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.
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, 13/4 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. 38-43. Diagrams showing differences in progress of molt in the three species of the genus Zapus. All approximately 1/2 natural size. Figs. 38, 40 and 42 lateral view. Figs. 39, 41 and 43 dorsal view.
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.
Skull.—Some of the structures useful for separating taxonomic entities may have little or no biological significance to the animals in nature. Characters mentioned by me are chosen simply for their significance taxonomically. The zygomata vary in degree of lateral bowing, being widely bowed in Z. princeps and Z. trinotatus, and less so in Z. hudsonius. Differences in zygomatic breadth owing to the degree of bowing are an aid in differentiating subspecies. The length of the skull from the occipital condyles to the tip of the longest nasal bone is useful in separating Z. hudsonius from Z. trinotatus and Z. princeps. The narrowness of the base of the zygomatic process of the squamosal is useful in distinguishing between Z. hudsonius and Z. princeps, but shows no variation of subspecific worth. The shape and dimensions of the incisive foramina provide specific and subspecific characters. The position of the anterior margin of the postpalatal notch, in relation to the last molars, provides subspecific characters in Z. princeps. In the species Z. princeps the median projection on the inferior ramus of the zygomatic process of the maxillary is absent in some subspecies, small in others, and large in some. Shape and inflation of the auditory bullae, shape of the pterygoid fossae, and shape of the nasals are useful in determining specific and subspecific relationships.
Teeth.—The alveolar length of the upper maxillary tooth-rows aids in distinguishing Z. hudsonius from Z. princeps and Z. trinotatus. Nearly parallel versus anteriorly divergent upper tooth-rows is a subspecific difference in Z. princeps. Variations in the dimensions of P4 and M1 aid in estimating the relationships of species. The occlusal pattern shows little variation and was of no use in separating species.
A knowledge of variation resulting from age, individual, or secondary sexual differences, as opposed to geographic variation between two or more populations of a single species is important in determining the reliability of taxonomic characters.
The largest population-sample of Zapus available to me for the study of nongeographic variation was 63 individuals from various localities in Keweenaw and Menominee counties, Michigan. Thirty-nine were females and 24 were males. It is on these specimens that this discussion is based.
Teeth.—The teeth provide a valuable standard for age determination in that they wear at a measurable rate. The molars erupt in sequence from front to back, and wear shows first on M1 and last on M3. The peglike permanent P4, of which I have not seen the deciduous precursor, receives wear at the same time that the molars are being worn. Wear proceeds at approximately the same rate in the teeth of both the upper jaws and lower jaws.
In order to be more nearly certain that specimens used in making racial comparisons were comparable as to age, six age-groups were established, from youngest to oldest. These groups were based on the degree of wear on the occlusal surface of the upper cheek-teeth, and are as follows: group 1, in which M1 and M2 have not reached full and equal height and show no occlusal wear, and M3 has not erupted or is just breaking through the alveolus; group 2, in which M1 and M2 have reached full and equal height and show slight wear, and M3 may be almost or quite equal in height to M1 and M2 and, when equal, sometimes shows slight wear; group 3, in which M1 and M2 show wear on all cusps but cusps are visible, and M3 shows slight wear; group 4, in which P4 shows slight wear, M1 has cusps and re-entrant folds between cusps mostly gone, M2 shows considerable wear but re-entrant folds are visible, and M3 has most re-entrant folds and cusps gone; group 5, in which P4 shows considerable wear, M1 has cusps completely worn away, M2 has re-entrant folds and cusps worn away, and M3 lacks occlusal pattern except for one or two lakes; group 6, in which all upper cheek-teeth are without occlusal pattern.
These groupings are based on continuously variable features, and, therefore, when the teeth are at certain stages of wear a specimen is difficult to place in one of two groups.
Age group 1 and 2 include juvenal and subadult animals. Animals of age groups 3 through 6 are considered adult. Individuals of age groups 3 through 5, including as they do the great majority of the adult population, were the only age classes used in measuring geographic variation.
Quimby’s (1951:69) data indicate that some mice produce litters at the age of approximately 2 months, when four-fifths grown. Therefore, sexual maturity is not always synonymous with morphological maturity.
Measurements of external parts.—Data presented here on Z. hudsonius are those recorded by Quimby (1951) on specimens from Anoka County, Minnesota, and those obtained by me from museum specimens from Menominee and Keweenaw counties, Michigan.
According to Quimby (1951:65-66) the mean length [= body length] for three newly born Z. hudsonius is 24.8 mm (24.0-25.5); at the end of the fourth week of growth the mean length averaged 64.4 mm and at the 13th week 77.6 mm. Rapid growth occurs during the first four weeks, with the mean length increasing approximately 2.6 times the size at birth. After the fourth week of development, growth proceeds at a slower rate; the mean length at 13 weeks is only 3.1 times greater than the mean length at birth.
