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.
[Illustration:
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).
[Illustration: 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.
NONGEOGRAPHIC VARIATION
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.
Age Variation
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
---------------+------------+------------+------------
Number | | |
examined | 4 | 13 | 33
| | |
Occipitonasal | 20.5 | 21.2 | 22.0
length | 20.0 21.2 | 20.8 21.8 | 21.5 23.2
| | |
Mastoid | 9.8 | 10.04 | 10.12
breadth | 9.7 10.0 | 9.6 10.4 | 9.5 10.5
| | |
Length of | 8.07 | 9.02 | 9.07
zygomatic arch | 8.0 8.2 | 8.5 9.3 | 8.5 9.4
| | |
Breadth of | 3.36 | 3.33 | 3.37
palate at P4 | 3.3 3.5 | 3.1 3.4 | 3.1 3.8
| | |
Breadth of | 2.4 | 2.55 | 2.66
palate at M3 | 2.3 2.6 | 2.3 2.7 | 2.3 3.2
| | |
Palatal | 8.67 | 8.98 | 9.38
length | 8.4 9.1 | 8.8 9.2 | 9.3 9.8
| | |
Distance from | | |
incisors to | 8.53 | 8.98 | 9.08
postpalatal | 8.4 8.7 | 8.5 9.5 | 9.0 9.8
notch | | |
| | |
Interorbital | 4.25 | 4.19 | 4.2
breadth | 4.2 4.3 | 4.0 4.4 | 4.0 4.4
| | |
Average length | | |
of upper | 3.2 | 3.2 | 3.21
molar series | 3.2 3.4 | 3.2 3.4 | 2.9 3.5
| | |
Breadth of | 9.5 | 9.58 | 9.61
braincase | 9.3 9.7 | 9.2 9.7 | 9.1 10.0
| | |
Zygomatic | 10.33 | 10.49 | 10.55
breadth | 10.0 10.7 | 10.4 10.9 | 10.1 11.2
| | |
Condylobasal | 16.9 | 18.33 | 18.80
length | 16.6 17.1 | 17.4 19.2 | 18.2 19.5
---------------+------------+------------+------------