagransto 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 thevagransgroup 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 thevagransgroup on the Pacific coast were divided into three species by H. H. T. Jackson in his revision of the North AmericanSorexin 1928. The large reddish shrews of the coast of California and southern Oregon were calledS. pacificus. The somewhat smaller ones from the coast of central Oregon were calledS. yaquinae. Still smaller shrews from northwestern Oregon and from the rest of the Pacific coast north into Alaska were calledS. 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 thevagransshrew 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 thevagransshrew 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 thevagransshrew 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 ofS. vagransfrom 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.
As mentioned above, the shrews from east of the continental divide in Montana are smaller than those of the other mountains of the state, and it is upon such small animals that the nameSorex vagranshas been based in this area. It is clear, however, that these smaller animals intergrade with the larger shrews of the more western mountains. The small size might be an adaptation to the lesser precipitation and harsher continental climate east of the continental divide in Montana.
Great Basin and Columbia Plateau Section
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. Those from nearest the Rockies are sometimes slightly larger and those from some places in Nevada are slightly paler than the average. This small reddish shrew is found all the way to the Pacific coast of California, Oregon, and Washington. In these coastal areas it is somewhat darker and sometimes a trifle larger than elsewhere. It intergrades with a somewhat larger, grayer shrew in the Sierra Nevada of California. Along the Wasatch front in Utah, this Great Basin shrew intergrades with the larger, grayer shrew of the Rockies. Owing to the abrupt change in elevation, the zone of intergradation is rather narrow horizontally. In the latitude of Salt Lake City, populations of intergrades occur at between 8,700 and 9,000 feet elevation. The lowland shrew occurs in the eastern part of the Snake River Plains, and along the valleys of the Bear and Salt rivers into Wyoming. Along the northern edge of the Snake River Plains and on the western edge of the mountains of central Idaho the transition from lowland to montane habitats is abrupt and in consequence the zone of contact between small and large shrews is narrow. In northern Idaho and northwestern Montana the transition from lowland to highland is more gradual. Tributaries of the Columbia River system, especially the Clark Fork, provide a path for movement of lowland forms into intermontane basins of western Montana. In addition, the vegetational zones are found at lower elevations, and there are boreal forests in the lowlands rather than only inthe mountains as is the case in Utah and Colorado. In this area, therefore, the zone of intergradation between the smaller lowland shrew and the larger montane shrew is more gradual and gradually intergrading populations are found over a relatively large area. This has been well demonstrated for northwestern Montana by Clothier (1950). In southern British Columbia and northern Washington this shrew in the mountains is large and in the intermontane valleys is small. There is extensive interdigitation of valleys and mountain ranges, and, consequently, of life-zones in this region. In a few places, recognizably distinct populations of the vagrant shrew occur within a few miles of one another, but in other places there are populations of intergrades. West of the Cascades no evidence of intergradation has been found and the two kinds occur almost side by side and maintain their distinctness.
These Great Basin shrews dwell in hydrosere communities as do their Rocky Mountain counterparts. In this arid region such a habitat obviously is the only one habitable for a shrew of thevagransgroup. These shrews often maintain their predilection for such habitats when they reach the Pacific coast, and are commonly found in such places as coastal marshes, marshy meadows, and streamsides, while the woodlands are inhabited by other species.
These small shrews of the Great Basin and the small vagrant shrews of the Pacific Coast were calledSorex vagransby Jackson.
Summary of Geographic Variation
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. Moving inland from the coast the shrews become markedly smaller in Alaska and British Columbia. The smaller inland and montane form occurs south through the Rocky Mountains, becoming slightly smaller in central Montana, slightly larger in southeastern New Mexico, and slightly smaller in western New Mexico and in Arizona. This montane form intergrades with a smaller more reddish Great Basin shrew, the zone of intergradation roughly following the western slope of the Rocky Mountains. The Great Basin shrew occurs westward to the Pacific Coast; there the Great Basin shrew occurs with, although in part it is ecologically separated from, the large reddish coastal shrews.
