at about the same time, a _P. maniculatus_ and all but one of her young
inexplicably died. Since the remaining young _maniculatus_, a male, was
just weaned and was considered expendable, I placed him in the cage with
the female _truei_ and her 33-day-old, male offspring. The reaction to
the newcomer was unexpected. The female immediately covered the _P.
maniculatus_ and her own young and prepared to defend them against me.
Later, when the _P. maniculatus_ was disturbed, he had only to emit a
squeak and the female _truei_ would run to cover and protect him. When
the young male of _P. truei_ was 69 days old the female kept him out of
the nest, but still kept the male _maniculatus_ in the nest with her.
Although the female was somewhat antagonistic to her own young, she did
not injure him, but only kept him out of the nest. The male _truei_ was
left in the cage with his mother and the _P. maniculatus_ from September
23 to December 10. None of the mice had any apparent cuts on the ears or
tail to indicate fighting. As much as seven months after the _P.
maniculatus_ was introduced into the cage, the female _truei_ continued
to cover him with her body whenever there was a disturbance. The male
_maniculatus_ not only tolerated this attention, but ran under the
female _truei_ when frightened. "Adoption" of young of another species
has been reported for a number of animals, but, without further
evidence, it is not possible to postulate that such adoptions occur
between species of _Peromyscus_ in nature.
Young males are tolerated by their mothers after weaning. One young male
_maniculatus_ was left in the cage with his mother from the time of his
birth in autumn until late February of the following year. A litter was
born on February 24. A young male _P. truei_ was also left in the cage
with his mother until he had acquired most of his postjuvenal pelage;
the female and male usually sat together in the cage.
Females of both species sometimes eat their young when the young die
shortly after birth. One female of each species killed three of her four
young, and ate their brains and viscera. In one of these cases, the
female, of _P. maniculatus_, also died; the female of _P. truei_ was the
same one that adopted the surviving _P. maniculatus_. The female _truei_
continued to nurse her one remaining young for at least several days
after killing three of his litter mates. A reason for this cannibalism
might have been that I had fed these mice for several weeks on a mixture
of grains low in protein content. Inadequacy of this diet for nursing
females may have caused them to become cannibalistic. The feed of all
captives was changed to Purina Laboratory Chow after the young were
killed.
Transportation of Young
Females of both species transported their young either by dragging them
collectively while the young were attached to mammae, or by carrying
them one at a time in the mouth. Since mice of the subgenus _Peromyscus_
have three pairs of nipples, they probably transport only six young
collectively. Svihla (1932:13) has stated that both pectoral and
inguinal teats are used in transporting young, in contrast to Seton's
reputed assertion that only inguinal nipples were used. But Svihla
neglected to cite Seton's complete statement. Seton (1920:137) recorded
a litter of three as using only the inguinal mammae, but on the
following page recorded the use of both inguinal and pectoral mammae by
another litter of four. My findings agree with those of Svihla. Nursing
females of both species were removed periodically from cages by lifting
them by the tail. The young would hang onto the mammae and the female
would clutch the young to her with all four feet. Young two weeks old or
older crawled behind the mother while nursing.
The method of transporting young in the mouth has been mentioned by
Seton (1920:136) and described by Lang (1925) and Hall (1928:256). These
authors report that the mother picks the young up in her paws, and
places it ventral-side up in her mouth, with her incisors around it. The
young are not picked up by the skin on the nape of the neck, as are the
juveniles of dogs and cats. I have found that females of both species of
_Peromyscus_ carry their young ventral-side up in their mouth while the
young are small, and sometimes when the young are older. Generally, when
females of _P. truei_ moved young weighing more than 10 grams, the
female grasped the young from the dorsal side, across the thorax just
posterior to the shoulders, and held them with the incisors more or less
around the animal. Perhaps this method was used with older young because
of the observed tendency of the larger young to resist being turned over
and grasped from the ventral side, and because their increased weight
would have made it difficult, if not impossible, for the mother to pick
them up with her paws. The young rarely resisted the efforts of the
mother to move them by this method; when grasped across the thorax by
the mother, the young would remain limp until released. Some females of
_P. truei_ would drag almost fully grown young back into the nest in
this manner. I have not observed older young of a comparable age to be
moved by females of _P. maniculatus_. The females of _P. maniculatus_
appear to be somewhat less concerned than those of _P. truei_ for the
welfare of their young once they are mobile and close to being weaned.
