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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
Volume 7, No. 4, pp. 349-472, 47 figures in text, 4 tables
April 21, 1954
North American Jumping Mice
(Genus Zapus)
BY
PHILIP H. KRUTZSCH
UNIVERSITY OF KANSAS
LAWRENCE
1954
UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
Editors: E. Raymond Hall, Chairman, A. Byron Leonard,
Robert W. Wilson
Volume 7, No. 4, pp. 349-472, 47 figures in text, 4 tables
Published April 21, 1954
UNIVERSITY OF KANSAS
LAWRENCE, KANSAS
PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1954
25-1128
North American Jumping Mice (Genus Zapus)
by
Philip H. Krutzsch
CONTENTS
PAGE
Introduction 351
Materials, Methods, and Acknowledgments 352
Paleontology of the Genus 355
Relationships, Distribution, and Speciation 356
Annotated List of Specific and Subspecific Names 369
Characters of Taxonomic Worth 371
Nongeographic Variation 376
Check-List of the Species and Subspecies of the Genus Zapus 382
Genus Zapus 382
Artificial Key to the Species of the Genus Zapus 384
Systematic Accounts of Species and Subspecies 385
_Zapus trinotatus_ 385
_Zapus princeps_ 394
_Zapus hudsonius_ 420
Tables of Measurements 455
Literature Cited 466
INTRODUCTION
The jumping mice (Genus _Zapus_) are widely distributed over northern
North America, occurring as far north as the Arctic Circle and as far
south as Georgia, Missouri, Oklahoma, New Mexico, Arizona, and central
California. In some years these small rodents are locally common in
moist places that are either grassy or weedy; the jumping mice are
notable for the much enlarged hind legs and the exceptionally long tail.
Members of the Genus as a whole have received no serious comprehensive
taxonomic attention in the 54 years since Preble's (1899) revisionary
work. In this time 15 new names have been proposed, mostly for
subspecies, and only a few attempts have been made at grouping related
named kinds.
In the present account it is aimed to record what is known concerning
geographic distribution, taxonomically significant characters, and
interrelationships of the known kinds as well as to provide means for
recognizing the species and subspecies in the genus. In addition,
attention is given to the probable center of origin of the subfamily
Zapodinae and to the relationships and taxonomic positions of the genera
_Zapus_, _Napaeozapus_, and _Eozapus_.
MATERIALS, METHODS, AND ACKNOWLEDGMENTS
The present report is based on a study of approximately 3,600 specimens
that were assembled at the Museum of Natural History of the University
of Kansas or that were examined at other institutions. Most of these
specimens are stuffed skins with skulls separate. Skulls without skins,
skins without skulls, entire skeletons, and separately preserved bacula
are included as a part of the total. Almost every specimen is
accompanied by an attached label, which bears place and date of capture,
name of collector, external measurements, and sex.
Specimens used in the study of geographic variation were arranged by
season of capture and according to geographic location; then they were
segregated as to sex, and, under each sex, by age. Next, individual
variation was measured in comparable samples of like age, sex, season,
and geographic origin. Finally, comparable materials were arranged
geographically in order to determine variations of systematic
significance.
The only external measurements used were total length, length of tail,
and length of hind foot; these measurements were recorded by the
collectors on the labels attached to the skins. Height of the ear was
not used since it was not recorded by many of the collectors.
In order to determine which cranial structures showed the least
individual variation but at the same time showed substantial geographic
variation, a statistical analysis was made of the 30 measurements, of
cranial structures, heretofore used in taxonomic work on _Zapus_. The
following measurements of the skull showed the least individual
variation but showed some geographic variation and therefore, were used
in this study. See figs. 1-3 which show points between which
measurements were taken:
_Occipitonasal length._--From anteriormost projection of nasal
bones to posteriormost projection of supraoccipital bone.
_a_ to _a´_
_Condylobasal length._--Least distance from a line connecting
posteriormost parts of exoccipital condyles to a line connecting
anteriormost projections of premaxillary bones. _b_ to _n_
_Palatal length._--From anterior border of upper incisors to
anteriormost point of postpalatal notch. _b_ to _b´_
_Incisive foramina, length._--From anteriormost point to
posteriormost point of incisive foramina. _c_ to _c´_
_Incisive foramina, breadth._--Greatest distance across incisive
foramina perpendicular to long axis of skull. _f_ to _f´_
_Zygomatic length._--From anteriormost point of zygomatic process
of maxillary to posteriormost point of zygomatic process of
squamosal. _d_ to _d´_
_Zygomatic breadth._--Greatest distance across zygomatic arches of
cranium at right angles to long axis of skull. _j_ to _j´_
_Breadth of inferior ramus of zygomatic process of
maxillary._--Greatest distance across inferior ramus of zygomatic
process of maxillary taken parallel to long axis of skull.
_d_ to _e_
_Palatal breadth at M3._--Greatest distance from inside margin of
alveolus of right M3 to its opposite. _g_ to _g´_
_Palatal breadth at P4._--Same as above except taken at P4.
_g_ to _g´_
_Mastoid breadth._--Greatest distance across mastoid bones
perpendicular to long axis of skull. _h_ to _h´_
_Breadth of braincase._--Greatest distance across braincase taken
perpendicular to long axis of skull. _i_ to _i´_
_Interorbital breadth._--Least distance across top of skull between
orbits. _k_ to _k´_
_Length of maxillary tooth-row._--From anterior border of P4 to
posterior border of M3. _l_ to _l´_
_Breadth of base zygomatic process of squamosal._--Greatest
distance across base of zygomatic process of squamosal taken
parallel to long axis of skull. _m_ to _m´_
[Illustration: FIGS. 1-3. Three views of the skull to show points
between which measurements of the skull were taken. Based on
_Z. t. montanus_, adult, female, No. 22165 KU, Cascade Divide,
6400 ft., Crater Lake Nat'l Park, Klamath County, Oregon. × 4.]
