Genes, Behaviour and Fitness in Mice: Concepts and Confusions

It is a truism that mice - particularly male mice - wantonly fight each other, with their aggression conditioned by body size, social relationships, hormonal status, and a host of other factors. It is also well-attested that mice of some inbred laboratory strains are quicker and more likely to fight than others under similar circumstances 1–4 which implies that aggressive behaviour is to some extent an inherited trait. A key question that follows from these considerations is: “What effect does aggression have on biological fitness?” In other words, how [and why] does aggressiveness influence the probability of successful reproduction [taking reproduction in the evolutionary sense of raising young to the stage where they themselves are capable of breeding]? Answers to this question must address the significance of fighting at all stages of a mouse’s life-cycle, and analyse the social, energetic and genetic costs of aggression to a pair. This paper is an attempt to cope with the question by reviewing the factors which are important in linking aggression in mice with the evolution of the species.