Affiliation in periadolescent rats: Behavioral and corticosterone response to social reunion with familiar or unfamiliar partners

The aim of this study was to investigate whether the social relationship between periadolescent rats, which is characterized by a high expression of play behavior, resembles attachment or whether it reflects a nonspecific social attraction between peers. Sprague-Dawley rats have been kept in pairs with a conspecific of the same age and sex from the time of weaning (PND 21). On PND 34, the members of each pair were separated for a 24-h period and randomly assigned to one of three groups: a) immediately sacrificed (NT = nontreated); b) reunited for 30 min with their previous cagemate (FAM); or c) with an unfamiliar conspecific of the same age and sex (UNF). During the reunion, the occurrence of both social and nonsocial behaviors was scored. Overall, periadolescent UNF animals spent more time in social investigation than FAM rats, which were conversely more involved in cage-oriented activities. In addition, higher levels of rough-and-rumble play were expressed in encounters between UNF subjects than between FAM rats. Finally, within the female group, UNF rats were more involved in play soliciting and less in digging activity than FAM animals. Blood corticosterone levels, which were measured at the end of the separation period, were higher in females than in males and were significantly lower than following reunion in a novel environment. The degree of familiarity did not affect hormonal changes. Results suggest that periadolescent rats do not form attachment relationships with each other. However, partner familiarity appears to be an important variable affecting social relationships.