The baculum has a characteristic size and shape according to the
species, and the following significant measurements of the structure
were taken:
_Greatest length._--From posteriormost border of base to anteriormost
point on tip.
_Greatest breadth at base._--Greatest distance across base taken
parallel to long axis of bone.
_Greatest breadth at tip._--Greatest distance across tip taken parallel
to long axis of bone.
In the descriptions of color the capitalized color terms refer to those
in Ridgway (1912). Any color term that does not have the initial letter
capitalized does not refer to any one standard.
In the description of the subspecies the two sexes are treated as one
because no significant secondary sexual variation was found. Only fully
adult specimens of age groups 3 to 5, as defined on pages 377 and 388,
have been considered.
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.
CAS. California Academy of Science.
CM. Carnegie Museum.
Chic. AS. Chicago Academy of Science.
Clev. MNH. Cleveland Museum of Natural History.
LMH. Collection of Lawrence M. Huey.
JKJ. Collection of J. Knox Jones, Jr.
CMNH. Colorado Museum of Natural History.
FM. Chicago Museum of Natural History.
HM. Hastings Museum, Hastings, Nebraska.
ISC. Iowa State College.
MCZ. Museum of Comparative Zoology.
MO. University of Missouri Museum of Zoology.
MVZ. Museum of Vertebrate Zoology, Berkeley, Calif.
NMC. National Museum of Canada.
NGFP. Nebraska Game, Forestation, and Parks Commission.
NCS. North Carolina State College.
OHIO. Ohio Wildlife Research Unit, Ohio State University.
OKLA. Oklahoma Agricultural and Mechanical College.
PM. Provincial Museum of British Columbia.
ROM. Royal Ontario Museum of Zoology.
SDM. San Diego Natural History Museum.
SITC. Southern Illinois Teachers College.
USBS. United States Biological Surveys Collection.
USNM. United States National Museum.
UCM. University of Colorado Museum.
UIM. University of Illinois Museum of Natural History.
UM. University of Michigan Museum of Zoology.
UU. University of Utah Museum of Zoology.
The species are arranged from least to most progressive, and the
subspecies are arranged alphabetically.
The synonymy for each subspecies includes first a citation to the
earliest available name then one citation to each name combination that
has been applied to the subspecies and, finally, any other especially
important references.
Marginal records of occurrence for each subspecies are shown on the maps
by means of hollow circles and these localities are listed in clockwise
order beginning with the northernmost locality. If more than one of
these localities lies on the line of latitude that is northernmost for a
given subspecies the western-most of these is recorded first. Marginal
localities have been cited in a separate paragraph at the end of the
section on specimens examined in the account of a subspecies. Localities
that are not marginal are shown on the maps by solid black circles.
Localities that could not be represented on the distribution map because
of undue crowding or overlapping of symbols are italicized in the lists
of specimens examined and in the lists of marginal records.
The localities of capture of specimens examined are recorded
alphabetically by state or province, and then by county in each state or
province. Within a county the specimens are recorded geographically from
north to south. The word "County" is written out in full when the name
of the county is written on the label of each specimen listed for that
county, but the abbreviation "Co." is used when one specimen or more
here assigned to a given county lacks the name of the county on the
label.
The following account has been made possible only by the kindness and
cooperation of those persons in charge of the collections listed above.
For the privilege of using the specimens in their care I am deeply
grateful, as I am also to Prof. A. Byron Leonard for assistance with
figures 35-37, to Dr. Rufus Thompson for figures 16-21, and to Mr.
Victor Hogg who made all of the other illustrations. My wife, Dorothy
Krutzsch, helped untiringly in assembling data, in typing the
manuscript, and gave me continued encouragement. Finally, I am grateful
to Professor E. Raymond Hall for guidance in the study and critical
assistance in the preparation of the manuscript and to Professors Rollin
H. Baker, Robert W. Wilson, and Robert E. Beer for valued suggestions.
PALEONTOLOGY OF THE GENUS
The fossil record of the genus _Zapus_ is scanty. All of the known
fossils of it are lower jaws of Pleistocene Age. The Recent species _Z.
hudsonius_ was recorded by Cope (1871:86) in the Port Kennedy Cave fauna
(pre-Wisconsinian) of Pennsylvania. Gidley and Gazin (1938:67) reported
a single mandibular ramus bearing m1-m3 recovered from the Cumberland
Cave (pre-Wisconsinian) of Maryland. The teeth are not typical of modern
_Zapus_ in that m1 and m2 are shorter crowned and m1 has a longer
anterior lobe. Gidley and Gazin, nevertheless, considered their material
insufficient for establishing a new species.
Two extinct species have been described: _Zapus burti_ Hibbard
(1941:215) from the Crooked Creek formation (= Meade formation of the
State Geological Survey of Kansas) mid-Pleistocene of Kansas and _Zapus
rinkeri_ Hibbard (1951:351) from the Rexroad formation (= Blanco
formation of the State Geological Survey of Kansas) of Blancan Age of
Kansas. Both species resemble _Zapus hudsonius_, but differ from it in
broader crowned more brachydont cheek-teeth. _Z. rinkeri_ differs from
_Z. burti_ and _Z. hudsonius_ by a more robust ramus, broader molars,
and three instead of two internal re-entrant valleys posterior to the
anterior loop on m1. The three species _Z. rinkeri_, _Z. burti_, and _Z.
hudsonius_ are in a structurally, as well as a geologically, progressive
series. The trend in dentition is from broad, brachydont cheek-teeth to
narrow, semi-hypsodont cheek-teeth.
