Social Factors in Dispersal of Deer Mice (Peromyscus maniculatus) from Their Natal Site

In a laboratory situation, social interactions and individual differences in Peromyscus maniculatus bairdi influence the rate at which juveniles leave their natal site. In a family group the father attracted the juveniles and did not expel them. The social influence of the mother depended upon the presence of a subsequent litter and her aggressiveness in defending her litters against conspecifics. Females were classified as aggressive or nonaggressive depending on whether or not they attacked a strange juvenile. An aggressive mother with a subsequent litter increased the rate of departure of her previous litter. Litter mates tended to leave together and no sex differences were found. Family differences in the rate of departure from the natal site were found when the litter members were tested separately. Social behaviors within family groups of P. m. bairdi influenced the rate at which juveniles leave their natal site. INTRODUCTION Social behavior probably mediates many of the factors influencing the initiation of dispersal in mammals, specifically leaving the natal site. The social interactions contributing to the departure of juveniles from their natal site have only been postulated. Individual differences among juveniles may reflect differences in social stimuli or differences in their sensitivity to these stimuli. Perornyscus maniculatus bairdi was chosen as the experimental species because it is socially organized into family groups, it is adaptable to the laboratory, and a relatively large amount of information is available on its movements in the field (Howard, 1949; Dice and Howard, 1951; Stickel, 1968) and on its population dynamics (Terman, 1968). However, several decades of field work on Peromyscus have contributed almost nothing to our knowledge of the behavioral interaction between family members in this genus. This paper presents the results of an experimental laboratory investigation of the hypothesis: Social factors and individual differences determine the rate of natal site departure of young P. m. bairdi. METHODS AND MATERIALS This work was carried out in the laboratory where weather, light, habitat and physical barriers could be controlled. A shock grid served as a barrier between two identical cages, thus maintaining a constant barrier to the departure of juveniles from their home cage. Several types of barriers (water, maze and shock) were considered. Shock permitted control of the intensity of the barrier and was the most effective in suppressing travel between the two, cages as determined in preliminary studies by photoelectric cell counts. The shock level used 1 Present address: Biomathematics Program, Department of Statistics, North Carolina State University, Raleigh 27607.