Aggression and Self‐Regulation of Population Size in Deermice

Abstract

Sadleir (1965) proposes that seasonal changes in the survival of juvenile deermice are determined by seasonal changes in the aggressiveness of the adult population. The purpose of the study is to examine some of the consequences of Sadleir's hypothesis experimentally. Laboratory studies confirmed Sadleir's observations on seasonal changes in the aggressiveness of male deermice. Similarly, in the laboratory, juvenile deermice grew poorly when competing with adults in their home cage. Further analysis showed that only aggressive males are capable of inhibiting juvenile growth. In order to avoid the crowded conditions and confinement implicit in the laboratory experiments, the relations between adult aggressiveness and juvenile growth and survival was reexamined in field experiments. Artificial populations or docile male deermice were established in field plots. Juveniles were then released onto the plots, and their growth and survival followed. Juveniles again grew poorly when competing with aggressive adults, but in the field juveniles also disappeared rapidly from the experimental plots when the adult population was aggressive. The success of immigrants onto trapped out plots and plots with a resident population was examined as well. Immigrants were more successful in establishing themselves on trapped out plots. All the data collected support Sadleir's hypothesis, and it is concluded that the correlation he drew between adult aggressiveness and juvenile survival is real. The data also provide some clues to the organization of deermouse populations. An organization is proposed in which the social units is an animal and its immediate neighbours. Within the social unit mutual antagonism is reduced. But the members of the unit remain highly aggressive, and are intolerant of any stranger that wanders into their home ranges. The system proposed prevents immigrants from settling, the helps to regulate population size, while conserving energy by reducing antagonism between familiar animals.