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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
Volume 9, No. 1, pp. 1-68, figures 1-18
-------------------- December 10, 1955 ---------------------
Speciation of the Wandering Shrew
BY
JAMES S. FINDLEY
UNIVERSITY OF KANSAS
LAWRENCE
1955
UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
Editors: E. Raymond Hall, Chairman, A. Byron Leonard,
Robert W. Wilson
Volume 9, No. 1, pp. 1-68, figures 1-18
Published December 10, 1955
UNIVERSITY OF KANSAS
Lawrence, Kansas
PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1955
[Illustration: union label]
25-7903
Speciation of the Wandering Shrew
BY
JAMES S. FINDLEY
CONTENTS
PAGE
INTRODUCTION 4
MATERIALS METHODS AND ACKNOWLEDGMENTS 4
NON-GEOGRAPHIC VARIATION 7
CHARACTERS OF TAXONOMIC WORTH 8
PELAGE CHANGE 9
GEOGRAPHIC DISTRIBUTION AND VARIATION 9
Pacific Coastal Section 9
Inland Montane Section 11
Great Basin and Columbia Plateau Section 12
Summary of Geographic Variation 13
ORIGIN OF THE _Sorex vagrans_ RASSENKREIS 16
RELATIONSHIPS WITH OTHER SPECIES 26
CONCLUSIONS 60
TABLE OF MEASUREMENTS 62
LITERATURE CITED 66
FIGURES
FIGS. 1-2.--CRANIAL MEASUREMENTS 5
FIG. 3.--GRAPH ILLUSTRATING WEAR OF TEETH 8
FIG. 4.--GRAPH ILLUSTRATING HETEROGONIC GROWTH OF ROSTRUM 10
FIG. 5.--PRESENT GEOGRAPHIC DISTRIBUTION OF _Sorex vagrans_ 15
FIG. 6.--SKULLS OF _Sorex vagrans_ 17
FIGS. 7-10.--PAST GEOGRAPHIC DISTRIBUTION OF SHREWS 19-20-22-27
FIGS. 11, 12.--MEDIAL VIEW OF LOWER JAWS OF TWO SHREWS 30
FIGS. 13, 14.--SECOND UNICUSPID TEETH OF SHREWS 30
FIG. 15.--DIAGRAM OF PROBABLE PHYLOGENY OF SHREWS 32
FIGS. 16-18.--GEOGRAPHIC DISTRIBUTION OF SUBSPECIES 33-40-53
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 assigned 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 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 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').
[Illustration: FIGS. 1 AND 2. Showing where certain cranial
measurements were taken. x 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 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 hours to
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. In _Sorex_ I 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 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 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 the _Sorex vagrans_ group.
Separation of the two age classes in an August-taken series of _Sorex
vagrans_ from coastal Washington is shown in figure 3, in which two
tooth-measurements that are dependent upon wear are plotted against
one another.
First year animals are more abundant in collections than are second
year animals. 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.
[Illustration: 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 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 the _vagrans_ group (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 (see fig. 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 places along the coast successively
farther north of southwestern British Columbia the shrews become
larger again, the largest individuals being those from near Wrangell,
Alaska. From that place northwesterly along the coast of Alaska, size
decreases again.
[Illustration: FIG. 4. Condylobasal length (in millimeters)
plotted against palatal index (palatal length/condylobasal
length x 100) in several subspecies of _Sorex vagrans_ to 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 the _vagrans_ group
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 the _vagrans_ group on the Pacific coast were
divided into three species by H. H. T. Jackson in his revision of the
North American _Sorex_ in 1928. The large reddish shrews of the coast
of California and southern Oregon were called _S. pacificus_. The
somewhat smaller ones from the coast of central Oregon were called _S.