The following listing describes changes in postnatal development of
young, of each species, from birth to nine weeks of age.
_P. maniculatus_ _P. truei_
----------------------------------+-------------------------------------
FIRST WEEK: At birth, young are | At birth, young are helpless, red
helpless, red overall, small | overall, smaller than _P. truei_,
with wrinkled skin. Pinna of ear | skin wrinkled. Ear, eyes, and
folded over and closed; eyes | digits as in _P. truei_.
closed; digits not separated |
from rest of foot. |
|
Redness diminished by fourth day. | Redness decreases and disappears by
| fourth day.
|
Hair apparent by fifth day; | Hair apparent by fourth day; body
dorsal one-half or two-thirds of | bicolored by end of week.
body more darkly pigmented than |
venter by fourth day. |
|
Young squeak loudly and suck; | Young squeak loudly; sucking more
sometimes crawl, but drag hind | pronounced than in _P. truei_; may
legs. | crawl, but drag hind legs.
----------------------------------+-------------------------------------
|
SECOND WEEK: Appreciable increase | As in _P. truei_.
in size; head about 60 percent |
larger than at birth, by 14th |
day, and still large in |
proportion to body. |
|
Toes on hind foot separated more | As in _P. truei_, but somewhat more
from foot. | advanced.
|
Body well haired by end of week; | Body well haired by end of week;
dorsum dark gray, venter whitish; | dorsum dark gray with brownish
tail bicolored in most, but not | tint; venter whitish; tail
haired. | bicolored in most, but not haired.
|
Pinna of ear unfolded and open by | As in _P. truei_, but development
end of week. | somewhat more advanced.
|
Through day 10, use hind legs to | Crawl well by end of week;
push, but by end of week use legs | difficult to hold, squirm but do
to crawl; difficult to hold, | not bite; agile.
squirm but do not bite. |
|
Walk behind mother while nursing; |
agile. |
----------------------------------+-------------------------------------
|
THIRD WEEK: Eyes open on 16th to | Eyes open on 16th to 20th day,
21st day. | partly open earlier.
|
Gray pelage of dorsum brownish. | Pelage of dorsum brownish; molt
Apparently there is a molt line | line across shoulders progressing
progressing posteriorly from | posteriorly; browner anterior to
nose; the molt line has moved to | line, grayer posterior to it.
shoulder region by end of week; |
pelage anterior to line browner, |
grayer posterior to it. |
|
Tail haired and weakly bicolored | Tail haired and bicolored in all
in some individuals by end of | individuals.
week. |
|
Young walk and jump well; squirm | Young walk and jump well; fight and
but rarely bite. | bite when handled.
----------------------------------+-------------------------------------
|
FOURTH WEEK: Begin to eat solid | Some young eat grain by 24th day;
foods at 23-29 days, but also | others continue to nurse.
nurse. |
|
Molt line about 3/4 inch | Juvenal pelage complete; no sign of
posterior to head. Juvenal pelage | postjuvenal molt.
completed by end of week. Some |
young have brownish hair on front |
legs. |
|
Young roll over on backs and use | As in _P. truei_; also, all jump
feet to ward off litter mates | well, and fight fiercely when
that are dropped into nest, or | handled.
into container, with them. |
----------------------------------+-------------------------------------
|
FIFTH WEEK: Young weaned on 30th | All young weaned before or by end
to 40th day; some nurse beyond | of week; none observed to nurse
30th day if female is lactating. | beyond 30th day, even if female is
| lactating.
|
Juvenal pelage complete and no | Juvenal pelage complete;
postjuvenal molt apparent on | postjuvenal pelage not apparent on
dorsum. | most, but probably present on all,
| and concealed under juvenal pelage.
----------------------------------+-------------------------------------
|
SIXTH WEEK: Postjuvenal pelage | Postjuvenal molt apparent in most
apparent in most individuals | young; almost complete in some,
under juvenal pelage, especially | except above tail and on flanks.
along lateral line. |
----------------------------------+-------------------------------------
|
SEVENTH WEEK: Postjuvenal pelage | Postjuvenal pelage apparent in all
apparent in most young; in some | young; less distinct molt line than
the molt line has progressed well | in _P. truei_.
up on the sides, but not to |
mid-dorsum. |
----------------------------------+-------------------------------------
|
EIGHTH WEEK: All individuals | Growth completed in some
growing; total lengths of 156-170 | individuals; those in larger
millimeters; weight 17-22 grams. | litters have total lengths of
| 128-144 millimeters; weight
| 14-17 grams.