RELATIONSHIPS, DISTRIBUTION, AND SPECIATION
Relationships in the Subfamily Zapodinae
The subfamily Zapodinae is known from Pliocene and Pleistocene deposits
of North America and now occurs over much of northern North America and
in Szechuan and Kansu, China. The living species occur among grasses and
low herbs in damp or marshy places both in forested areas and in plains
areas.
The early Pliocene _Macrognathomys nanus_ Hall (1930:305), originally
described as a Cricetid, is actually a Zapodid as shown by the structure
of the mandibular ramus, shape of the incisors, and occlusal pattern of
the cheek-teeth.
If _Macrognathomys_ can be considered a member of the subfamily
Zapodinae (possibly it is a sicistine) then it represents the oldest
known member of this subfamily. Judging from the published
illustrations, _Macrognathomys_ seems to be structurally ancestral to
the Mid Pliocene _Pliozapus solus_ Wilson; the labial re-entrant folds
are wider and shorter and on m2 and m3 fewer. The difference in stage of
wear of the teeth in _Macrognathomys_ and _Pliozapus_ is a handicap in
comparing the two genera but they are distinct. Wilson (1936:32) points
out that _Pliozapus_ clearly falls in the Zapodinae and stands in an
ancestral position with respect to the structurally progressive series
_Eozapus_, _Zapus_, and _Napaeozapus_. Nevertheless, _Pliozapus_ cannot
be considered as directly ancestral to _Eozapus_ because of the
progressive features in the dentition of _Pliozapus_. Wilson (1937:52)
remarked that if _Pliozapus_ is ancestral to _Zapus_ and _Napaeozapus_,
considerable evolution must have taken place in the height of crown and
in the development of the complexity of the tooth pattern. In contrast
to Wilson's opinion, Stehlin and Schaub (1951:313) placed _Pliozapus_
and _Eozapus_ in the subfamily Sicistinae because certain elements in
the occlusal pattern of the cheek-teeth are similar. I disagree with
those authors and hold with Wilson; I consider _Pliozapus_ and _Eozapus_
in the subfamily Zapodinae. In dental pattern _Pliozapus_, as Wilson
(1936:32) pointed out, resembles the Recent Eurasiatic sicistid,
_Sicista_ more than do _Zapus_ or _Napaeozapus_. Nevertheless, from
_Sicista_ Wilson distinguishes _Pliozapus_ and relates it to the
subfamily Zapodinae by: "more oblique direction of protoconid-hypoconid
ridge, anterior termination of this ridge at buccal portion of
protoconid rather than between protoconid and metaconid as in _Sicista_;
cusps more compressed into lophs; cheek-teeth somewhat broader; greater
development of metastylid; greater development of hypoconulid ridge, ...
absence of anteroconid...."
_Eozapus_ is more closely related to _Pliozapus_ than to either _Zapus_
or _Napaeozapus_ (Wilson, 1936:32) but all four genera are in the
subfamily Zapodinae. Stehlin and Schaub (op. cit.:158 and 311) relate
_Eozapus_ to the subfamily Sicistinae on the basis of similarity in the
occlusal pattern of the cheek-teeth of _Eozapus_ and various sicistines.
Stehlin and Schaub do not consider other structures such as the elongate
hind limbs, the shape of malleus and incus, and the shape of the
baculum, in which there is close resemblance to the Zapodinae. It is
these structural similarities as well as those, pointed out by Wilson
(_loc. cit._), in dentition that leads me to place _Eozapus_ in the
subfamily Zapodinae. The early Pleistocene _Zapus rinkeri_ Hibbard shows
that the _Zapus_ stage of development had already been achieved perhaps
as early as the late Pliocene. Hibbard (1951:352) thought that _Zapus
rinkeri_ was not structurally intermediate between _Pliozapus_ and any
Recent species of _Zapus_; although the teeth of _Z. rinkeri_ have the
broader, shallower, re-entrant folds of _Pliozapus_, these teeth are
higher crowned and have an occlusal pattern resembling that of the
Recent species of _Zapus_. The middle Pleistocene species, _Zapus burti_
Hibbard, progressed essentially to the structural level of the Recent
_Zapus hudsonius_, but the molars were more brachydont, broader crowned,
and their enamel folds less crowded. Pleistocene material of
pre-Wisconsin age obtained from cave deposits in Pennsylvania and
Maryland is most nearly like _Zapus hudsonius_. One such cave deposit in
Maryland contained an example of the Recent genus _Napaeozapus_,
indicating that its history dates from at least middle Pleistocene time.
The Asiatic Recent Genus, _Eozapus_, has not progressed much beyond the
Pliocene stage in zapidine evolution if _Pliozapus_ be taken as a
standard; the North American Recent Genus _Zapus_ essentially achieved
its present form by early Pleistocene times, and the Recent Genus
_Napaeozapus_ achieved its more progressive structure by middle
Pleistocene times.
Perhaps _Pliozapus_ and _Eozapus_ represent one phyletic line and
_Zapus_ and _Napaeozapus_ a second line, both of which lines evolved
from a pre-zapidine stock in the Miocene. As mentioned earlier, Wilson
(1936) thinks that _Pliozapus_ is not directly ancestral to _Eozapus_.
Possibly these two genera diverged at an early date; nevertheless, they
are closely related primitive forms.
_Zapus_ and _Napaeozapus_ closely resemble each other and both are
structurally advanced; _Napaeozapus_ seems to have differentiated at a
more rapid rate.
