Individual differences in mouse behavioural development: effects of precocious weaning and ongomg sexual segregation

Abstract

Individual differences in behaviour have been related to variations in early social milieu, such as those at the age of weaning and in litter gender composition. To understand better the respective role that these two variables have in the ontogeny of behaviour, the effects of their manipulation in developing CD-1 mice, Mus domesticus, were assessed. On postnatal day 15, 32 litters were split in half. Each half litter contained four males or four females, or two of each sex. At the same time, one half litter was weaned (Precocious Weaning, PW), while the other remained with the dam (Regular Weaning, RW). The behaviour of each half litter was scored (single 15-min session) on days 16, 20 and 24. Cage exploration increased and affiliative behaviour decreased with age. Solitary play showed a non-monotonic profile with a peak on day 20. Early weaning was associated with both an increased expression of jumping and eating, and shorter self-grooming periods, and also induced a female primacy in exploration and solitary play. Females also appeared to be more involved than males in an affiliative item, pushing the snout under another pup's body. Finally, isosexual subjects spent more time in cage exploration and less time in social inactivity than sex-balanced controls. In conclusion, dam and sibling effects seemed to act independently of each other, stressing the importance of different facets of early social milieu for mouse behavioural development.