In specimens assigned to age groups 1 and 2 the length of the body averaged 70 and 74.8 mm, respectively. The individuals of both groups are less than 13 weeks old if we assume that growth proceeds at the same rate in Michigan as it does in Minnesota.
In the specimens from Michigan of age groups 3, 4, 5, and 6 the average length of the body is 80.9, 83.7, 89.0, and 83.6, respectively.
According to Quimby (loc. cit.), the average length of the tail for three Z. hudsonius at birth was 9.2 mm. (8.5-10.0). During the first four weeks of development the tail grew rapidly and reached an average length of 92.0 mm, which was 10 times the length at birth. By the end of 13 weeks of development the average length of the tail for these three individuals was 119.6 mm or 12 times the average length at birth. The most rapid growth was early in development: 80 per cent of the growth of the tail occurred during the first month, after which growth proceeded at a much slower rate.
Quimby (loc. cit.) records an average dimension of 4.7 mm (4.5-5.0) for the length of the hind foot in three newly born Z. hudsonius. The hind foot grew rapidly in length and by the fourth week had increased 5.6 times in its length and averaged 26.3 mm. Growth was much less rapid from the fourth to the thirteenth week when the hind foot averaged 27.7 mm, only five per cent more than in mice four weeks old. Assuming the average length of the hind foot of the adults to be 29.0 mm, the hind foot in individuals 13 weeks old is 96 per cent of the adult size.
According to Quimby (loc. cit.), the pinna of the ear at birth is small and folded over the external auditory meatus. The length of the ear increases proportionately more (29 per cent) than any other external dimension after the first four weeks of growth.
If the average length of the ear (measured from the crown) of adults is 14.7 mm, the animals from Michigan in age groups 1 and 2 are 91.8 per cent and 96.5 per cent as large as adults.
Table 1.—Average Dimensions (in Millimeters) for Specimens of Z. h. hudsonius of Various Ages (Specimens from Michigan).
| Age groups | 1 | 2 | 3 | 4 | 5 | 6 |
| No. examined | 4 |
13 |
33 |
12 |
3 |
3 |
| Body | 70.0 |
74.8 |
80.9 |
83.7 |
89.0 |
83.6 |
| Tail | 113.8 |
118.5 |
122.9 |
125.0 |
125.0 |
118.3 |
| Hind foot | 28.8 |
28.6 |
28.9 |
29.1 |
28.9 |
29.3 |
| Ear | 13.5 |
14.2 |
14.7 |
14.8 |
15.0 |
14.3 |
From these data, concerning growth of external parts, it seems that: growth is most rapid during the four weeks following parturition; specimens from Michigan, assigned to age groups 1 and 2 on the basis of tooth wear, are less fully developed and probably younger than mice from Minnesota, with a known age of 13 weeks; individuals with sufficient wear on the teeth to be placed in age group 3, if they were obtained in the late fall, may be young from the first litters of the year or, if they were obtained in early spring, may be at least one year old; individuals in age groups 4, 5, and 6 are at least one year old.
Skull.—The post-embryonic development of the skull is rapid. Animals in age groups 1 and 2 have skulls which average more than 80 per cent of the size that is here considered adult (an average size obtained from age groups 3, 4, and 5). The actual increase in size of certain cranial elements for various age groups is given in table 2.
In age group 1 the rostrum is relatively short as it is in Neotoma micropus (J. A. Allen, 1894:235) and juveniles of Peromyscus truei (Hoffmeister, 1951:7). The rostrum lengthens rapidly and there is a general increase in actual and relative size of the entire preorbital region; the increase after age group 3 is slower and of lesser magnitude. Changes with age in the size of the braincase are slight. In age group 1 the average depth of the braincase is 99.6 per cent of the adult size; the average breadth of the braincase is 98 per cent of the adult size, and the average width across the mastoid region is 96.4 per cent of the adult size. These dimensions indicate that the braincase reaches full size early. The zygomatic arch, however, undergoes change with age; there is a gradual increase in breadth owing to lateral bowing and a gradual lengthening which is in keeping with a general elongation of the skull anterior to the braincase.
The incisive foramina in age group 1 are short (4.0 mm), broad (2.2 mm in the middle), and taper to a point at each end. In age group 2 the foramina have elongated (4.2 mm) and are less pointed posteriorly, but there is no change in breadth. In age groups 3, 4, 5, and 6 the foramina become progressively longer (4.5 mm in age group 6), have a relatively constant breadth (2.2 mm), and become more nearly truncate anteriorly.