According to Simpson (1947), the occurrence of the same group of mammals
on two different land masses is to be taken as prima facie evidence that
migration has occurred. Keeping in mind then the present geographic
distribution, unspecialized condition of the dentition of _Eozapus_, and
its resemblance to the extinct _Pliozapus_ known from North America but
not from Asia, it may be that _Eozapus_ descended from primitive stock
of a North American jumping mouse that was forced to the periphery
(across the Asiatic North American land bridge) by the more specialized
zapidine stock.
Subsequently or perhaps during the migration of the pre-_Eozapus_ stock
the zapidine stock may have dispersed transcontinentally, occupying most
of northern North America. The unprogressive _Macrognathomys_ and
_Pliozapus_ line which remained in North America may have become
extinct. Any such period of dispersal and climatic equilibrium ended
when glaciers came to cover most of the northern part of the continent
and the mammals living there were forced southward by the ice or
remained in ice-free refugia within the glaciated area. Later, with
melting and retreat of the ice, the jumping mice could have again spread
enough to occupy the northern part of the continent. Such glaciation
isolated segments of the population and aided their evolution into
distinct species.
If it be assumed, as Matthew (1915) did and as Hooper (1952:200) later
on the generic level did, that the region of origin and center of
dispersal for a given group of animals is characterized by the presence
of the most progressive forms, then southeastern Canada and the
northeastern United States make up the area of origin and center of
dispersal in relatively late time of the subfamily Zapodinae. This area
is inhabited by _Zapus hudsonius_ and _Napaeozapus_, the most
progressive members of the subfamily.
As I visualize it, the evolution of the Zapodinae occurred in two
stages: the first stage involved the movement of the primitive
pre-_Eozapus_ stock to Asia and the second stage involved the dispersal,
isolation, and specialization in North America of the more progressive
basic zapidine stock into the present genera _Zapus_ and _Napaeozapus_.
Status of the genera _Eozapus_, _Zapus_, and _Napaeozapus_
The genus _Zapus_ is one of three living genera in the subfamily
Zapodinae. These genera _Zapus_ and _Napaeozapus_ from North America
and _Eozapus_ from China have been variously considered as subgenera of
the genus _Zapus_ (Preble, 1899) or as three separate genera (Ellerman,
1940).
[Illustration: FIGS. 4-15. Three views of the skull and a lateral
view of the left lower jaw of each of the Recent genera of the
subfamily Zapodinae. × 1.5.
FIGS. 4-7. _Eozapus s. vicinus_, adult, male, No. 240762 USNM,
Lanchow, Kansu, China.
FIGS. 8-11. _Zapus h. pallidus_, adult, male, No. 240762 KU,
5-1/2 mi. N, 1-3/4 mi. E Lawrence, Douglas County, Kansas.
FIGS. 12-15. _Napaeozapus i. insignis_, adult, male, No. 41109 KU,
Shutsburg Rd., at Roaring Creek, 600 ft., Franklin County,
Massachusetts.]
[Illustration: FIGS. 16-21. Occlusal views of upper and lower right
cheek-teeth, of the three Recent genera of the subfamily Zapodinae.
× 12-1/2.
FIGS. 16 and 19. _Eozapus s. vicinus_, adult (age group 3), male,
No. 240762 USNM, Lanchow, Kansu, China.
FIGS. 17 and 20. _Zapus h. alascensis_, adult (age group 2), female,
No. 29073 KU, E side Chilkat River, 9 mi. W and 4 mi. N Haines,
Alaska.
FIGS. 18 and 21. _Napaeozapus i. insignis_, adult (age group 3),
male, No. 41109 KU, Shutsburg Rd., at Roaring Creek, 600 ft.,
Franklin County, Massachusetts.
Note especially the variation in complexity of occlusal pattern,
width of re-entrant folds, and degree of tubercularity.]
The remarkable similarity of the body form, post-cranial skeleton,
mandibular rami, and general structure of the cranium of _Zapus_,
_Napaeozapus_, and _Eozapus_ indicate their relationship (see figs.
4-15); however, dissimilarity between the groups in the dentition (tooth
number and occlusal pattern), bacula, and ear ossicles provides basis
for considering them distinct genera. As pointed out earlier, _Zapus_
and _Napaeozapus_ appear to be more closely related and progressive and
the Asiatic _Eozapus_ somewhat removed and less progressive.
_Teeth._--According to the complexity in dental pattern and in number
and size of the cheek-teeth, these genera can be arranged in a
structurally progressive series with _Eozapus_ showing the least
complexity and _Napaeozapus_ the most (see figs. 16-21). There are three
distinct molar patterns; one is simple (_Eozapus_) and the others
(_Zapus_ and _Napaeozapus_) are more complex. The complexity is greatest
in _Napaeozapus_, which is characterized by numerous additional flexures
in the enamel and dentine. The simplicity of the molars of _Eozapus_ is
evident in the tuberculate rather than flat-crowned occlusal surface;
the wide, simple, re-entrant bays; the small (or sometimes absent)
anteroconid; and the essentially quadritubercular nature of the teeth.
The molars of _Zapus_ and _Napaeozapus_ are flat crowned; however,
_Zapus_ has wider and fewer re-entrant bays, a smaller anteroconid, and
less complexity in the occlusal pattern. The characteristics of the
molar teeth would tend to indicate a close relationship between _Zapus_
and _Napaeozapus_ and to place _Eozapus_ as primitive.
