North American Jumping Mice (Genus Zapus)

Chapter I: Part 1

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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)

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)

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´_

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).

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.]

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).

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_.

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.

1942 _brevipes_ (_Zapus hudsonius_) Bole and Moulthrop [= Zapus
hudsonius americanus], Sci. Publ. Cleveland Mus. Nat. Hist., 5:168,
September 11, 1942, based on specimens from Bettsville, Seneca
County, Ohio, which are inseparable from _americanus_ that has
priority.

1942 _rafinesquei_ (_Zapus hudsonius_) Bole and Moulthrop [= _Zapus
hudsonius americanus_], Sci. Publ. Cleveland Mus. Nat. Hist.,
5:169, September 11, 1942, was applied to jumping mouse of
southeastern Ohio but is here regarded as a synonym of
_americanus_.

1943 _ontarioensis_ (_Zapus hudsonius_) Anderson [= _Zapus
hudsonius canadensis_], Ann. Rept. Provancher Soc. Nat. Hist.,
Quebec, 1942:52, September 7, 1943, was applied to animals from
eastern Ontario but is here regarded as a synonym of _canadensis_.

1950 _pallidus_ (_Zapus hudsonius_) Cockrum and Baker, Proc. Biol.
Soc. Washington, 63:1, April 26, 1950, refers to the jumping mouse
from Kansas, Missouri, Oklahoma, Nebraska, and south-central South
Dakota.

1951 _rinkeri_ (_Zapus_) Hibbard, Jour. Mamm., 32:351, August,
1951, refers to single incomplete right ramus of upper Pliocene
age, Rexroad formation and fauna, from Loc. UM-UK-47, Fox Canyon,
sec. 25, T. 34S, R. 30W, XI Ranch, Meade County, Kansas.

1953 _intermedius_ (_Zapus hudsonius_) described as new on page 447
of this paper.

1953 _preblei_ (_Zapus hudsonius_) described as new on page 452 of
this paper.

CHARACTERS OF TAXONOMIC WORTH

EXTERNAL PARTS.--The total length, the length of the tail, and the length of the hind foot are useful to some extent in distinguishing species and subspecies. Geographic variation in these measurements is clinal in some species. For example, _Zapus trinotatus_, which inhabits the western coast of North America, decreases in size from the northern to the southern part of its range. There is considerable overlap in external measurements, in specimens of the same age, between the species _Z. trinotatus_ and _Z. princeps_, but only slight overlap between _Z. princeps_ and _Z. hudsonius_ and between _Z. trinotatus_ and _Z. hudsonius_. If all collectors measured external parts in the same way the measurements would be more useful for differentiating one species from another.

PELAGE.--The pelage, both in its entirety and as individual hairs, provides taxonomic characters as has been pointed out by Moojen (1948:324) for the genus _Proechimys_, by Williams (1938:239) for the Insectivora, and by Hausman (1920:496) for several groups of mammals. In addition to the sensory hairs, facial vibrissae, nasal hairs, and carpal vibrissae, there are three kinds of hairs in the normal coat of _Zapus_: guard hairs, overhairs, and underfur. The guard hairs and underfur differ in different species (see figs. 35-37).

The guard hairs taper at both ends, are elliptical in cross section, and are wider and longer than the other two kinds of hair. The bases of the guard hairs are grayish, and the amount of pigment gradually increases distally to a dark brownish or blackish shade. The guard hairs vary in greatest diameter from 96 microns to 168 microns, depending upon the species, and variation in diameter provides characters of taxonomic worth. No clinal variation in diameter of the guard hairs was detected. In _Z. hudsonius_ the guard hairs average 115 microns (96-140) and are significantly narrower than those of _Z. princeps_ and _Z. trinotatus_, which average 142 microns (130-168) and 141 microns (133-154), respectively. Pigmentation of the guard hairs contributes little information useful in separating the species of _Zapus_. All of the species have a prominent compounded medulla in which the pigment cells anastomose to form a labyrinthine column.

The individual hair of the underfur is cylindrical and tapers abruptly at each end; it is short, thin, flexible, and usually is bicolored on the back and sides of the mouse. The apical zone is yellow-brown (for example, Ochraceous-Buff) and the proximal part is whitish or grayish, which gradually darkens to near black subapically.

The width of a hair in the underfur is of no taxonomic significance, in that individual variation exceeds that between species.

The pattern of the pigment in the medulla of the hair, however, does vary specifically. Comparable samples from _Z. trinotatus_, _Z. princeps_, and _Z. hudsonius_ of the same age, sex, and season reveal a pattern characteristic for each species (see figs. 35-37).

All species of _Zapus_ agree closely in color pattern. A broad longitudinal dorsal band of some shade of yellow-brown flecked with black hairs is bordered by a lateral band of a lighter color usually containing fewer black hairs than on the dorsum. The underparts are usually white but are sometimes suffused with color resembling that on the sides. Between the white underparts and the darker color of the sides there is often a narrow, clear ochraceous stripe. Dorsal and lateral hairs are uniformly grayish-white at their bases; only the distal parts of the hairs are responsible for the external color of the animal.

The pelage of juveniles is usually finer and softer than the pelage of adults. The lateral and dorsal bands are not so conspicuously marked in young animals, and individual hairs are not so long or so wide as in adult animals.

