rostrum pointed and short; mastoid region relatively narrow; incisive
foramina short; base of zygomatic process of squamosal narrow; coronoid
process of mandible short, relatively weak. Upper premolar usually small
(averaging .30 mm in length and .35 mm in breadth) sometimes functional
(most often so in old adults), occlusal surface divided by single
shallow re-entrant fold, which in worn teeth forms centrally located
lake; tooth-row short as compared to that of other species; individual
cheek-teeth usually smaller than those of other species; lower
cheek-teeth shorter and narrower than those of other species; angle of
mandible strongly inflected.
GEOGRAPHIC VARIATION
The species _Z. hudsonius_ is divisible into 11 subspecies based on
differences in color, relative proportions of the tail, hind feet, body,
and size and shape of parts of the skull (zygomata, braincase, incisive
foramina, auditory bullae, pterygoid fossae, rostrum, and interorbital
breadth).
Color of the pelage varies, as a general rule, from dark-backed,
dull-sided individuals in the northern parts of the geographic range of
the species to light-backed, bright-sided individuals in the southern
parts of the range.
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.
Seemingly there is no clinal variation in the several qualitative
features of the cranium, for instance in the shape of the auditory
bullae, shape of the incisive foramina, and shape of the postpalatal
notch. On the other hand, the dimensions of the entire skull show that
the larger crania are of the northernmost subspecies and the smaller of
the southernmost subspecies.
NATURAL HISTORY
_Habitat._--_Zapus hudsonius_ occurs in low undergrowth usually of
grasses or forbs or both, in open coniferous forests, deciduous hardwood
groves, or in stands of tall shrubs and low trees, but most frequently
in open, moist areas.
Quimby (1951:75) notes that jumping mice were more common in the moist
lowlands than in the drier uplands. More were in the open type lowlands
than in the forested type, and these mice favored habitats normally
bordered by small streams affording moist to semi-aquatic living
conditions. The reports of Goodwin (1924:255), Christian (1936:416), G.
S. Miller (1899:329), Cory (1912:249), Lyon (1936:277), Stoner
(1918:123), and others, although concerning widely different parts of
North America, indicate that _Z. hudsonius_ selects habitats in
vegetation of like form, even though different assemblages of plant
species may be involved.
An average of 11.91 mice per acre was recorded by Quimby (1951:91) from
a study plot at Itasca Park, Clearwater County, Minnesota. He gives the
monthly population densities per acre for _Z. hudsonius_ at Centerville,
Anoka County, Minnesota, as follows: June 2.78, July 3.57, August 3.10,
and September 1.81. Blair's (1940:248) data on bi-monthly population
density per acre for _Z. hudsonius_ on the Edwin S. George Reserve,
Livingston County, Michigan, are remarkably similar, when adjusted on a
monthly basis, to those obtained by Quimby (_loc. cit._). Blair's (_loc.
cit._) monthly population densities per acre are as follows: June 3.90,
July 3.85, August 3.10, and September 2.00. Townsend (1935:90) estimated
population densities per acre for _Z. hudsonius_ in central New York
state, at 11 to 72 individuals. As Quimby (1951:92) points out,
Townsend's figures are probably too high, as commonly is the case when
the moving quadrat technique is used because animals from neighboring
areas enter the trapped area to take over the niches made available by
their predecessors' removal.
The population of _Z. hudsonius_ may vary considerably from year to year
as well as seasonally. Blair (1940:249) found notably fewer jumping mice
on the George Reserve in 1938 than in 1939. Quimby (1951:94) found the
numbers of _Zapus_ to be highly variable and thought that there was a
rapid turnover. Young animals were not caught until July when 25 per
cent were either juveniles, young, or subadults; from this time on these
age classes increased to a high of sixty-one per cent in September.
Quimby (_loc. cit._) found that separating the individuals into their
proper age classes was more difficult in September, since the young
from early litters are adultlike in appearance. His data indicate as he
remarked, "That the over-wintering adults are, for the most part,
gradually replaced by the young of the year as the summer progresses."
The sexes in _Z. hudsonius_ vary only slightly from a one to one ratio.
