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Aggression and self-regulation of population size in deermice Healey, Michael Charles

Abstract

Sadleir (1965) proposes that the survival of juvenile deermice is determined by the aggressiveness of the adult population. During the summer, when adult aggression is high, juvenile survival is poor, but in the fall, when adult aggression is low, juveniles survive well. The purpose of this study is to examine some of the consequences of Sadleir's hypothesis experimentally. Sadleir bases his hypothesis on the observation that the aggressiveness of males changes seasonally. This premise has been reexamined and confirmed. How adult aggression affects juveniles was studied first in the laboratory. Juveniles grow poorly when competing with adults in their home cage. Males appear to be more active aggressors than females, but only aggressive males are capable of inhibiting juvenile growth. Even though juveniles grew slowly when competing with aggressive adults, they seldom died from encounters with adults. In order to avoid the crowded conditions and confinement implicit in the laboratory experiments, the relationship between adult aggressiveness and juvenile growth and survival was reexamined in field experiments. Two partly isolated plots of habitat were used, and on these plots artificial populations of aggressive or docile male deermice were established. Juveniles were then released onto the plots, and their growth and survival followed. In the field, as in the laboratory, juveniles grew poorly when competing with aggressive adults. Since emigration was not restricted in the field, however, juveniles disappeared in significantly greater numbers when the adult population was aggressive than when the adult population was docile. In addition to these experiments, the success of immigrants onto trapped out plots and plots with a resident population was examined. Immigrants were more successful in establishing themselves on trapped out plots. All the data collected support Sadleir's hypothesis, and it seems reasonable to conclude that the correlation he drew between adult aggressiveness and juvenile survival is real. However, the data collected also provide some interesting clues as to the organization of deermouse populations. An organization is proposed in which the social unit is an animal and its immediate neighbours. Within the social unit mutual antagonism is reduced. But the members of the unit maintain a high level of aggressiveness, and are intolerant of any stranger that wanders into their home ranges. The system proposed would prevent immigrants from settling, while conserving energy by reducing antagonism between familiar animals. The system would also effectively regulate population size.

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