Sex preference and body orientation preference in huddling behavior of rat pups

The social huddling of six litters of rat pups (Rattus norvegicus) was observed on Days 6–9 postpartum without the dam present. Each litter was reduced to three male and three female pups. Thirty-six pups were used as subjects (18 male, 18 female). Tests for the effects of sex of subject, age of subject, sex of littermate huddled against, and body orientation during huddling were carried out. Results indicated that subjects huddled against opposite-sex Uttermates more frequently than against same-sex littermates. Huddling was a curvilinear function of age, with average frequency of huddling reaching a peak on the second day of observation (Day 7 postpartum) and decreasing thereafter. Results also showed that subjects, regardless of sex, huddled using an orientation that allowed their heads to touch more frequently than an orientation in which the head was touching the body of a sibling. These data on early sex preference and body orientation pReferences during huddling suggest that rat pups are competent in aspects of behavior having attachment potential. The findings also help define the age of social competence in this species.