Quimby (1951:63) found a sex ratio of 110 females to 100 males and Blair
(1940:245) records a sex ratio of 113 males to 100 females. Townsend
(1935:42) records a sex ratio in central New York of 155 males to 100
females. Such a wide variation from a one to one ratio suggest that the
moving quadrat technique, which Townsend (1935:90) employed in obtaining
his data, may be, in some way unknown to me, more selective for the
males.
_Behavior._--The saltatorial powers of _Z. hudsonius_ are well developed
and often have been described in the literature. Stoner (1918:123)
remarks that, "When disturbed _hudsonius_ moves away by a series of
leaps ... the distance traversed in one of these leaps is from six to
eight feet."; Cory (1912:249) observed these mice to make surprisingly
long leaps, and, according to him, a distance of 10 feet is by no means
unusual; Handley and Patton (1947:49) credit these animals with jumping
eight to ten feet at a single bound; Hamilton (1935:190) remarked that
he noted an average of not more than four to six feet per jump; Townsend
(1935:91) observed one individual make jumps of about two feet; and
Harper (1932:29) records a jumping mouse leaping for distances of two to
three feet. Quimby (1951:72) notes that he had never seen one jump
farther than three feet. He found that the greatest jumps occurred
initially and normally covered a distance of two to three feet;
subsequent leaps were shorter but more rapid. 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.
Concerning the use of the tail as a balancing organ, G. S. Miller
(1899:330) describes the behavior of a jumping mouse from which the tail
had been severed by the sickle of a mowing machine. "When I approached,
it made violent efforts to escape, but the moment it was launched in the
air, its body, deprived of its balancing power, turned end over end so
that it was as likely as not to strike the ground facing the direction
from which it had come."
Riparian animals such as _Z. hudsonius_ need enter the water to escape
from enemies or perhaps in search of food. _Zapus hudsonius_ can and
does swim. Hamilton (1935:190) found it to be a strong swimmer capable
of remaining in the water for from four to five minutes. According to
Hamilton (_loc. cit._), when the mouse is swimming the head is held
high, the tail is arched near its middle, and only the hind limbs are
employed in propulsion. According to Sheldon (1938:327), Philip Allan,
in northern Minnesota, saw many _Z. hudsonius_ swimming three or four
inches under the surface of the water. The mice swam upstream and only
the hind legs were employed in the swimming movements. N. A. Preble
(1944:200), at Archer's Pond, 3 miles southeast of Center, Ossipee
County, New Hampshire, observed a jumping mouse swimming rapidly under
water toward another portion of the shore 30 or 40 feet away. The mouse,
swimming less than a foot beneath the surface, was vigorously using both
forefeet and hind feet, but the long tail trailing limply behind,
contributed in no way to the animal's movements. Quimby (1951:72)
released five of the mice, one at a time, in the open water of a lake.
He followed alongside in a boat and observed that, "In all instances the
animals proved to be excellent swimmers both on and underneath the
surface. The methods of progression were similar to land movements; i.
e., the limbs were employed differently at various times depending upon
the speed. When first placed in water they moved rapidly by lunges
produced by sweeping strokes of the hind limbs employed simultaneously.
This movement was accomplished similarly to the long jumps made on
land ... Following the first excited lunges, they settled down to a
steadier and slower gait using all four limbs one at a time. The
anterior part of the body was held high in the water ... When approached
too closely, they attempted to escape by diving. The maximum distance
noted was about four feet ... One was able to swim vigorously for
approximately three minutes after which it tired greatly and was in
danger of drowning."
As concerns digging ability, Goodwin (1935:148) reports that _Z.
hudsonius_ makes its own burrows; these are short and close to the
surface in the summer but longer, deeper, and below the frost-line in
winter. Two captives used their forefeet and nails in digging a tunnel
in the foot of soil that Goodwin (_loc. cit._) had placed in their cage.
Quimby (1951:72) remarks that captives excavate soil by means of the
front feet and throw the soil out behind; as the burrow deepened the
hind feet were also utilized to throw the loose soil out of the burrow.
_Zapus hudsonius_ climbs; Sheldon (1934:293) observed captive animals to
climb over small evergreen trees in their cages. They moved with
surprising sureness and agility, chasing each other among the branches
or sitting for several minutes at a time on one of the limbs. Hamilton
(1935:190) found that the mice ran over limbs and brush which were
placed in their outdoor enclosure.
