Gender composition of the litter affects behavior of male mice

Abstract

Swiss—Webster mice were reared in litters composed of six males, six females, or five females and one male. Maternal and pup behaviors were observed prior to weaning and recorded by checklist using the instantaneous scan method. Some preweaning social behaviors occurred earlier among pups in the mixed litters than in litters of one sex. No differences in maternal behavior were observed. The males were tested for aggressiveness at 60–65 days of age by caging them in groups of four, two each from all-male and single-male litters, for a period of 6 days. Aggressiveness ranks were assigned within each cage on the basis of body scars and behavioral observation. It was found that males reared as the single male in the litter engaged in more intermale aggression than those from all-male litters. Aggressiveness was also highly related to weight loss during the group housing period, the most aggressive animals losing the least weight. The preweaning environment affects later intermale aggression in mice, perhaps by way of intralitter social interactions, olfactory cues, or hormonal levels.