Adult-young interactions in island and mainland populations of the deermouse Peromyscus maniculatus

Summary The demographic and ecological characteristics of island populations of small mammals have received increasing attention in recent years, but few studies have compared the behavioral characteristics of island populations with those of mainland populations. Behavior is considered an important variable because it is believed by many to be a crucial factor affecting the population dynamics and demography of natural populations. In particular, among many species of rodents, the social behavior of adults towards juveniles is cited as an important factor influencing dispersal patterns and population regulation. The present study compares social interactions between adults and juveniles of island and mainland populations of the deermouse Peromyscus maniculatus, and attempts to relate differences in behavior to the demographic differences between the two populations. Adult mice were trapped on the mainland of British Columbia and on one of the Gulf Islands off the British Columbia coast, and allowed to breed in the laboratory. Male and female juveniles from both populations were then tested with their own parents and with unrelated male and female adults. The results demonstrate that island adults show almost no aggression towards either own or unrelated young. Mainland adults likewise show little aggression towards their own young, but a proportion of the population, consisting of both male and female adults, shows severe aggression towards unrelated juveniles of both sexes. These results suggest four major conclusions: 1) behavior may be the mechanism responsible for the demographic differences reported for these island and mainland populations; 2) female aggression may be a more important factor in deermouse population dynamics than has been previously recognized; 3) since parents show little aggression towards their own young, adult aggression may be a significant factor in juvenile mortality and emigration only after juveniles have initiated dispersal away from their natal sites; and 4) adult aggression controls the number of both male and female juveniles which are recruited into the population.