The absence of P4 in _Napaeozapus_ would lead one to suspect that this
genus has evolved at a more rapid rate than the historically older
_Zapus_ and _Eozapus_ which still retain this structure. The small size
of P4, even in the primitive _Eozapus_, indicates that it has long been
of little use to the mouse. An even greater reduction of P4 in the more
complex dentition of _Zapus_ argues for complete loss of this tooth as
the next step in specialization, such as is seen in the more progressive
_Napaeozapus_. The following parallel columns show selected differences
between the occlusal patterns of the cheek-teeth of the three genera:
BACULUM.--The baculum (os penis) of _Eozapus_ is known to me only from
Vinogradov's (1925) figures of the dorsal and lateral aspects. The
proximal end (base) is laterally expanded, and the shaft tapers
gradually toward the distal end where it expands abruptly into the
spade-shaped tip. In lateral aspect the bone is relatively thick; it is
curved downward slightly from the proximal end to the base of the tip
where it curves upward to a rounded point.
The baculum of _Zapus_ differs from that of _Eozapus_ as follows: base
less expanded horizontally; shaft slenderer; distal end less
spade-shaped except in _Z. trinotatus_. The tip is less expanded in _Z.
princeps_ and is still less so in _Z. hudsonius_. In _Napaeozapus_ the
tip is lanceolate, the base is narrow, and in lateral view the shaft is
slender and curved (see figs. 22-31).
-----------------------+-----------------------+-----------------------
_Eozapus_ | _Zapus_ | _Napaeozapus_
-----------------------+-----------------------+-----------------------
P4--Small | Smaller | Absent
| |
M1--Four wide labial | Four moderately | Three narrow labial
re-entrant folds | narrow labial | re-entrant folds
of equal length; | re-entrant folds | of unequal length,
paracone and | of unequal length; | 1st long, 2d and 3d
metacone largest | 1st and 3d longer | shorter; paracone
cusps; anterior | than 2d, 4th | and metacone larger
cingulum large. | shortest; paracone | than in _Zapus_ and
| smaller than in | _Eozapus_; anterior
| _Eozapus_; metacone | cingulum absent.
| largest cusp; |
| anterior cingulum |
| small. |
| |
| |
M2--Four wide labial | Four moderately | Narrow labial
re-entrant folds; | narrow labial | re-entrant folds,
2d short, others | re-entrant folds | variable in number,
of equal length | of unequal length, | often as many as 6;
but longer than | 1st and 3d long, | anterior and
2d; anterior and | 2d and 4th short; | posterior cingula
posterior cingula | anterior and | small; occlusal
large; occlusal | posterior cingula | pattern complex.
pattern simple. | moderately large; |
| occlusal pattern |
| moderately complex. |
| |
| |
M3--Three wide labial | Two moderately | Three narrow labial
re-entrant folds | narrow labial | re-entrant folds
of unequal length, | re-entrant folds | of unequal length,
1st short, 2d and | of equal length; | 1st long, 2d and
3d long; anterior | anterior and | 3d short; anterior
and posterior | posterior cingula | and posterior
cingula low, | moderately large; | cingula large;
small; occlusal | occlusal pattern | occlusal pattern
pattern simple. | moderately complex. | complex.
| |
| |
m1--Anterior oblique | No anterior | No anterior
re-entrant fold | re-entrant fold; 4 | re-entrant fold;
separating equal | moderately narrow | narrow lingual
sized protoconid | lingual re-entrant | re-entrant folds
and metaconid | folds of equal | variable in
cusps; 3 wide | length, 1st joining | number, often as
lingual re-entrant | 1st labial | many as 4;
folds of equal | re-entrant fold, | anteroconid well
length; | 4th joining 2d | developed, encloses
anteroconid absent;| labial re-entrant | 1 or 2 small lakes;
occlusal pattern | fold; anteroconid | occlusal pattern
simple; mesoconid | well developed, | complex; mesoconid
present. | encloses | absent.
| small lake; occlusal |
| pattern moderately |
| complex; |
| mesoconid absent. |
| |
| |
m2--Four wide lingual | Four moderately | Narrow lingual
re-entrant folds | narrow lingual | re-entrant folds,
of unequal length, | re-entrant folds, | variable in number,
1st short, other | 1st and 2d long, 3d | may be as many as 5;
3 equal and long; | and 4th short, 1st | anteroconid large,
anteroconid | joins 1st labial | encloses
moderately large; | re-entrant fold | complex folds from
occlusal pattern | and 4th joins 2d | 1st labial re-entrant
simple. | labial re-entrant | fold; occlusal
| fold; anteroconid | pattern complex.
| large; occlusal |
| pattern moderately |
| complex. |
| |
| |
m3--Three wide lingual | Three moderately | Narrow lingual
re-entrant folds | narrow lingual | re-entrant
of near equal | re-entrant folds | folds variable in
length; antero- | of unequal length, | number, as many as 3;
conid absent; | 1st and 2d long, 3d | anteroconid present;
occlusal pattern | short; anteroconid | occlusal pattern
simple; 1 labial | absent; occlusal | complex; 2 labial
re-entrant fold. | pattern moderately | re-entrant folds.
| complex; 1 labial |
| re-entrant fold. |
-----------------------+-----------------------+------------------------
[Illustration: FIGS. 22-31. Dorsal and lateral views of the bacula
of the Recent genera (and species of the genus _Zapus_) of the
subfamily Zapodinae. × 10.
FIGS. 22 and 27. _Eozapus setchuanus_ (after Vinogradov, 1925:585).
FIGS. 23 and 28. _Zapus t. trinotatus_, adult, No. 94596 MVZ,
1-1/4 mi. ENE Amboy, 350 ft., Clark County, Washington.
FIGS. 24 and 29. _Zapus p. princeps_, adult, No. 20870 KU, 3 mi. S
Ward, Boulder County, Colorado.
FIGS. 25 and 30. _Zapus h. pallidus_, adult, No. 22954 KU, 4 mi. N,
1-3/4 mi. E Lawrence, Douglas County, Kansas.
