Philip H. Krutzsch

“ 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. ”

“ Remarks.—Zapus hudsonius intermedius has a large geographic range. There is some variation detectable when individuals from widely separate localities are compared, but where there is much variation it is obviously the result of intergradation. All characters differentiating Z. h. intermedius from any contiguous subspecies are not present in every specimen even in the type series. Nevertheless, a certain series of cranial characters (narrow incisive foramina, short rounded auditory bullae, parallel lateral margins of nasals and narrow pterygoid fossae) is diagnostic. ”

“ GEOGRAPHIC VARIATION
There are four subspecies currently recognized, all of which are confined to the Pacific coastal region of North America (See fig. 45) . The features that vary geographically are external size, color of pelage (shade and tone of upper parts and tint of lower parts) , and dimensions of certain cranial structures (zygomata, braincase, incisive foramina, palatal bridge, auditory bullae, and pterygoid fossae) .
External size is smallest in the southernmost geographic race (Z. t. orarius) and largest in the northernmost geographic race (Z. t. trinotatus) . ”

“ 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. ”

“ 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. ”

“ Only the head and body of the malleus and the short and long limbs and body of the incus are sufficiently consistent [365] 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 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 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. ”

“ Zapus luteus australis, based on a single individual taken in a riparian thicket along the Rio Grande at Socorro, New Mexico, is referable to Z. p. luteus. The diagnostic characters, referred to in the original description, are as follows: Small, slender, and very narrow skull; especially narrow braincase; slender rostrum; and light dentition. These are expressions of age, rather than of geographic variation, in that the individual is a subadult (young of the year) . ”

“ A jumping mouse in full retreat progressed by jumps of about one foot.
Statements concerning the gait of Z. hudsonius are not in agreement but the consensus of opinion is that these animals when unfrightened progress by a series of hops of one to six inches, or, occasionally, with a slow creeping motion while the animal is on all fours. When frightened, however, their progress is by long bounds; the mice make a series of two or three such leaps to the nearest protective cover, and then sit motionless until pursued. ”

“ Animals at the northern periphery of the range (lower peninsula of Michigan to the west and southeastern Massachusetts to the east) are largest and darkest; to the southward there is a progressive reduction in size and a change to a lighter, brighter color. Animals from Maryland, Virginia, and North Carolina are more nearly average representatives of the subspecies than are those from the region of the type locality.
A jumping mouse allegedly of this subspecies has been recorded by Coleman (1941:91) from Caesars Head, 300 ft., South Carolina. ”

“ 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. ”

“ In the fourth week the juvenal pelage is replaced by adult pelage. The eyes open between the 22nd and 25th days. The color of the incisors changes from white to yellowish-orange as in the adults. P4, M1, M2, m1 and m2 have emerged from the maxillary and dentary bones; M3 and m3 have not yet erupted. A mouse 33 days old had all teeth well developed. By the end of the 4th week the young, except for size, are adultlike and capable of independent existence.
The greatest increase in dimensions of the body is in the first four weeks. A slowing down of growth is simultaneous with weaning. ”

“ 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) . ”

“ 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 ”

“ 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 ”

“ From Zapus princeps minor, Z. p. idahoensis differs in: Size larger; color of underparts less ochraceous; lateral line indistinct or wanting; skull averaging larger in all measurements taken except that the two subspecies are approximately same in least interorbital constriction, length of zygomatic arch, and distance from anterior face of incisors to postpalatal notch; nasals, in profile, straight instead of with proximal third depressed; postpalatal notch anterior to posterior face of last molar, instead of even with, or usually posterior to, same. ”

“ 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 [374] 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. ”

“ 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. ”

“ Much of this area is unsuitable to jumping mice but local marshy places might be inhabited. Much territory inhospitable to Zapus intervenes also between the ranges of Z. h. preblei and Z. h. campestris. This area (northern Platte, Goshen, eastern Converse, Niobrara, and southern Weston counties, Wyoming) is chiefly rolling hills and short grass prairie and, like that to the east, is only locally suitable for Zapus. If jumping mice do occur in suitable places in these intervening areas it is to be expected that they will show intergradation between the subspecies concerned. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 [364] 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 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. ”

“ Upper parts generally dull, averaging lighter, less black-tipped hair; dorsal band less distinct; sides duller; averaging smaller in most cranial measurements taken; least interorbital constriction narrower; auditory bullae smaller, less well inflated; incisive foramina narrower, not truncate posteriorly; frontal region usually more inflated.
From Zapus hudsonius pallidus, Z. h. preblei differs as follows: Upper parts generally duller (less ochraceous) ; dorsal band less distinct; sides paler (not bright Ochraceous-Buff) ”

“ It seems that ecological difference rather than [369] 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. ”

“ Zapus hudsonius ontarioensis Anderson (1942:59) from eastern Ontario was based chiefly, in comparison with Z. h. canadensis, upon, “dorsal stripe less distinct and sides somewhat duller yellowish with more admixture of blackish hairs.” Examination of 68 of the 69 specimens from the type locality shows that 58 are subadult and in subadult pelage. Individuals which are adult are indistinguishable in color of pelage and in cranial features from comparable material from southern Quebec. Z. h. ontarioensis is, therefore, considered to be a synonym of Z. h. canadensis. ”

“ Individuals from the southernmost geographic races (Z. h. americanus and Z. h. pallidus) are the smallest for the species and those from the northernmost subspecies (Z. h. alascensis) are the largest. One subspecies, Z. h. campestris, from the central part of the range of the species, however, seems to be out of the cline. This form inhabits the eastern foothills of the Rocky Mountains and is a robust animal approaching Z. princeps in size. ”