yaquinae_. Still smaller shrews from northwestern Oregon and from the
rest of the Pacific coast north into Alaska were called _S. 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 the
_vagrans_ shrew decreases rapidly. Specimens from western Alaska,
central Alaska, and the interior of British Columbia are uniformly
smaller than coastal specimens. In addition the red of the hair is
masked more by neutral gray than by black with the result that the
pelage is grayish rather than brownish or reddish. Shrews of this
general appearance are found southward through the Rocky Mountain
chain to Colorado and New Mexico. On the more or less isolated
mountain ranges of Montana east of the continental divide the
_vagrans_ shrew 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 the _vagrans_ shrew 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 of _S.
vagrans_ from 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 name _Sorex vagrans_ has
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 in the
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 the _vagrans_
group. 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 called _Sorex vagrans_ by 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 one species,
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 is
_Sorex vagrans_ Baird, 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 as _Sorex obscurus_. The name _S. vagrans_, in the specific
sense, must therefore apply to all the shrews discussed which have
heretofore been known by the names _S. pacificus_, _S. yaquinae_, _S.
obscurus_, and _S. 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 of _Sorex vagrans_ really 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 of _S. v. vagrans_ and several of _S. v.
setosus_ in 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 artemesiae_ and
_P. m. osgoodi_ come together without interbreeding in Glacier
National Park, Montana. _P. m. artemesiae_ is almost entirely a
forest-dwelling subspecies, whereas _osgoodi_ is 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).
[Illustration: FIG. 5. Probable present geographic distribution
of _Sorex vagrans_. The range of _S. v. vagrans_ and its
derivatives _S. v. vancouverensis_, _S. v. halicoetes_, and
_S. v. paludivagus_, is shown by lines slanting in a different
direction than those which mark the range of all the other
subspecies of _S. vagrans_. The region in which _S. v. vagrans_
occurs together with other subspecies of _S. vagrans_ is 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 snake _Thamnophis ordinoides_.
The geographic distribution of the species _Sorex vagrans_ is shown in
figure 5. The geographic range of the Great Basin subspecies is shown
by a different pattern of lines than the other subspecies of _S.
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 THE _SOREX VAGRANS_ RASSENKREIS
The distribution of the species _Sorex vagrans_ and 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 of_ S. vagrans_ were at one
time habitable. If we are to speculate on the manner in which the
_Sorex vagrans_ rassenkreis 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 many of
these isolated ranges today. A boreal tree squirrel, such as
_Tamiasciurus_, could hardly be suspected of crossing a treeless,
intermontane desert valley, miles wide.
[Illustration: FIGS. 6_a_-6_f_. Fig. 6_a_. _Sorex vagrans
pacificus_, 1 mi. N Trinidad, Humboldt Co., California,
FC 1442. Fig. 6_b_. _S. v. yaquinae_, Newport, Lincoln Co.,
Oregon, AW 707. Fig. 6_c_. _S. v. yaquinae_ (near _bairdi_),
McKenzie Bridge, Lane Co., Oregon, AW 82. Fig. 6_d_. _S. v.
setosus_, Reflection Lake, Jefferson Co., Washington, CMNH
4275. Fig. 6_e_. _S. v. obscurus_, 10 mi. SSW Leadore, Lemhi
Co., Idaho, FC 1499. Fig. 6_f_. _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.
Wisconsinan
_Sangamonian_
Illinoian
_Yarmouthian_
Kansan
_Aftonian_
Nebraskan
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 vagrans_
probably 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 the
_Sorex vagrans_ rassenkreis 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 to _Sorex vagrans_ may 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 will be explained
beyond; see fig. 7). At that time the species probably was a
continuously interbreeding unit.
[Illustration: FIG. 7. Possible distribution in Illinoian
(inset) and Sangamonian times of the ancestor of the _Sorex
vagrans-ornatus-longirostris-veraepacis_ complex. Approximate
southern boundary of Illinoian glaciation marked by heavy
line.]