FIGS. 26 and 31. _Napaeozapus i. insignis_, adult, No. 41110 KU,
Shutsburg Rd., at Roaring Creek, 600 ft., Franklin County,
Massachusetts.]
EAR OSSICLES.--The auditory ossicles are of three types which differ
only slightly. These ossicles possibly are more conservative than some
other structures because the ossicles are not so much affected by the
molding influence of the environment.
Instances of variation in the auditory region in mammals in general are
small, even at the family level; therefore, these differences in the
subfamily Zapodinae are offered as additional support for recognizing
_Eozapus_, _Zapus_, and _Napaeozapus_ as distinct genera. The
distinctive features are chiefly in the malleus and incus; the stapes,
however, differs slightly and, therefore, it too is described (see figs.
32-34).
In _Eozapus_ the head of the malleus is narrow, oblong, and rounded
dorsally and attaches to the body by a long, slender, abruptly recurved
neck. The body is weakly pointed ventrally and rounded dorsally. A
beaklike manubrium malleus composed of anterior projecting external and
internal spines extends from the body to the tympanum. The incus has a
dorsally rounded body with an anterior downward snoutlike projection
with which the malleus articulates. The short limb of the incus is broad
basally and narrows somewhat distally. The long limb is narrow and its
articulating lenticular process is a flat circular structure. The limbs
of the stapes are wide-spread and heavy. The neck is short and wide with
a large circular articulating surface.
In _Zapus_ the head of the malleus is angular with an anterior
projecting point and is flattened in dorsal aspect. The neck is slender,
elongate, and gently curved away from the long limb of the incus. The
body is pointed dorsally and rounded ventrally, the reverse of the
condition in _Eozapus_. There is a beaklike manubrium malleus composed
of internal and external anteriorly projecting spines extending from the
body to the tympanum as in _Eozapus_. The incus has a rounded body with
a long angular limb articulating via a small lenticular process with the
stapes. The short limb is narrow but does not taper distally as in
_Eozapus_. The limbs of the stapes are relatively narrow, weak, and
gently curved. The neck is longer and more slender than that of
_Eozapus_.
In _Napaeozapus_ the head and neck of the malleus resemble those of
_Zapus_ but are less robust. The body is more rounded dorsally, having
the curved dorsal surface directed anteriorly rather than posteriorly
(as in _Zapus_) and the lateral surface is nearly flat instead of curved
as in the other genera. The manubrium resembles that of _Eozapus_ and
_Zapus_. The body of the incus is flattened dorsally but otherwise
rounded. The long limb of the incus is angular and longer than that of
_Zapus_. The short limb of the incus is broad at the base and tapers
distally. The limbs of the stapes are narrow, weak, and abruptly curved.
The neck is more slender and elongate than in _Zapus_.
In summary: Only the head and body of the malleus and the short and long
limbs and body of the incus are sufficiently consistent within a given
group to be of taxonomic importance. The similarity in the morphology of
these ossicles indicates a close relationship between all three genera.
_Zapus_ and _Napaeozapus_ resemble one another more than either
resembles _Eozapus_. The differences recorded are constant between the
described groups and, therefore, are considered to be of taxonomic
significance. The differences give basis for dividing the subfamily
Zapodinae into the three genera _Eozapus_, _Zapus_, and _Napaeozapus_.
[Illustration: FIGS. 32-34. Lateral views of the left ear ossicles
(articulated) of the Recent genera of the subfamily Zapodinae. × 20.
FIG. 32. _Eozapus s. vicinus_, adult, male, No. 240762 USNM,
Lanchow, Kansu, China.
FIG. 33. _Zapus p. princeps_, adult, male, No. 32858 KU, Medicine
Wheel Ranch, 28 mi. E Lovell, Big Horn County, Wyoming.
FIG. 34. _Napaeozapus i. insignis_, adult, male, No. 9544 KU, 3 mi.
W Base Station, Coos County, New Hampshire.]
Distribution of and Speciation in the Genus _Zapus_
Many of the described kinds of the genus _Zapus_ were initially named as
distinct species (see Preble, 1899). Subsequently (see Hall, 1931), some
of the nominal species were reduced to the rank of subspecies. Only
three species in the genus _Zapus_ are recognized in the following
account. The concept of species adopted here is, in Mayr's (1942:120)
words, this: "Species are groups of actually or potentially
interbreeding natural populations, which are reproductively isolated
from other such groups." The three species are _Z. trinotatus_, _Z.
princeps_, and _Z. hudsonius_. No hybridization is known where two occur
together or where their ranges are adjacent. Each of these species has
several geographically contiguous subspecies.
The three species of _Zapus_ are closely related but are not equally
progressive. If eastern North America is considered to be the region of
origin and center of dispersal of _Zapus_ (see pp. 368-369) the
geographically distant species would be expected to be the least
progressive, and such seems to be the case. _Zapus trinotatus_ is
geographically farthest removed and structurally least progressive.
_Zapus hudsonius_ occurs at the center of dispersal and is the most
progressive structurally whereas _Z. princeps_ is geographically and
structurally intermediate. Structural progressiveness is postulated for
the species that has the simplest (in this instance specialized) baculum
and smallest fourth upper premolar. The phyletic branches of the genus
_Zapus_ possibly developed from geographic segments of a population
radiating from the centrally located progressive group. On continental
areas where a species with a wide and continuous range gives rise to
several daughter species, geographic isolation is thought to be
important in bringing about the formation of species. The unspecialized
populations conceivably occupied an area west of the present Rocky
Mountains and south of latitude 50°. From later Miocene times on,
climatic and geological differentiation occurred in this area, and with
the growth of geological barriers and differentiation of habitat these
unspecialized populations may have been separated into two ecological
groups, one inhabiting the more arid area between the present Rocky
Mountains and the present Cascade Range and Sierra Nevada and the other
group inhabiting the Pacific coastal region. Isolation of each of these
groups probably was not complete. How far differentiation might have
proceeded with incomplete isolation can only be guessed, but at least
incipient differences probably were present and possibly the animals
approached in character those found in these areas today in that the
ecology of the region was much the same as now.