----------------------------------+-------------------------------------
|
NINTH WEEK: Testes partly scrotal | Scrotum in season usually large,
in one male on 59th day. | vaginae open, evidence of coitus
| common. (McCabe and Blanchard,
| 1950:39).
|
New brown pelage encroaching on | Postjuvenal molt completed in some
saddle and on hind legs; | individuals by end of week. New
postjuvenal molt completed in | pelage tends to be concealed under
some individuals by eleventh | juvenal pelage longer than in _P.
week. | truei_.
----------------------------------+-------------------------------------
CHANGES OWING TO INCREASE IN AGE
Increase in length of limb bones, changes in proportion of bones in the
skull, eruption and degree of wear of teeth, and changes in pelage can
be used to ascertain relative age. Different investigators might choose
different limits for the three categories young, subadult, and adult.
Museum specimens were assigned to one of five age groups listed below
mostly on the basis of tooth wear, essentially as described by
Hoffmeister (1951:1).
Juvenile: M3 just breaking through bony covering of jaw or showing
no wear whatsoever.
Young: M3 worn smooth except for labial cusps, and M1 and M2
showing little or no wear.
Subadult: M3 worn smooth; labial cusp may persist, but is well
worn; M1 and M2 having lingual cusps worn, but not smooth; labial
cusps showing little wear.
Adult: Lingual cusps worn smooth and labial cusps showing
considerable wear; labial cusp of M3 may persist.
Old: Cusps worn smooth; not more than one re-entrant angle per
tooth discernible, frequently none.
For live animals examined in the field, criteria based on pelage and
breeding condition were used, as follows:
Juvenile: Only gray, juvenal pelage present.
Young: Subadult pelage apparent on lateral line or on sides; body
usually smaller than in adults.
Subadults: Subadult pelage having mostly replaced juvenal pelage;
mice often as large as adults; testes of males often abdominal in
breeding season; gray juvenal pelage may persist on head of some
individuals.
Adult: Adult pelage present; body usually largest of all animals in
population; females may have enlarged mammae from nursing previous
litters; testes of males usually scrotal in breeding season; gray
pelage may be present on head of some individuals.
Old individuals in the field could not be distinguished from adults;
hence any animals that appeared older, or more developed, than subadults
were classified as adults.
In _P. truei_, subadult pelage appears first on the lateral line or on
the flanks; new pelage is ochraceous and contrasts markedly with the
gray juvenal coat. In _P. maniculatus_, the subadult pelage contrasts
less with the juvenal coat; the new pelage progresses from anterior to
posterior over the body in the same manner as in _truei_, but replaces
the juvenal coat in a less distinct manner than in _truei_. As a result,
contrast often is lacking between juvenal and subadult pelages in
_maniculatus_ making it difficult to assign an individual to one of
these two age categories when examined in the field. In museum
specimens, the subadult pelage is much more noticeable because it can be
compared with the pelages of other specimens. The subadult pelage in _P.
maniculatus_ is duller than the adult pelage: In _P. truei_ the subadult
and adult pelages appear to have an equal sheen.
In early winter, the postjuvenal pelage acquired by young individuals of
_P. truei_ was thick and luxuriant and indistinguishable from the winter
pelage of adults. My observations lead me to conclude that individuals
born late in the breeding season molt from juvenal summer pelage
directly into winter adult pelage. Technically, this new coat is the
postjuvenal one, yet it cannot be distinguished as such after the molt
is completed.
ANOMALIES AND INJURIES
Anatomical anomalies were rare in the individuals of _Peromyscus_ that I
examined. When anomalies were found they were striking, principally
because of their low rate of occurrence.
One female of _P. truei_, born in captivity, had a congenital defect of
the pinna of the right ear, noted on the fifteenth day after birth.
Closer examination then and later revealed that the pinna was normal in
all respects except that the tip was missing. The tip showed no evidence
of injury. When the mouse was subadult, this defective pinna was
approximately half as long as the normal pinna. The topmost part of the
defective pinna was somewhat more constricted in circumference than the
normal one.