There seems to be an intergrading chain of subspecies of onespecies, the end members of which (the small Great Basin form and the large coastal form) are so different in size and ecological niche that they are able to coexist without interbreeding. In southern British Columbia the morphological differences are not so marked as farther south along the Pacific Coast. 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 oldest name applied to a shrew of the group under consideration isSorex vagransBaird, 1858, the type locality of which is Willapa Bay, Pacific County, Washington. The name applies to the small vagrant shrew of this area, rather than to the larger forest dweller which has been known asSorex obscurus. The nameS. vagrans, in the specific sense, must therefore apply to all the shrews discussed which have heretofore been known by the namesS. pacificus,S. yaquinae,S. obscurus, andS. vagrans.
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 ofSorex vagransreally do coexist; specimens of the overlapping subspecies have been taken together at the same localities from California to British Columbia. I have taken a specimen ofS. v. vagransand several ofS. v. setosusin the same woodlot at Fort Lewis, Pierce County, Washington. Two subspecies of deer,Odocoileus hemionus, in the Sierra Nevada of California, occur together over a sizeable area but for only a part of each year that does not include the breeding season (Cowan, 1936:156-157). In the deer mouse,Peromyscus maniculatus, the geographic ranges of several pairs of subspecies meet at certain places without intergradation of the two kinds. In these instances well marked ecological differences exist between the subspecies involved. In western Washington, for example, the geographic range of the lowland subspecies,P. m. austerus, interdigitates to the east and west with the range of the montane and coniferous forest-inhabiting subspecies,P. m. oreas, and the two kinds have not been shown to intergrade.Peromyscus maniculatus artemesiaeandP. m. osgoodicome together without interbreeding in Glacier National Park, Montana.P. m. artemesiaeis almost entirely a forest-dwelling subspecies, whereasosgoodiis an inhabitant of open country. The two kinds do not actually occur together ecologically although they occur together in buildings at the edge of the woods (A. Murie, 1933:4-5).
Fig. 5.Probable present geographic distribution ofSorex vagrans. The range ofS. v. vagransand its derivativesS. v. vancouverensis,S. v. halicoetes, andS. v. paludivagus, is shown by lines slanting in a different direction than those which mark the range of all the other subspecies ofS. vagrans. The region in whichS. v. vagransoccurs together with other subspecies ofS. vagransis shown by the superposition of one pattern upon the other.
Fig. 5.Probable present geographic distribution ofSorex vagrans. The range ofS. v. vagransand its derivativesS. v. vancouverensis,S. v. halicoetes, andS. v. paludivagus, is shown by lines slanting in a different direction than those which mark the range of all the other subspecies ofS. vagrans. The region in whichS. v. vagransoccurs together with other subspecies ofS. vagransis shown by the superposition of one pattern upon the other.
Cases of sympatric existence of two subspecies of one species are known in birds and in reptiles. Notable examples are in the gull,Larus argentatus(Mayr, 1940), in the Old World warbler,Phylloscopus trochiloides(Ticehurst, 1938), and in the great titmouse,Parus major(Rensch, 1933), of the Old World. In the first species the two end-members, the herring gull and the lesser black-backed gull, occur together over an extensive region from northern Europe and the British Isles throughout Fennoscandia. Fitch (1940) described a rassenkreis with overlapping subspecies in the garter snakeThamnophis ordinoides.
The geographic distribution of the speciesSorex vagransis shown in figure 5. The geographic range of the Great Basin subspecies is shown by a different pattern of lines than the other subspecies ofS. vagrans. In the region in which the geographic range of the Great Basin subspecies overlaps those of the subspecies of the Pacific Coast, the pattern of shading for the Great Basin subspecies is superimposed on the patterns for the other subspecies.
ORIGIN OF THESOREX VAGRANSRASSENKREIS
The distribution of the speciesSorex vagransand that of its immediate ancestors obviously has not always been the same; during glacial ages much of the present range of the species in Canada and in some of the higher mountains of the United States was covered with ice and not available to the shrew. Furthermore, large areas that are now too hot and dry to permit the existence ofS. vagranswere at one time habitable. If we are to speculate on the manner in which theSorex vagransrassenkreis originated we must inquire into the nature and extent of these climatic changes.