Ordinarily _Z. hudsonius_ is nocturnal, appearing in the early dusk and
remaining active until pre-dawn. Occasional individuals are abroad in
daylight hours. Sheldon (1934:293) found in Nova Scotia that _Z.
hudsonius_ is most active from early dusk through the night, but that it
may be abroad in daylight as well. Her statements are based on trapping
results, field observations, and observations made on captive
individuals. Quimby (1951:73) found that _Z. hudsonius_ in Michigan is
mostly nocturnal; however, he saw mice on a few occasions in the
daytime. Diurnal activity seems to be increased in cloudy or damp
weather; Quimby (_loc. cit._) almost invariably trapped more of these
mice on cloudy, damp days than on other days.
This jumping mouse usually is silent but does utter various sounds.
Sheldon (1934:295) records squeaking and clucking noises. Quimby
(1951:73) records the clucking noise described by Sheldon (_loc. cit._)
and mentions also the squeaking and suckling sounds produced by the
small young. This mouse is most vociferous when young or when about to
go into hibernation. Sheldon (1938:327) writes that _Z. hudsonius_ makes
a drumming noise by vibrating the tail against dry leaves.
Many data are available concerning the hibernation of _Z. hudsonius_. In
general it seems necessary for the mice to put on a certain amount of
fat preparatory to hibernation. This fat is deposited in a thin layer
over the inside of the skin, over the back, and in the body cavities.
The thickest deposits are in and about the inguinal region.
Quimby (1951:83) noted that gain in weight was accelerated in a brief
period prior to entrance into hibernation. This relationship of rapid
gain in weight to hibernation allows a person to estimate the date of
hibernation. Cold weather seems to hasten hibernation, but less so than
the correct physiologic condition which is foreshadowed by a rapid gain
in weight. For example, Quimby's (1951:84) data reveal that mice that
were moved to a heated room gained weight and hibernated in a fashion
similar to those in unheated surroundings. Hamilton (1935:193) states
that, "It seems necessary for the mouse to lay on a certain amount of
fat before it is capable of hibernation." Hamilton (_loc. cit._)
reported that 18 specimens of _Z. hudsonius_ taken [presumably in an
active state] near Ithaca, New York, on November 13, were without a
trace of fat.
Data that are available concerning the hibernation sites of _Z.
hudsonius_ show that almost invariably these mice seek shelter in
burrows beneath the surface of the ground and there construct nests of
grass, leaves, or some other vegetation. Nicholson (1937:103) found a
hibernating _Z. hudsonius_ on the George Reserve, Livingston County,
Michigan, on October 20. The mouse was in a nest, composed of 10 to 12
damp elm leaves, in a sand bank two feet three inches vertically and
three feet nine inches horizontally from the surface. On April 11, 1948,
Schwartz (1951:228) found five nests (three with occupants) of _Z.
hudsonius_ at Jefferson City, Cole County, Missouri. All nests were one
foot beneath the surface of a pile of coal-ash, which was about three
and one-half feet high and five feet in diameter. The nests were
spherical, approximately four inches in diameter and consisted of dried
oak leaves and bits of dried grass. Grizzell (1949:74) found two
hibernating jumping mice at the Patuxent Research Refuge, Laurel,
Maryland, in January, 1948. The mice were in separate woodchuck dens;
one mouse was 40 inches below the surface and the other was 26 inches
below the surface. The mice were curled up in the center of masses of
dead leaves, and thus, were well insulated against the cold. On April
29, 1944, at Ithaca, New York, Eadie (1949:307) uncovered a hibernating
jumping mouse. The nest, about the size of a baseball, was compactly
made of fine grasses and was 10 inches below the surface of the ground
in a mound of earth that was approximately six by four feet at the base
and three feet high.
From the foregoing reports on hibernation sites it is evident that well
drained areas are utilized. Sheldon (1934:300) remarks that the burrows
used for hibernating are dug in a bank or some place from which the rain
water and melted snow probably drains off.