On September 11, 1963, a subadult male of _P. truei_ was captured that
had five functional toes on its right front foot, the only one of more
than 175 individuals caught and handled in the field that exhibited
polydactyly. The front foot was examined closely in the field, but it
could not be determined how or where the extra bones of the sixth toe
articulated. _Peromyscus_ normally has four full-sized toes on each
front foot, and a small inner toe hardly more than an enlarged tubercle,
having no nail.
A few mice of both species had broken toes or claws torn off. Such
injuries were more common on toes of the hind foot. In several instances
the toes were shortened, as if by marking, although the animals
concerned had been marked earlier by clipping toes other than the
injured toes. The reason for these injuries is not apparent, although
they could have been caused by fighting, or from having been caught in
doors of Sherman live traps.
Toes of several mice were swollen and inflamed due to small glochids of
cacti that were stuck in them. Apparently the mice had stepped on the
glochids by chance, for I found no evidence that _Peromyscus_ of either
species eats cacti.
One _P. truei_ had a broken tail; three other individuals had tails
about one-half normal length. One _P. maniculatus_ had a shortened tail.
Some of these injuries probably were caused by the Sherman live traps;
several individuals of _P. truei_ were released after having been caught
by the tail by the spring-loaded door of these traps.
On October 17, 1963, an adult _P. truei_ had a bleeding penis; when this
mouse was recaptured on October 25, the injury was healed.
Losses Attributed to Exposure in Traps
Observations of wild mice caught in live traps suggest that metabolic
maturity is reached later than physical and reproductive maturity. In
such trapping, it became apparent that juvenal and young mice suffered
from exposure to cold and to heat much more than did subadult or adult
mice. Although traps were carefully shaded and ample nesting material
and food provided, some mice died in the traps. An overwhelming majority
of these mice were juveniles and young.
Traps were checked in the morning, both in the summer and autumn, yet
mice died in traps that were barely warm to the touch, in summer, and
cool to the touch in autumn. Older mice frequently were found in traps
that were warm, or even hot, to the touch; yet the older mice rarely
died in such traps. Apparently the tolerance of adults is much greater
to heating and chilling. Greater bulk and perhaps longer pelage in
adults might provide sufficiently better insulation to account for this
difference.
Occasionally juvenal mice were found in traps in a sluggish and weakened
condition, especially in autumn when nights were cool. In such cases the
mice were either cupped in the hands and warmed until lively enough to
fend for themselves, or, if especially weakened, were taken to the
laboratory. None of such animals that were returned to the laboratory
lived for more than two weeks. Most of those released in the field did
not reappear in the traps.
I conclude that juvenal and young mice placed under stress by
overheating or cooling die immediately or live only a few days. Subadult
and adult animals tolerate more extreme conditions of overheating or
cooling, presumably because they are able to regulate their internal
temperature better, by either losing or retaining heat more effectively.
Mice found dead in overheated traps had salivated heavily, and may also
have licked the fur on their chests to increase heat dissipation. One
such adult, of _P. truei_, had a wet chest when he was taken from a warm
trap; when released, this mouse ran to a nearby plant of _Comandra
umbellata_, and ate a few of the succulent leaves before running off.
This individual was trapped several times later in the summer, and
apparently suffered no ill effects from the exposure.
Dental Anomalies
Abnormalities in the formation and occlusion, or decay of teeth, are
relatively rare in wild mammals. Of all bodily structures, the teeth
apparently are under the most rigid genetic controls; they form early in
the embryo and follow rigidly specified patterns in their ontogeny.
Apparently any deviation from the normal pattern of tooth formation is
quickly selected against. All specimens of _P. m. rufinus_ and _P. t.
truei_ in the collection of the Museum of Natural History at the
University of Kansas, and in my collection, were examined for dental
anomalies. A total of 317 specimens of _P. m. rufinus_ and 54 specimens
of _P. t. truei_ were examined. The following specimens were found to
have abnormalities:
K. U. 69361, _P. maniculatus_, adult: Small bundles of plant fibers are
lodged between all upper teeth and have penetrated the maxilla anterior
to the left M1. The maxillary bone is eroded away from the roots of all
teeth. The anteriormost roots of both lower first molars are almost
completely exposed, because the dentary has been abraded away.