The most recent epoch of geological time, the Pleistocene, is known to have been divided into a series of alternating glacial and interglacial ages. During the glacial ages continental and montane glaciers are judged to have covered much of Canada and the northern United States. Concurrently the major storm tracks of the west probably were shifted southward; in any event much of the now arid intermontane west was much better watered than it is today.The increased precipitation, and probably glacial meltwater, formed large lakes in the closed basins of the Great Basin. There were boreal forests at lower elevations than there are today in comparable latitudes and continuous boreal habitat probably connected many of the isolated mountain ranges of the southwest. That probability is supported by the presence of boreal animals and plants on manyof these isolated ranges today. A boreal tree squirrel, such asTamiasciurus, could hardly be suspected of crossing a treeless, intermontane desert valley, miles wide.
Figs. 6a-6f. Fig. 6a.Sorex vagranspacificus, 1 mi. N Trinidad, Humboldt Co., California, FC 1442. Fig. 6b.S. v. yaquinae, Newport, Lincoln Co., Oregon, AW 707. Fig. 6c.S. v. yaquinae(nearbairdi), McKenzie Bridge, Lane Co., Oregon, AW 82. Fig. 6d.S. v. setosus, Reflection Lake, Jefferson Co., Washington, CMNH 4275. Fig. 6e.S. v. obscurus, 10 mi. SSW Leadore, Lemhi Co., Idaho, FC 1499. Fig. 6f.S. v. vagrans, Baker Creek, White Pine Co., Nevada, 88042 (after Hall, 1946:113).
Figs. 6a-6f. Fig. 6a.Sorex vagranspacificus, 1 mi. N Trinidad, Humboldt Co., California, FC 1442. Fig. 6b.S. v. yaquinae, Newport, Lincoln Co., Oregon, AW 707. Fig. 6c.S. v. yaquinae(nearbairdi), McKenzie Bridge, Lane Co., Oregon, AW 82. Fig. 6d.S. v. setosus, Reflection Lake, Jefferson Co., Washington, CMNH 4275. Fig. 6e.S. v. obscurus, 10 mi. SSW Leadore, Lemhi Co., Idaho, FC 1499. Fig. 6f.S. v. vagrans, Baker Creek, White Pine Co., Nevada, 88042 (after Hall, 1946:113).
Interglacial ages were characterized by warmth and aridity as compared to the glacial ages. Glaciers retreated or disappeared, boreal forests became montane in much of the United States, and the lakes in the Great Basin were reduced or disappeared. One can envision that during such times boreal mammals were isolated, their geographic ranges were restricted, and Sonoran mammals expanded their ranges.
Evidence is more extensive concerning the number and extent of glacial ages in the eastern than in the western part of North America. This evidence suggests a division of the Pleistocene into four glacial ages and four interglacial ages, the fourth interglacial age corresponding to the present time. More information is available about the Wisconsinan, or last, glacial age, than about the earlier ones, because the last glaciation in many montane areas destroyed evidence of earlier glaciations. The names of currently recognized glacial and interglacial ages of the Pleistocene are listed below. The names of interglacial ages are in Italic type.
WisconsinanSangamonianIllinoianYarmouthianKansanAftonianNebraskan
We may think of these ages as an alternating series of cool moist and warm dry periods during which boreal mammals, and other organisms, alternately moved southward (disappearing in the glaciated regions) and northward into previously glaciated areas (while disappearing from southern areas except on isolated mountain ranges).Sorex vagransprobably followed this pattern of movement and now is restricted to forested or well-watered places.
One possible series of events culminating in the formation of theSorex vagransrassenkreis may be thought of as having begun during the Illinoian age. With much of Canada, and perhaps also many areas in the Rockies, Cascades, and the Sierra Nevada covered with glacial ice, the shrew-stock ancestral toSorex vagransmay well have occupied a more or less continuous range over the Colorado Plateau, the Columbian Plateau, the Great Basin, and in the forests of the Pacific Coast (as well as over part of eastern United States, as willbe explained beyond; seefig. 7). At that time the species probably was a continuously interbreeding unit.