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
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.
The present report is based on a study of approximately 3,600 specimens that were assembled at the Museum of Natural History of the
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´
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:
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.
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.
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
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.
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. 4-15. 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, 51/2 mi. N, 13/4 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. 16-21. Occlusal views of upper and lower right cheek-teeth, of the three Recent genera of the subfamily Zapodinae. × 121/2.
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
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 NapaeozapusP4—
Small
Smaller
Absent
M1—
Four wide labial re-entrant folds of equal length; paracone and metacone largest cusps; anterior cingulum large.
Four moderately narrow labial re-entrant folds of unequal length; 1st and 3d longer than 2d, 4th shortest; paracone smaller than in Eozapus; metacone largest cusp; anterior cingulum small.
Three narrow labial re-entrant folds of unequal length, 1st long, 2d and 3d shorter; paracone and metacone larger than in Zapus and Eozapus; anterior cingulum absent.
M2—
Four wide labial re-entrant folds; 2d short, others of equal length but longer than 2d; anterior and posterior cingula large; occlusal pattern simple.
Four moderately narrow labial re-entrant folds of unequal length, 1st and 3d long, 2d and 4th short; anterior and posterior cingula moderately large; occlusal pattern moderately complex.
Narrow labial re-entrant folds, variable in number, often as many as 6; anterior and posterior cingula small; occlusal pattern complex.
M3—
Three wide labial re-entrant folds of unequal length, 1st short, 2d and 3d long; anterior and posterior cingula low, small; occlusal pattern simple.
Two moderately narrow labial re-entrant folds of equal length; anterior and posterior cingula moderately large; occlusal pattern moderately complex.
Three narrow labial re-entrant folds of unequal length, 1st long, 2d and 3d short; anterior and posterior cingula large; occlusal pattern complex.m1—
Anterior oblique re-entrant fold separating equal sized protoconid and metaconid cusps; 3 wide lingual re-entrant folds of equal length; anteroconid absent; occlusal pattern simple; mesoconid present.
No anterior re-entrant fold; 4 moderately narrow lingual re-entrant folds of equal length, 1st joining 1st labial re-entrant fold, 4th joining 2d labial re-entrant fold; anteroconid well developed, encloses small lake; occlusal pattern moderately complex; mesoconid absent.
No anterior re-entrant fold; narrow lingual re-entrant folds variable in number, often as many as 4; anteroconid well developed, encloses 1 or 2 small lakes; occlusal pattern complex; mesoconid absent.
m2—
Four wide lingual re-entrant folds of unequal length, 1st short, other 3 equal and long; anteroconid moderately large; occlusal pattern simple.
Four moderately narrow lingual re-entrant folds, 1st and 2d long, 3d and 4th short, 1st joins 1st labial re-entrant fold and 4th joins 2d labial re-entrant fold; anteroconid large; occlusal pattern moderately complex.
Narrow lingual re-entrant folds, variable in number, may be as many as 5; anteroconid large, encloses complex folds from 1st labial re-entrant fold; occlusal pattern complex.
m3—
Three wide lingual re-entrant folds of near equal length; anteroconid absent; occlusal pattern simple; 1 labial re-entrant fold.
Narrow lingual re-entrant folds variable in number, as many as 3; anteroconid present; occlusal pattern complex; 2 labial re-entrant folds.
Three moderately narrow lingual re-entrant folds of unequal length, 1st and 2d long, 3d short; anteroconid absent; occlusal pattern moderately complex;
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, 11/4 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, 13/4 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.