Speciation of the Wandering Shrew Part 1

Speciation of the Wandering Shrew.

by James S. Findley.

INTRODUCTION

The purpose of this report is to make clear the biological relationships between the shrews of the _Sorex vagrans-obscurus_ "species group." This group as defined by H. H. T. Jackson (1928:101) included the species _Sorex vagrans_, _S. obscurus_, _S. pacificus_, _S. yaquinae_, and _S.

durangae_. The last mentioned species has been shown (Findley, 1955:617) to belong to another species group. _Sorex milleri_, also a.s.signed to this group by Jackson (1947:131), seems to have its affinities with the _cinereus_ group as will be explained beyond. The position of the _vagrans_ group in relationship to other members of the genus will be discussed.

Of this group, the species that was named first was _Sorex vagrans_ Baird, 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 monticola_ to _Sorex obscurus obscurus_ in the Rocky Mountains, (2) _Sorex pacificus_, _S. yaquinae_, and _S. obscurus_ to one another on the Pacific Coast, and (3) _S. o. obscurus_ to _S. v. amoenus_ in California. Few studies have been made of these relationships. Clothier (1950) studied _S. v. monticola_ and _S. o.

obscurus_ in 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 the _vagrans-obscurus_ groups have avoided the problems in one way or another. Recently Rudd (1953) has examined the relationships of _S. vagrans_ to _S. ornatus_.

MATERIALS METHODS AND ACKNOWLEDGMENTS

Approximately 3,465 museum study skins and skulls were studied. Most of these were a.s.sembled at the University of Kansas Museum of Natural History, but some were examined in other inst.i.tutions.

Specimens were grouped by geographic origin, age, and s.e.x. Studies of the role of age and s.e.x in variation were made. Because it was discovered that secondary s.e.xual 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 2 show 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').

[Ill.u.s.tration: FIGS. 1 AND 2. Showing where certain cranial measurements were taken. 3-1/2. (Based on _Sorex vagrans obscurus_, from Stonehouse Creek, 5-1/2 mi., W junction of Stonehouse Creek and Kelsall River, British Columbia, [Female], 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:

AMNH American Museum of Natural History CM Carnegie Museum ChM Chicago Museum of Natural History CMNH Cleveland Museum of Natural History FC Collection of James S. Findley HC Collection of Robert Holdenreid SGJ Collection of Stanley G. Jewett CDS Collection of Charles D. Snow AW Collection of Alex Walker NMC National Museum of Canada OSC Oregon State College PMBC British Columbia Provincial Museum of Natural History SD San Diego Natural History Museum BS United States Biological Surveys Collection USNM United States National Museum UM University of Michigan Museum of Zoology OU University of Oregon Museum of Natural History UU University of Utah Museum of Zoology WSC Washington State College, Charles R. Conner Museum

In nature, the subspecies of _Sorex vagrans_ form 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 lat.i.tude 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 ill.u.s.trations. My wife, Muriel Findley, devoted many hours to secretarial work and typing of ma.n.u.script. Finally I am grateful to Professor E. Raymond Hall for guidance in the study and for a.s.sistance in preparing the ma.n.u.script. During the course of the study I received support from the University of Kansas Endowment a.s.sociation, 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. In _Sorex_ I have detected no significant secondary s.e.xual differences between males and females; accordingly the two s.e.xes are here considered together.

Variation with age must be considered in order to a.s.semble 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 of _Sorex cinereus_ of different ages.

Several students of American shrews, notably Pearson (1945) on _Blarina_, Hamilton (1940) on _Sorex fumeus_, and Conaway (1952) on _Sorex pal.u.s.tris_, have shown that young are born in spring and summer, usually reach s.e.xual 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 cla.s.ses, first year and second year animals. These two age cla.s.ses are readily separable on the basis of differences in the skull as well as on the decreased p.u.b.escence 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 cla.s.ses 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. 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 the _Sorex vagrans_ complex, 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 the _S. vagrans_ group 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"_k_ is grayer than 17'_k_ and 17"_m_ is blacker than 17"_k_. In subspecific diagnoses in this report, color and size, and sometimes relative size, are the characters usually mentioned.

[Ill.u.s.tration: FIG. 3. Two measurements (in millimeters) reflecting tooth-wear plotted against one another. First year and second year individuals of _Sorex 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 a.s.sumed 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 lat.i.tude and alt.i.tude. 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