The Amount of Maternal Care in Peromyscus leucopus and Its Thermal Significance for the Young

Female Peromyscus leucopus noveboracensis raised litters from birth to 16 days of age in nest boxes within large enclosures in the laboratory and forest. Movement of the female into and out of the nest box was recorded and supplied indirect evidence on the amount of care provided to the young. Outside activity of all females was primarily nocturnal. Four mothers in laboratory enclosures spent an estimated average of 3.0 to 6.2 hours per day away from their young. Five mothers in forest enclosures tended to spend more time away from their young than the laboratory mothers, averaging 5.0 to 8.4 hours per day. The difference between laboratory and forest mothers was statistically significant. Estimated time spent away from the young per day varied significantly from mother to mother in both laboratory and forest situations, but did not vary with the age of the young in either situation. Young reared in the forest enclosures were significantly smaller at 16 days of age than young reared in the laboratory enclosures, whereas the latter young were similar in weight to young raised in a laboratory breeding colony associated with these experiments. It is suggested that the small size of the forest young resulted partly or wholly from the diminished care provided by their mothers. The thermal status of young with their mothers in nest boxes with cotton nests was examined in a separate set of experiments. Four- to 14-day-old, laboratory-reared young were exposed singly and in groups of four, with their mothers, to ambient temperatures of 0 to 30°C for 2.5 to 3.0 hours. Body temperatures of the young at the end of such exposure were nearly always at 34°C or above, and it was concluded that young P. leucopus are maintained at homeothermic temperatures when in the company of their mother, regardless of their capacities to thermoregulate when alone. Thus, litters raised in the laboratory enclosures were at homeothermic temperatures for at least 17.8 to 21 hours per day, and litters in the forest enclosures, for at least 15.6 to 19 hours per day. Potential implications of these results for the growth of the young in nature and for the reproductive biology of the species are discussed.