Table 2.—Average and Extreme Measurements (in Millimeters) of Skulls of Six Age-groups in Specimens of Zapus hudsonius from Michigan.
| Age groups | 1 | 2 | 3 | 4 | 5 | 6 |
| Number examined | 4 | 13 | 33 | 14 | 3 | 3 |
| Occipitonasal length | 20.5 20.0 21.2 |
21.2 20.8 21.8 |
22.0 21.5 23.2 |
22.7 21.8 23.4 |
22.9 22.7 23.3 |
23.0 22.4 23.7 |
| Mastoid breadth | 9.8 9.7 10.0 |
10.04 9.6 10.4 |
10.12 9.5 10.5 |
10.12 9.6 10.7 |
10.3 10.0 10.8 |
10.36 10.1 10.8 |
| Length of zygomatic arch | 8.07 8.0 8.2 |
9.02 8.5 9.3 |
9.07 8.5 9.4 |
9.25 9.2 9.4 |
9.5 9.5 9.5 |
9.35 9.1 9.6 |
| Breadth of palate at P4 | 3.36 3.3 3.5 |
3.33 3.1 3.4 |
3.37 3.1 3.8 |
3.44 3.1 3.7 |
3.66 3.6 3.7 |
3.45 3.4 3.5 |
| Breadth of palate at M3 | 2.4 2.3 2.6 |
2.55 2.3 2.7 |
2.66 2.3 3.2 |
2.74 2.5 3.0 |
3.11 3.0 3.2 |
2.77 2.6 2.9 |
| Palatal length | 8.67 8.4 9.1 |
8.98 8.8 9.2 |
9.38 9.3 9.8 |
9.59 9.0 10.0 |
9.73 9.5 9.9 |
9.8 9.6 10.1 |
| Distance from incisors to postpalatal notch | 8.53 8.4 8.7 |
8.98 8.5 8.5 |
9.08 9.0 9.8 |
9.68 9.2 10.0 |
9.73 9.5 9.9 |
9.80 9.6 10.1 |
| Interorbital breadth | 4.25 4.2 4.3 |
4.19 4.0 4.4 |
4.2 4.0 4.4 |
4.2 4.0 4.4 |
4.23 4.1 4.4 |
4.2 4.2 4.2 |
| Average length of upper molar series | 3.2 3.2 3.4 |
3.2 3.2 3.4 |
3.21 2.9 3.5 |
3.22 2.9 3.5 |
3.2 3.2 3.2 |
3.16 3.1 3.2 |
| Breadth of braincase | 9.5 9.3 9.7 |
9.58 9.2 9.7 |
9.61 9.1 10.0 |
9.68 9.3 10.0 |
9.83 9.5 10.2 |
9.63 9.3 9.9 |
| Zygomatic breadth | 10.33 10.0 10.7 |
10.49 10.4 10.9 |
10.55 10.1 11.2 |
10.80 10.7 11.2 |
11.0 10.5 11.5 |
11.25 11.2 11.3 |
| Condylobasal length | 16.9 16.6 17.1 |
18.33 17.4 19.2 |
18.80 18.2 19.5 |
19.33 18.5 19.9 |
19.6 19.4 19.8 |
19.9 19.5 20.3 |
Measurements of external parts in Zapus are more variable than are measurements of most parts of the skull. As Hoffmeister (1951:16) points out for Peromyscus truei, this variation in external features results in part from “the difficulties in accurately measuring soft parts of the anatomy” and also from inconsistencies on the part of collectors in making these measurements.
A comparison of coefficients of variation (see table 3) for cranial measurements between populations of like age and sex for the species Z. hudsonius, Z. princeps, and Z. trinotatus shows that variation of approximately the same degree is recorded in corresponding elements in all species; that is to say, structures which are most variable individually in Z. princeps are also most variable in Z. trinotatus and Z. hudsonius.
Individual variation in the occlusal pattern of the molariform teeth is slight. In several specimens, however, the re-entrant fold is absent from the lingual surface of M1. Teeth in addition to the normal number were recorded for five specimens. In all instances they are in the upper dentition and usually at the posterior end of the maxillary tooth-row. In each of four specimens (KU No. 34852, KU No. 32852, MVZ No. 52105, all Z. princeps, and USBS No. 22921, Z. hudsonius), there is only a single additional tooth. One individual (USBS No. 264388, Z. princeps) possessed two extra molars, one in each maxillary tooth-row. The extra teeth vary in size from those which are only slightly smaller than the adjacent normal molars to those which are simple, peglike structures. In four of the five animals the extra teeth are posterior to the normal M3; in the fifth (MVZ No. 52105) the added tooth is anteriormedial to M3.