K. U. 76041, _P. maniculatus_, young: A piece of plant fiber is wedged
between the left M2 and M3. The maxillary bone has eroded away from
around the roots of M3, indicating the presence of an abscess in this
area.
K. U. 69362, _P. maniculatus_, adult: All teeth in the lower right
tooth-row are greatly worn, especially on the lingual side. The labial
half of the right M1 is all that remains; decay is apparent both in the
crown and roots on the lingual side of this tooth.
K. U. 69397, _P. maniculatus_, old: The maxillae have eroded away from
around the anterior roots of each first upper molar, leaving these roots
unsupported.
C. L. D. 231, _P. maniculatus_, old: The teeth in this female are
greatly worn; re-entrant angles are not visible in any teeth. A circular
hole, 0.1 millimeter in diameter, exists in the dentine immediately over
(when viewed from the underside of the skull) the posterior root of the
right M1. The crowns of the teeth are greatly reduced in height, and the
dentine is thin.
Anomalies in the Skull
Wormian bones and other abnormalities in the roofing bones are noted, as
follows:
K. U. 76090, _P. maniculatus_, young: The interparietal is divided; the
divided suture is in line with the suture between the parietals. The
interparietal is 7.8 millimeters long.
K. U. 76091, _P. maniculatus_, young: A wormian bone, 0.5 millimeter by
0.2 millimeter, lies between the anterior border of the interparietal
and the posterior border of the left parietal, at a point midway between
the center line of the skull and the posterolateral border of the
parietal bone.
C. L. D. 248, _P. maniculatus_, adult: An oval wormian bone, 1.1
millimeters long and 0.6 millimeter wide, lies between the parietals at
their posterior margin; the long axis of the bone is parallel to the
long axis of the skull.
C. L. D. 246, _P. maniculatus_, juvenal: The interparietal is divided
equally by a suture. An oval wormian bone, 0.3 millimeter long and 0.1
millimeter wide, lies between the frontals, midway between the anterior
and posterior borders of these bones.
C. L. D. 656, _P. maniculatus_, young: A small, rounded wormian bone
lies between the right parietal and interparietal, lateral to the
posterior junction of the suture between the parietals. This bone
extends anteriorly into the parietal bone from the suture of the
interparietal and parietal. This bone is 0.7 millimeter wide, and
extends 0.6 millimeter into the parietal.
C. L. D. 662, _P. maniculatus_, subadult: An elongated, diamond shaped
wormian bone closes the suture between the parietal bones. This bone is
2.3 millimeters long and 0.8 millimeter wide.
K. U. 34735, _P. truei_, old: The anterior one-quarter of the left
parietal bone is slightly depressed; and the posterior one-third of the
left frontal and anterior one-quarter of the left parietal are thin and
sculptured. This malformation of the roofing bones posterior to the
orbit probably is not the result of a break, for the orbital part of the
frontal bone is normal. The frontal-parietal sutures are in the normal
positions on both sides of the skull.
The above-mentioned anomalies do not appear to be correlated with age or
locality at which the specimens were taken. Apparently such anomalies
are present throughout the population, but in a small percentage of
specimens.
FOOD HABITS
Mice of the genus _Peromyscus_ are known to eat a wide variety of plants
and arthropods, and to be highly opportunistic in selection of food
(Cogshall, 1928; Hamilton, 1941; Williams, 1955, 1959a; Jameson, 1952;
Johnson, 1962). In order to determine possible food preferences, captive
mice of both species were fed plants indigenous to Mesa Verde. Entire
plants were used whenever possible; available seeds also were offered
(Tables 5, 6). All feeding experiments were replicated with at least six
different individuals in order to minimize the trends resulting from
individual preferences or dislikes. The mice of each species tended to
be consistent in their feeding.
The plant species listed in Tables 5 and 6 were those that were eaten or
rejected by a majority of the individuals tested.
Plant material eaten by _P. maniculatus_ and refused by _P. truei_
included only the leaves and stem of _Viguiera multiflora_. Plant
material eaten by _P. truei_ and refused by _P. maniculatus_ included
the leaves of _Calochortus gunnisonii_ and the leaves and stem of
_Erigeron speciosus_.
TABLE 5--Plants, or Parts of Plants, Eaten by Captive Individuals of
_P. truei_ in Mesa Verde National Park, Colorado. 0 = not eaten,
+ = eaten, - = not offered.