Eadie (1949:307), Grizzell (1949:75), Sheldon (1934:299), Schwartz
(1951:228), and Sheldon (1938:331) all agree that the hibernating mouse
rolls up into a ball-like shape (resting on its head and pelvis) with
the head between the hind legs, the nose against the lower belly, the
forefeet curled on the chest, and the tail curled around the head and
body.
A marked loss of weight occurs immediately after hibernation begins,
and then reduction in weight is slow and regular. (See Hamilton,
1935:194 and Quimby, 1951:84.)
Sheldon (1934:297) cites a letter from Vernon Bailey in which he remarks
on the necessity of abundant moisture and saturate air for hibernating
jumping mice. Bailey writes "... they will awaken at times famished for
water and will drink and drink before going back to sleep."
Hamilton (1935:195) thinks that in the Ithaca area of New York these
mice probably leave their winter quarters in the second half of April
and that in southern New York and Long Island they emerge considerably
earlier. Quimby (1951:82) and Bernard Bailey (1929:163) report that
males appear earlier in the spring than do the females. Quimby (_loc.
cit._), by recording the sequence and dates of phenological events and
appearance of _Zapus_ in several years, was able to predict fairly
accurately the time of emergence of _Zapus_ in a succeeding year. In
Minnesota, jumping mice emerged late compared to other hibernating
rodents.
_Enemies._--V. Bailey (1927:119) reports that A. K. Fisher found 50
skulls of _Zapus_ in barn owl pellets taken from the towers of the
Smithsonian Institution, Washington, D. C. Dearborn (1932:32) reported
mink as having fed on jumping mice. Surface (1906:197) records taking a
_Zapus_ from the stomach of a rattlesnake. Pearson and Pearson
(1947:138) found remains of _Z. hudsonius_ in pellets of barn owls.
Quimby (1951:74) reports two cases of predation on _Z. hudsonius_; one
was by a northern pike, _Esox lucius_ Linnaeus and the other was by a
weasel, _Mustela_ sp. Vergeer (1948:91) collected a green frog, _Rana
clamitans_ Latreille, which had eaten a jumping mouse.
Quimby (1951:74) frequently found the fleas, _Megabothris quirini_
Rothschild, and _Megabothris wagneri_ (Baker), and occasionally a larval
tick, _Dermocenter variabilis_ (Say), on _Z. hudsonius_. Sheldon
(1934:296) remarks that captive animals are burdened with numerous
fleas. Hamilton (1935:191) removed a louse from a jumping mouse. One
mouse had a hole in the throat and three others had holes in the
inguinal region; presumably bot-flies had emerged from these holes. Test
(1943:507) found a single _Cuterebra_ larva in the inguinal region of a
_Z. hudsonius_, and Sheldon (1938:328) found _Z. hudsonius_ infested by
larvae of _Cuterebra fontinella_ Clark. Here, as in other cases, these
larvae were found immediately below the skin. Erickson (1938:252)
examined 18 _Z. hudsonius_ obtained in Minnesota, and found that three
were parasitized. He found a bot-fly larva, _Cuterebra_ sp., nematodes
of the genera _Subulura_ and _Spirocerca_, and a fluke of the genus
_Notocotylus_.
_Food._--Quimby (1951:85-86) studied the food preferences, by presenting
to caged _Z. hudsonius_ the plants and invertebrate animals normally
available to these mice in nature, and indicates that in general, the
starchy fruits of the Gramineae and the less fleshy fruits of various
groups of plants are more heavily utilized than other plant materials.
His observations indicate that these rodents are highly insectivorous
and that they consume many insects under natural conditions. Goodwin
(1935:148) reports that the stomach contents of several individuals
obtained at South Woodstock, Connecticut, consisted exclusively of
blackberries, and that others had subsisted principally on cranberries.
Hamilton (1935:197) remarks that seeds are the favored food but that
berries, nuts, fruits of various kinds, roots, and insects are also
utilized. Stoner (1918:123) writes that the food in cultivated areas of
Iowa is various grains as well as grass and weed seeds; in wooded places
the mice feed on seeds and nuts of trees. Vernon Bailey (1927:118)
states that the examination of a great many stomachs of these jumping
mice [in North Dakota] revealed nothing "but the fine white pulp of
carefully shelled, well-masticated seeds. Generally these are from
grasses, although grain and a variety of other plant seeds are eaten."