Table 3.—Coefficients of Variation for Dimensions of Corresponding Parts of the Skull of Three Species of Zapus. The Specimens of Zapus hudsonius are from Menominee and Keweenaw counties, Michigan, the Zapus princeps are from the Vicinity of Encampment, Wyoming, and the Zapus trinotatus from Huntingdon, British Columbia.
| Species | Z. h. hudsonius | Z. p. princeps | Z. t. trinotatus |
| No. examined | 52 | 46 | 19 |
| Mastoid width | 2.85 | 1.98 | 2.21 |
| Occipitonasal length | 2.64 | 1.37 | 1.20 |
| Incisors to postpalatal notch | 3.02 | 2.56 | 2.56 |
| Interorbital constriction | 2.75 | 3.66 | 3.22 |
| Zygomatic breadth | 2.74 | 2.54 | 1.94 |
| Maxillary tooth-row | 4.50 | 4.44 | 3.82 |
The size and shape of certain cranial elements vary individually even between right and left sides of the same animal. The paired parietal bones in some animals are nearly square and identical. In other animals these bones are approximately equal and straight on three sides with the fourth side forming an anterolateral projection; this projection may be slightly or greatly produced, and opposite elements in a single individual differ in this respect.
The interparietal also is variable; the lateral arms may be blunted and not included in the fusion of the squamosal, parietal, and occipital elements, or the interparietals may be elongated and fused with these elements. Posterior and anterior borders of the interparietal may be straight, produced anteriorly, produced posteriorly, or produced anteriorly and posteriorly.
There is frequently variation in the degree of taper of the nasals. They may be parallel sided, narrowed distally, or narrowed proximally. There is some variation in the degree of inflation, in the size, and in the shape of the frontal bones. The anterior surface of the postpalatal notch varies individually and may be truncate, anteriorly convex, or anteriorly concave.
Individual variation in the color of the pelage of animals that are in the same stage of molt or non-molt is by my observation slight. The presence of oil in the hair results in a false impression of sleekness and seemingly darker pigmentation. Abnormal white-spotting dorsally occurs as does yellow and melanistic coat color. These mutations are considered in the discussion concerning pelage.
In specimens of the two sexes from similar age groups of hudsonius from Michigan, the mean values for each measurement for the two sexes differ only slightly or are essentially the same (see table 4). In no species has secondary sexual variation been found to be greater than individual variation.
Table 4.—Mean Measurements for Adult Male and Female Z. hudsonius of Age Group 2 and Per Cent Difference of Females to Males (Specimens from Michigan).
| Sex | Male | Female | Per cent difference, females to males | |
| No. examined | 18 | 15 | ||
| Total length | 202.85 |
202.88 |
0.02% | larger |
| Hind foot | 122.85 |
122.10 |
0.60% | smaller |
| Mastoid width | 10.10 |
10.28 |
1.50% | larger |
| Occipitonasal length | 22.15 |
22.03 |
0.55% | smaller |
| Incisors to postpalatal notch | 9.39 |
9.33 |
0.64% | smaller |
| Zygomatic breadth | 10.47 |
10.57 |
0.95% | larger |
| Maxillary tooth-row length | 3.52 |
3.60 |
0.23% | larger |
| PAGE | |
| Zapus trinotatus | 385 |
| Zapus trinotatus eureka A. B. Howell | 389 |
| Zapus trinotatus montanus Merriam | 390 |
| Zapus trinotatus orarius Preble | 391 |
| Zapus trinotatus trinotatus Rhoads | 392 |
| Zapus princeps | 394 |
| Zapus princeps cinereus Hall | 399 |
| Zapus princeps curtatus Hall | 400 |
| Zapus princeps idahoensis Davis | 401 |
| Zapus princeps kootenayensis Anderson | 404 |
| Zapus princeps luteus Miller | 406 |
| Zapus princeps minor Preble | 407 |
| Zapus princeps oregonus Preble | 409 |
| Zapus princeps pacificus Merriam | 412 |
| Zapus princeps princeps Allen | 414 |
| Zapus princeps saltator Allen | 416 |
| Zapus princeps utahensis Hall | 418 |
| Zapus hudsonius | 420 |
| Zapus hudsonius acadicus (Dawson) | 432 |
| Zapus hudsonius alascensis Merriam | 435 |
| Zapus hudsonius americanus (Barton) | 436 |
| Zapus hudsonius campestris Preble | 441 |
| Zapus hudsonius canadensis (Davies) | 442 |
| Zapus hudsonius hudsonius (Zimmerman) | 443 |
| Zapus hudsonius intermedius Krutzsch | 447 |
| Zapus hudsonius ladas Bangs | 449 |
| Zapus hudsonius pallidus Cockrum and Baker | 450 |
| Zapus hudsonius preblei Krutzsch | 452 |
| Zapus hudsonius tenellus Merriam | 453 |