Variation in Aggressiveness and the Regulation of Numbers in House Mouse Populations

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Abstract
Three years of study were devoted to the search of a behavioural genetic mechanism for regulation of numbers in the house mouse (Mus. m.domesticus), following the lead of a pilot study suggesting applicability of the theory to this species. The hypothesis of a behavioural genetic mechanism, developed by Chitty and his school, maintains that (a) qualitative changes in behaviour are responsible for fluctuations in numbers of animal populations, and (b) these behavioural adjustments are underlain by shifts in frequency of genotypes. Chitty himself has always stressed the role of geno- types differing in aggressiveness: highly aggressive types having selective advantage under conditions of high density, and less aggressive (but more fertile) types being favoured in periods of low density. The first research problem posed by this hypothesis, is to investigate whether aggressiveness indeed has a genetic basis. Fortunately, in the house mouse a beha- vioural measure for aggressiveness (the Attack Latency) was available, for which a genetic component has been established by means of a selection experiment (VAN OORTMERSSEN & BAKKER, 1981). The emphasis of my work was to investigate the second premise of the hypothesis, that behaviour changes result from changes in density, as an outcome of shifts in genotype frequency. By establishing populations of feral house mice in small enclosures and following demographic changes in detail, conditions of high and low density were replicated several times. The aggressive level of the mice in these experimental populations was determined regularly by measuring Attack Latency (with the help of two methods: directly and testing offspring produced and raised in the laboratory), and periodi- cally behaviour observations on the feral populations were done to detect correlations between various aggressive behaviours and demographic features.

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