Young of each species grow rapidly for the first month, and attain, in that time, the largest percentage of their adult size; they grow rapidly up to sometime between the thirtieth and fiftieth days. Thereafter the rate of growth diminishes and the animals begin to gain weight rather than continuing to extend the lengths of the body and appendages.
[Figure 19] reveals that the appendages of young maniculatus attain most of their length about a week earlier than those of truei. Young truei acquire mobility and coordination somewhat later than young maniculatus, but both species are seemingly equal in these respects by about the end of the second week.
Length of gestation period, number and size of embryos, amount of time spent in the nest, and time required for bodily growth are all of major importance in determining the relative success of truei and maniculatus. These parameters will be considered further in the discussion.
In the laboratory, pregnant females were supplied with either kapok, cotton, or a piece of burlap with which to make a nest. The kapok or cotton was used directly by the mice in constructing a hollow, compact, moundlike nest. When burlap was used for nest building, the female first completely frayed the cloth by chewing it into a fluffy mass of fibers.
When the top of a nest was opened to inspect young, the female would attempt to pull the nesting material back into shape by means of forefeet and teeth. The mother's defensive posture was to cover the young with her body, often lying over them and facing upward, toward the investigator. In this semi-recumbent position, the female would attack the investigator's fingers with her forefeet and teeth. Often the female would stand bipedally and use the forefeet and teeth to mount the attack. If at this time a young chanced to wander away from the mother, she would quickly pick it up and place it in the nest at her feet.
When disturbed, females of both species, but especially P. maniculatus, often dove headlong under their nest or into the wood shavings on the floor of the cage. This type of retreat was most often used when young were nursing. Time is required even by the mother to disengage nursing young, and this mode of escape is the most expedient. The mother disengaged nursing young by licking around their faces and pushing with her paws.
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 participated 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.