In the ensuing Sangamonian interglacial age all glaciers retreated or
disappeared thereby opening up extensive areas in the north and in the
higher mountains which were occupied by a boreal fauna, including _S.
vagrans_. Concurrently the Great Basin, and probably also much of the
Columbian Plateau, became dry, and desert conditions developed,
perhaps much as they are today. Increasing aridity eliminated shrew
habitat in most places between the Rocky Mountains and the Sierra
Nevada-Cascade mountain chain with the result that the geographic
range of the species resembled an inverted "U", one arm lying along
the Rocky Mountains and the other along the Cascade-Sierra Nevada
axis; the connection between the two arms was in British Columbia (see
fig. 7). At present _Sorex vagrans_ does occur in isolated places in
the Great Basin, but its existence there is tenuous and seemingly
dependent upon the occurrence of permanent water such as Ruby Lake and
Reese River. With such an arrangement as this it can readily be seen
that gene flow between the eastern and western arms of the "U" would
be greatly reduced by distance; consequently differentiation between
the two might be expected.
[Illustration: FIG. 8. Possible distribution of _Sorex vagrans_
at two different times in the Wisconsinan Age. Left, early
Wisconsinan; right, mid-Wisconsinan.]
Wisconsinan glaciation again rendered Canada uninhabitable, and it is
quite possible that extensive areas in the Rocky Mountains, the
Cascades and the Sierra Nevada were heavily glaciated. With the
elimination of the northern part of the "U", the eastern and western
arms became isolated, if not by the width of the Columbian Plateau at
least by the glaciated Cascade Mountains. At the same time extensive
areas on the Colorado Plateau and much of the area south to the
Mexican highlands were again occupied by the species. Finally the
Great Basin, again being well-watered, provided suitable habitat for,
and was reoccupied by, _Sorex vagrans_ (see fig. 8). This reoccupation
of the Great Basin took place probably from the Colorado Plateau and
mountains of Arizona and Utah, since the present day shrews of the
species _S. vagrans_ in the Great Basin closely resemble Rocky
Mountain shrews but differ markedly from the large endemic subspecies
of the Pacific Coast.
Finally, with the waning of Wisconsinan ice, the species again was
able to occupy northern and montane areas as it had during Sangamonian
times. Again dessication of the Great Basin caused drastic restriction
of shrew habitat. The small, marsh-dwelling kind of wandering shrew
which had developed there around the lakes of Wisconsinan time
occupied suitable habitat all the way to the Pacific coast where its
range came into contact with that of the western arm of the
Sangamonian "U."-pattern of shrew distribution (see fig. 9). The
animals of this western segment and the new arrivals from the east
were by this time so different from one another that the two kinds
lived in the same areas without interbreeding. The descendants of the
original western arm now are known as _Sorex vagrans sonomae_, _S. v.
pacificus_, _S. v. yaquinae_, and _S. v. bairdi_. The newcomers from
the east are known as _S. v. vagrans_, _S. v. halicoetes_, _S. v.
paludivagus_ and _S. v. vancouverensis_.
In addition to occupying the Pacific Coast from San Francisco Bay
north to the Fraser Delta, the Great Basin subspecies populated the
Columbia Plateau and the western foothills of the central and northern
Rockies. By so doing that subspecies came into secondary contact with
its own parent stock with which it was still in reproductive
continuity in Utah. In some places in British Columbia differentiation
between the two kinds had proceeded to such an extent that some
reproductive isolation was effected, but in many other places the two
interbred. The Rocky Mountain form spread north and west and occupied
the Cascades and coastal lowlands in southwestern British Columbia and
in Washington. Here the differentiation between the Rocky Mountain
subspecies and the Great Basin subspecies was great enough to cause
complete reproductive isolation.
[Illustration: FIG. 9. Probable changes in the distribution of
_Sorex vagrans_ concurrent with and following the dissipation
of Wisconsinan ice. Dark arrows in Washington, Idaho, Oregon,
and California, shows_ S. v. vagrans_.]
Deglaciation of the Sierra Nevada opened it up for reoccupation from
the east by _Sorex vagrans_ of the Great Basin. In response to the
montane environment the subspecies _obscuroides_, resembling the
subspecies _obscurus_ of the Rockies, developed.