FIGS. 35-37. Photomicrographs of underhairs (middle third) from each
of the species of the genus _Zapus_. × 500.

FIG. 35. _Zapus t. orarius_, adult, female, No. 20293 MVZ, 3 mi. W
Inverness, 300 ft., Marin County, California.

FIG. 36. _Zapus p. oregonus_, adult, male, No. 47856 KU, Harrison
Pass R. S., Ruby Mt's, Elko County, Nevada.

FIG. 37. _Zapus h. pallidus_, adult, male, No. 22954 KU, 4 mi. N,
1-3/4 mi. E Lawrence, Douglas County, Kansas.]

Preble (1899:7) and Howell (1920:226) remark as to the noticeable difference between pelages of spring and early fall. The pelage in spring is described as bright and fresh whereas that in fall is dull and worn. Actually both bright and worn pelages can occur in any one population at any one time. Some newly molted individuals are in fresh unworn pelage; some individuals, which are molting, are in ragged, worn pelage; and other individuals perhaps could be found to represent intermediate stages.

Variations from the normal color of the pelage are rare. Among more than 3,000 specimens of _Zapus_ examined there were only 12 individuals (five _Z. princeps_, 6 _Z. hudsonius_, and 1 _Z. trinotatus_) that were abnormally colored. A single white spot was noted on each of 10 (5 _Z. princeps_, 4 _Z. hudsonius_, and 1 _Z. trinotatus_) of these individuals; the spots were on the dorsal, anterior half of the body. The skin beneath the patch of white hair was in each animal like that beneath the neighboring normally-pigmented hair. One specimen of _Z. hudsonius_ (NMC No. 6669) is everywhere black, excepting the dorsal surface of the toes of the forefeet. Most of the individual hairs from various areas of the body are black for their entire length; some, however, have non-pigmented silvery tips. One specimen of _Z. hudsonius_ (KU No. 645) lacks any black; dorsally the pelage is nearest to Ochraceous-Buff and it is white on the venter. Individual hairs of the dorsal area are white for the basal two-thirds of their length (as compared to gray and brown in the animals with normal pigmentation) and near Ochraceous-Buff on the distal third (as compared to hairs which are dark brown tipped with Ochraceous-Buff). The feet and tail are white.

MOLT.--The sequence of molt for _Zapus_ has been ascertained from examination of the study skins. In all species of this genus there seems to be only one annual molt in adults. In the young of the year this molt occurs after August first and before hibernation. All individuals of a single population do not molt at any one time; females continue to molt later in the autumn than do the males; some individuals begin the molt as early as mid-June and others show molt as late as the end of October; approximately three weeks are required for an individual to complete its molt (Quimby, 1951:74); readiness for molt and early stages in molt can be detected (in museum specimens) by the greater thickness of the skin. Hairs lost accidentally are quickly replaced, regardless of the condition of the molt.

In _Zapus hudsonius_, new hair appears simultaneously on the anterior dorsal surface of the nose and on the mid-dorsal surface between the scapulae. The molt proceeds anteriorly from the shoulders and posteriorly from the nose. At the same time that the head is covered, new hair appears on the sides of the body from the forelegs to the cheeks. New pelage then appears posteriorly, and molt continues as a wave from these points over the sides and back with the rump receiving new hair last (see figs. 42 and 43).

In _Zapus princeps_ new hair appears first on the mid-dorsal surface between the scapulae. From this starting point molt progresses anteriorly, laterally, and posteriorly. Progress over the head is rapid; the head receives its new hair sooner than the caudal region. Molt moves progressively nearer to the base of the tail and progressively nearer to the mid-ventral surface. The rump is the last area to complete its molt (see figs. 40 and 41).

The progress of molt in _Z. princeps_ might be likened to the flow of a drop of paint on the curved surface of a ball where the paint flows in all directions but is speeded at one point and slowed at the opposite by a slight tilting of the ball from the horizontal.

In the species _Zapus trinotatus_ new hair appears simultaneously on the anterior, dorsal surface of the nose and on the mid-dorsal surface between the scapulae. In this respect the progress of molt of _Z. trinotatus_ resembles that of _Z. hudsonius_. From these starting points molt progresses rapidly over the head, the molt moving anteriorly from the shoulders and posteriorly from the nose with the result that it covers the dorsal surface of the head; hair then appears on the cheeks and sides of the neck. The progress of molt on the remaining areas of the body is comparable to that of _Z. princeps_; molt progresses toward the tail and toward the mid-ventral line. The rump, as in _Z. princeps_, is the last area to complete its molt (see figs. 38 and 39).

FIGS. 38 and 39. _Zapus trinotatus._

FIGS. 40 and 41. _Zapus princeps._

FIGS. 42 and 43. _Zapus hudsonius._]

BACULUM.--The general shape and dimensions of the baculum (os penis) provide characters of taxonomic value for the species of _Zapus_ (see figs. 23-25 and figs. 28-30).

Three measurements--length, transverse diameter at the base, and transverse diameter at the tip--are easily obtained and are diagnostic. The bacula of all species are somewhat curved. The measurement of length used by me does not represent the actual length of the bone, but instead the chords of the arcs involved.

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North American Jumping Mice (Genus Zapus)Chapter I: Part 1

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