Sibling effects on the behavior of infant mouse litters (Mus domesticus).

We investigated whether the number of same- or other-sex littermates had a graded effect on the occurrence of early solitary play and social play by mouse (Mus domesticus) family units. Sixty litters, reduced at birth to 5 different sex ratios (6 males, 5 males and 1 female, 3 males and 3 females, 1 male and 5 females, and 6 females) were scored in 15-min sessions on postnatal Days 18 and 21. An increasing trend with age was found for run, pounce, popcorn, self-groom, and explore episodes. Type of family unit influenced the occurrence of specific social interactions: One male-5 female and 6 female litters showed more social play than similar male litters. Litters with a balanced sex ratio showed higher exploration than isosexual litters. The results extend previous reports of both social and solitary play in developing laboratory mice and, in contrast with rat data, indicate a marked female primacy in playful social behavior.