Desiccation of the intermontane parts of New Mexico, Arizona, and
Chihuahua, left "marooned" populations of _Sorex vagrans_ on suitable
mountain ranges. In this way _Sorex vagrans orizabae_ may have been
isolated in southern Mexico. The isolated populations of Arizona and
New Mexico differentiated _in situ_ into the subspecies _monticola_
and _neomexicanus_.
Western Canada and Alaska were populated by shrews which originated in
the habitable parts of the Rocky Mountains and Colorado Plateau during
Wisconsinan time (as opposed to shrews originating, as subspecies, in
the Great Basin or on the Pacific Coast). These shrews differentiated
into the currently recognized subspecies of the west coast and coastal
islands of British Columbia and Alaska in response to the different
environments in these places, many of which were isolated; the
subspecies _isolatus_, _mixtus_, _setosus_, _longicauda_, _elassodon_,
_prevostensis_, _malitiosus_, and _alaskensis_ are thought to have
originated in this fashion after the areas now occupied by them were
freed of Wisconsinan ice.
This group of shrews from the Rocky Mountains probably came into
contact with the Pacific coastal segment of the species somewhere in
northwestern Oregon. The clinal decrease in size from _S. v.
pacificus_ to _S. v. setosus_ seems steepest in this area. Upon the
establishment of this contact reproductive continuity was resumed,
probably because the temporal separation of the two stocks involved
was not so great as, say, that between _S. v. vagrans_ and _S. v.
pacificus_, and in addition the morphological differentiation was not
so great.
On the eastern side of the Rockies the montane stock moved
northeastward, occupying suitable territory opened up by the
dissolution of the Laurentide ice sheet. Still later changes in the
character of the northern plains owing to desiccation divided the
range of the species and isolated _S. v. soperi_ in Manitoba and
central Saskatchewan and a population of _S. v. obscurus_, in the
Cypress Hills. A number of semi-isolated stocks in central Montana
became differentiated as a recognizable subspecies there.
A number of other boreal mammals have geographic ranges which resemble
that of _Sorex vagrans_, except that the geographic ranges of
subspecies do not overlap. Because of the general similarities of
these geographic ranges, it is pertinent to examine the reasons
suggested by students to account for the present geographic
distributions of some of these other boreal species.
The red squirrel genus, _Tamiasciurus_, has a Rocky Mountain (and
northern coniferous forest) species, _T. hudsonicus_, that occurs all
along the Rocky Mountain chain and northward into Alaska. In the
Cascade Mountains of Washington and British Columbia this species
meets the range of a well marked western species, _T. douglasii_, with
no evidence of intergradation. Dalquest (1948:86) attributes the
divergence of the two species to separation in a glacial age but feels
that the degree of difference between the two is too great to have all
taken place during the Wisconsinan. Perhaps he has overemphasized the
importance of the differences between the two, but, be that as it may,
it seems that the two kinds differentiated during a glacial age when
they were isolated, perhaps by ice on the Cascades into a coastal
population and an inland population. One difference between the
distribution of the red squirrels and vagrant shrew is that the
squirrel of the Sierra Nevada is the species of the Pacific Coast,
whereas the vagrant shrew of the Sierra Nevada was derived from the
Great Basin population, which in turn was derived from the Rocky
Mountain kind. Red squirrels do not occur on any of the boreal montane
"islands" of Nevada. During the pluvial periods when hydrosere-loving
shrews populated the Great Basin, that region may have been a treeless
grassland. Vagrant shrews, then as now, probably depended on hydrosere
communities, while red squirrels required trees. Therefore the shrews
were able to traverse the Great Basin, while the Sierran red squirrels
were of necessity derived from the coastal population.