In the region between the Rocky Mountains and the present Cascade Range
and the Sierra Nevada, the flora (in late Pliocene) became semidesert,
which presumably made most of this region uninhabitable for jumping
mice. The aridity probably induced local concentration into boreal
montane islands, thus possibly displacing the populations of the two
species that were in contact.
In Pleistocene times continental glaciation must have interrupted the
contacts between the coastal, intermontane (the area between the present
Rocky Mountains and the present Cascade Range and the Sierra Nevada),
and northern and eastern groups of _Zapus_ or mammals of any genus that
occurred over all of this vast region. The advance of the ice southward
would have increased opportunity for evolution by interposing barriers
that isolated some populations. The populations possibly were
re-established in interglacial periods and then were isolated again by
another descent of glacial ice.
If a population occupied the unglaciated coastal region of Oregon and
Washington it may have been separated from other populations to the
north and east by an ice cap which covered most of the Cascade Range.
The population occupying the intermountain region probably was isolated
from the population to the north and west. The formation of glaciers
presumably reduced the size of areas available to the populations
occupying eastern North America, Alaska, and Canada with the result that
they persisted only in areas south of the ice or in ice-free refugia
(central and western Alaska) within the glaciated area. According to
Axelrod (1948), the flora in the eastern United States during the
Pleistocene furnished most of the stock for the revegetation of southern
and subarctic Canada east of the Rocky Mountains. Eastern populations of
_Z. hudsonius_ (or its progenitors) probably followed the spread of this
vegetation and, thus, extended their range into Canada where they
crossbred with populations advancing south and east from the refugia in
Alaska. Western montane floras, which extended north along the Rocky
Mountains and the Cascade and Coast ranges, probably paved the path for
a northward migration of populations of the intermountain _Z. princeps_
(or its progenitors). Populations of _Z. princeps_ moved eastward from
the present Rocky Mountains, inhabiting the high plains of southern
Canada and the north-central United States. In general, _Zapus
hudsonius_ occupies the region to the north and to the east of that
inhabited by _Zapus princeps_; however, the ranges of the two meet and
overlap in central and northern British Columbia and in the high plains
area of southern Alberta, Saskatchewan, eastern Manitoba, eastern
Montana, North Dakota, and northern South Dakota. In these places of
overlap, owing to range expansion following the retreat of the ice,
there is no sign of interbreeding, indicating that the populations have
attained specific rank.
Populations of both _Z. hudsonius_ and _Z. princeps_ occur together at
Indianpoint Lake, British Columbia. Specimens taken there are readily
sorted into two groups; none is intermediate. The difference in size
between these species there is especially marked; _Z. p. saltator_ there
is a large derivative of _Z. princeps_ and _Z. h. tenellus_ is a
medium-sized _Z. hudsonius_.
_Z. princeps_ minor and _Z. hudsonius intermedius_ have been taken at
several neighboring localities in North Dakota. Although these
geographic races are more nearly of the same size (_minor_ is a small
subspecies of _princeps_ and _intermedius_ is a moderately large
subspecies of _hudsonius_) they do not interbreed. Specimens of _Z. p.
minor_ and _Z. h. intermedius_ have been obtained from an ecologically
homogeneous area in the vicinity of Fort Totten and Devils Lake, North
Dakota. Values obtained from several measurements of the skull and
baculum allow for ready recognition of the two species. The populations
from North Dakota are, however, not so widely divergent as are those
populations from the area of contact in British Columbia. Perhaps the
difference in the degree of distinction between the species at the two
areas of contact is indicative of the length and completeness of
geographic isolation between neighboring populations.
The ranges of _Z. trinotatus_ and _Z. hudsonius_ are not at present in
contact, but the two species differ more strongly than do _hudsonius_
and _princeps_ or _princeps_ and _trinotatus_. Therefore, _trinotatus_
and _hudsonius_ are here considered to be two distinct species.
As pointed out earlier in this discussion, the separation between the
progenitors of _Z. trinotatus_ and _Z. princeps_ probably occurred when
the present Cascade Range and the Sierra Nevada were being formed. From
this time until Pleistocene glaciation an incomplete geographic
isolation was in effect between the populations of the Pacific coast and
the intermountain populations. Perhaps in the region north of the
present Cascade Range there was moderate interbreeding between these
populations and the transcontinental form. There may have been a similar
zone of interbreeding along the crest of the present Cascades where the
intermountain and coastal populations conceivably could have met. At
least incipient characters probably were present when in Pleistocene
time, continental glaciation further isolated the two populations. Since
the retreat of the last ice (Wisconsin) the unprogressive coastal _Z.
trinotatus_ has expanded its range only slightly, reaching as far as
southwestern British Columbia. It seems that ecological difference
rather than the barrier formed by the higher elevations is responsible
for the limited expansion of range. The population of _princeps_ has
extended its range northward to the southern part of the Yukon Territory
but does not occur in coastal southern British Columbia because that
area already was occupied by _Zapus trinotatus_. The ranges of the two
species meet and overlap in southwestern British Columbia. The species
occur sympatrically in Manning Park where, according to Carl _et al._
(1952:77), they occupy the same range in the region of Allison Pass,
Pinewoods, and Timberline Valley. These workers remark that no
intergradation was apparent between individuals of the two species
obtained in the same trap line.
