Part 8 (1/2)
Nursing females of both species tolerated the male parent in the nest. A male and female often sat side by side in the nest and by means of their bodies partic.i.p.ated in covering the young. Males were not observed to attempt any defense of the nest, or of the young. Females were tolerant of older young in the nest when another litter was born and was being nursed. In one nest, a female of _P. truei_ gave birth to a litter of three when her older litter was 29 days old. The three older young continued to nurse until they were 37 days old, at which time they were removed from the cage. The female appeared tolerant of this nursing by members of the older litter, but appeared to give preference to the wants of the younger offspring.
One female of _P. truei_ lost or killed all but one young of her litter; 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 a.s.sertion 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
p.r.o.nounced 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.