The ecological requirements of jumping mice, genus _Zapus_, and the
subspecies of _Sorex vagrans_ that dwell in hydroseres are essentially
similar. The species _Zapus princeps_ lives in the Rocky Mountains,
the Great Basin, the Sierra Nevada, and north to Yukon (Krutzsch,
1954:395). Its geographic range is similar to that of the montane and
basin segments of _S. vagrans_. The species _Z. trinotatus_ occurs
along the Pacific coast and in the Cascades north to southwestern
British Columbia. Its distribution thus coincides in general with that
of the large red coastal subspecies of _S. vagrans_. Krutzsch
(1954:368-369) thought that these two kinds of jumping mice were first
separated by the formation of the Cascade Mountains and the Sierra
Nevada and finally by Pleistocene glaciation. The Sierran jumping
mouse (_Zapus princeps_), as is the Sierran vagrant shrew, is more
closely related to the jumping mouse of the Great Basin and of the
Rocky Mountains than it is to the jumping mouse (_Z. trinotatus_) of
the Pacific Coast, just as the Sierran vagrant shrew is related to the
shrew of the Great Basin and Rocky Mountains. The jumping mouse also
is limited in its distribution by hydrosere communities, not by
forests.
In western North America there are two species of water or marsh
shrews: _Sorex palustris_ and _S. bendiri_. They have been placed in
separate subgenera, but, as pointed out beyond, are closely related
and here are placed in the same subgenus. The species _palustris_ is
found throughout the Rocky Mountains, north into Alaska, across the
Great Basin into the Sierra Nevada, and west to the Pacific coast in
Washington. The species _bendiri_ is found from northwestern
California north along the Pacific coast to southwestern British
Columbia and east to the Cascades. Where the ranges of the two species
overlap in western Washington they do not interbreed so far as is
known, and are somewhat different in their ecology, _bendiri_ being a
lowland, and _palustris_ being a montane, species. The two species
probably were separated in a glacial period as seems to have been the
case with the wandering shrews. Also, the water shrew of the Sierra
Nevada is derived from that of the Great Basin and Rocky Mountains.
_Sorex palustris_ is tied closely in its distribution to hydrosere
communities and is not dependent upon the presence of forests.
Red-backed mice, genus _Clethrionomys_, occur throughout the Rocky
Mountains and west to the Cascades in Washington as the species _C.
gapperi_. The species _C. californicus_ is found along the Pacific
Coast from California north to the Olympic Peninsula. Where the ranges
of the two species meet in Washington they seem not to intergrade. In
some glacial interval these two species may have evolved in the same
manner as has been described for the species of _Zapus_ and those of
_Tamiasciurus_. No _Clethrionomys_ are found in the Sierra Nevada, nor
are red-backed mice found in the boreal islands of the Great Basin. It
is not known why _Clethrionomys californicus_ does not occur in the
Sierra Nevada. Some boreal birds have distributional patterns similar
to those of the mammalian examples cited above. One kind of sapsucker,
_Sphyrapicus varius nuchalis_, occurs in the Rocky Mountains north
into British Columbia and west to the Cascades and Sierra Nevada. A
related kind, _S. varius ruber_, occurs along the Pacific Coast from
California north into British Columbia. Recently Howell (1952) has
shown that some intergradation takes place between _ruber_ and
_nuchalis_ in Washington and British Columbia, although they do not
intergrade freely. Previously the two kinds were thought not to
intergrade and were regarded as two species. The two kinds intergrade
also in northeastern California, although in that state _S. v.
daggeti_, rather than _S. v. ruber_, is involved in the
intergradation. Howell considered the two kinds to be conspecific with
one another as well as with the eastern _S. varius_. He attributed a
measure of the distinctness of _nuchalis_ and _ruber_ to their
separation during a glacial period, but felt that the separation was
much older than Wisconsinan. Whatever the time of separation, the
pattern seems clear: _nuchalis_ and _ruber_ (as well as _varius_) were
separated into montane, coastal, and eastern segments respectively,
probably by glaciation (it seems to me in the Pleistocene), and have
since re-established contact with one another.