I have examined material of both species from Allison Pass. There the
species differ in color, in the shape of the skull, and in the size and
shape of the baculum. Material from Timberline Valley, an area in which
Carl _et al._ (_loc. cit._) reported both species, here is assigned to
_Z. princeps_. Where bacula have been preserved the identity of the
species is instantly possible.
In summary: First, a population of jumping mice, possibly a monotypic
genus, occurred over most of North America; then this population partly
divided into Pacific northwest, intermountain (from the east slopes of
the present Rocky Mountains to the east slopes of the present Cascade
Range and the Sierra Nevada), and transcontinental (eastern and
northern) groups with the least progressive groups peripheral; a further
reduction or possibly a complete isolation of these populations followed
owing to Pleistocene glaciation (especially in the Wisconsin period);
and, finally, the present day contacts were established between these
populations which by now have differentiated into species. Conceivably,
_Z. burti_ (Blancan age) and _Z. rinkeri_ (mid Pleistocene) may
represent stages in the development of _Z. hudsonius_.
ANNOTATED LIST OF SPECIFIC AND SUBSPECIFIC NAMES
(Applied to the genus _Zapus_ since 1899)
Edward A. Preble's (1899) early revisionary account of the genus _Zapus_
provides an annotated list of the names which had been proposed for
American jumping mice to that date. The present account supplies in
chronological order the names proposed (including the new kinds
described by Preble) in the 54 years since Preble's revision. Detailed
synonymies are given for each kind under the accounts of the
subspecies.
1899 _campestris_ (_Zapus hudsonius_) Preble, N. Amer. Fauna,
15:20, August 8, 1899, applies to the jumping mouse of southeastern
Montana, and the Black Hills region of Wyoming and South Dakota.
1899 _minor_ (_Zapus princeps_) Preble, N. Amer. Fauna, 15:23,
August 8, 1899, originally applied to the jumping mouse of the
prairies of Saskatchewan, but now includes populations of this
species from the plains of Canada (southern Manitoba to Canadian
Rockies) and northern United States (Montana, North and South
Dakota).
1899 _oregonus_ (_Zapus princeps_) Preble, N. Amer. Fauna, 15:24,
August 8, 1899, originally applied to the jumping mouse of eastern
Oregon, but now applies also to populations from southeastern
Idaho, eastern and central Nevada, and extreme northeastern
California.
1899 _major_ (_Zapus_) Preble [= _Zapus princeps oregonus_], N.
Amer. Fauna, 15:25, August 8, 1899, arranged as a subspecies of
_Zapus princeps_ by Hall, Univ. California Publ. Zool., 37:10,
April 10, 1931; here considered a synonym of _Zapus princeps
oregonus_.
1899 _nevadensis_ (_Zapus_) Preble [= _Zapus princeps oregonus_],
N. Amer. Fauna, 15:25, August 8, 1899, arranged as a subspecies of
_Zapus princeps_ by Hall, Univ. California Publ. Zool., 37:10,
April 10, 1931; here considered a synonym of _Zapus princeps
oregonus_.
1899 _orarius_ (_Zapus_) Preble [= _Zapus trinotatus orarius_], N.
Amer. Fauna, 15:29, August 8, 1899, applies to the animals from
southwestern Marin County, California.
1911 _luteus_ (_Zapus_) Miller [= _Zapus princeps luteus_], Proc.
Biol. Soc. Washington, 24:253, December 23, 1911, applies to the
jumping mouse in north-central and southern New Mexico and eastern
Arizona.
1913 _australis_ (_Zapus luteus_) Bailey [= _Zapus princeps
luteus_], Proc. Biol. Soc. Washington, 26:129, May 21, 1913, was
applied to the jumping mouse of southern New Mexico, but is here
regarded as a synonym of _luteus_.
1920 _eureka_ (_Zapus trinotatus_) Howell, Univ. California Publ.
Zool., 21:229, May 20, 1920, applies to the jumping mouse of the
humid coastal district of northern California.
1931 _cinereus_ (_Zapus princeps_) Hall, Univ. California Publ.
Zool., 37:7, April 10, 1931, applies to the jumping mouse of
extreme northwest Utah and south-central Idaho.
1931 _curtatus_ (_Zapus princeps_) Hall, Univ. California Publ.
Zool., 37:7, April 10, 1931, applies to the jumping mouse of the
Pine Forest Mountains, Humboldt County, Nevada.
1931 _palatinus_ (_Zapus princeps_) Hall [= _Zapus princeps
oregonus_], Univ. California Publ. Zool., 37:8, April 10, 1931, was
applied to the jumping mouse of Lander and Nye counties, Nevada,
but is here regarded as a synonym of _oregonus_.
1932 _kootenayensis_ (_Zapus princeps_) Anderson, Ann. Rept. Nat.
Mus. Canada for 1931:108, November 24, 1932, applies to the jumping
mouse of southeastern and central British Columbia, northern Idaho,
and eastern Washington.
1934 _idahoensis_ (_Zapus princeps_) Davis, Jour. Mamm., 15:221,
August 10, 1931, applies to populations in parts of British
Columbia, Alberta, Idaho, Montana, and Wyoming.
1939 _utahensis_ (_Zapus princeps_) Hall, Occas. papers Mus. Zool.
Univ. Michigan, 296:3, November 2, 1934, applies to the jumping
mouse of southeastern Idaho, western Wyoming, and eastern Utah.
1941 _burti_ (_Zapus_) Hibbard, Univ. Kansas Publ., Bull. State
Geol. Surv. Kansas, 38:214, July 14, 1941, refers to two
fragmentary right rami of Pleistocene age (Borchers fauna) from
Loc. No. 9, Meade County, Kansas.