Schmidt (1931:116) examined the stomach contents of several _Z.
hudsonius_ taken in Clark County, Wisconsin, and in most stomachs found
the remains of finely chewed roots; however, two from Hewett had eaten
several geometrid caterpillars.
Lyon (1938:279), Stoner (1918:123), and J. W. Bailey (1946:263) present
information which indicates that _Z. hudsonius_ stores food in its nests
or burrows. Possibly these mice awaken at intervals from hibernation and
eat.
"These rodents characteristically seize the material to be eaten with
the front feet and devour it while reclining on their haunches. The
following observation of a caged animal is typical of their feeding
habits. The mouse selected a head of yellow foxtail, _Setaria glauca_
(Weig.) Stuntz, from several in the cage, separated it by gnawing
through the supporting stem, seized it with the front feet, held it up
to the mouth and began to gnaw at one end, stripping all parts from the
rachis. The grass head was slowly rotated and shifted sideways until
nothing remained but the rachis which was discarded. Actually the seeds
were the only parts eaten ..." (Quimby, 1951:73). Sheldon (1934:294)
remarks that _Z. hudsonius_ eats from a squatting position and holds the
piece of food in the forepaws. The mouse seems to bite off a seed, and
then, holding it in the forepaws, transfers it to the mouth.
According to Sheldon (_op. cit._:295) and Quimby (_loc. cit._), caged
jumping mice drink water. When drinking, the mouths of the mice are in
contact with the water, but neither observer determined whether the mice
lapped or sucked the water. Sheldon (_loc. cit._) observed these mice
passing stems of long grass through their mouths as though to squeeze
out moisture, and thought that the mice obtain most of their required
moisture from green plants.
_Reproduction._--The breeding season begins shortly after the jumping
mice emerge from hibernation in the spring, and reproduction continues
until a few weeks before they hibernate in the autumn. The extent of the
breeding period probably varies geographically and possibly seasonally.
For example, Quimby's (_op. cit._:70) information suggests that the 1947
period of parturition occurred between June 15 and August 30 in the area
of Centerville, Minnesota. In Michigan, Blair (1940:246) found a peak of
breeding activity in spring and another in late summer with little
activity in the intervening midsummer. Brimley (1923:263) records a
female in North Carolina, with eight embryos on June 13, 1895, and
another with seven embryos on September 17, 1891, indicating a strong
possibility of two litters per year there. Vernon Bailey (1927:118)
records young born in May or June in North Dakota and thinks that there
is time for only one litter per year. Petrides (1948:76) captured a
female on September 22, 1944, at Athens, Georgia, that gave birth to six
young on September 29. This late parturition date indicates a longer
breeding season in the southeastern part of the range of _Z. hudsonius_.
The gestation period of nonlactating, caged _Z. hudsonius_, Quimby
(1951:63) thinks, "is approximately 18 days ... [but] gestation is
prolonged in lactating females."
Data from museum labels indicate that embryos in 62 pregnant females
averaged 5.4 (2-8) per female. Quimby (1951:67) found the average number
of embryos per female for 14 females taken in Minnesota, to be 5.3 and
that litters of young found in nests averaged 5.8. Sheldon (1938:330)
reports two litters of seven young each and one of four young for _Z.
hudsonius_ in Vermont. Petrides (1948:76) records a litter of six young
for _Z. hudsonius_ in Georgia. Brimley (1923:263) records one lot of
seven and one lot of eight embryos for _Z. hudsonius_ in North
Carolina. Vernon Bailey (1923:120) reports six embryos for a female of
_Z. hudsonius_ taken in Washington, D. C. Ivor (1934:8) obtained a
litter of five young _Z. hudsonius_ from Erindale, Peel County, Ontario.
Hamilton (1935:195) records litters of two, four, and five young and
embryo counts of four, two, four, and four for _Z. hudsonius_ in New
York.
There seems to be two litters per year. According to Quimby (1951:69),
"most adult females breed soon after emergence from hibernation and
produce the first litters within a month. The remaining females do not
breed immediately but produce the first litter," he says, "in the second
month after emergence." Both early-breeding females and late-breeding
females produce at least 2 litters per year. Those that breed early may
have 3 litters.