Ontogeny of social behavior of pups of laboratory rats genetically selected for activity level

In pups of Wistar laboratory rats genetically selected for high (+A) and low (-A) locomotor activity in an open field social behavior was tested in a situation of paired interactions in a novel environment. Tests were performed on postnatal Days 16, 19, 23, and 27. They consisted of four five-minute trials with 50 min, intertrial intervals. Duration of active contacts, passive contacts, mutual grooming, following (chasing), and playful fighting was recorded. In pups of the +A line total duration of all social interactions increased between Days 16 and 23, while in the -A pairs preweaning maximum in most types of social activities was achieved already on Day 16, after this day the time spent in mutual contacts decreased. Analysis of dynamics of social behavior over trials in the -A line showed the tendency to habituate, maximum duration of paired interactions appeared in the first trial contrary to +A groups, in which duration of social contacts tended to increase with repeated trials. In both lines, duration of following responses increased from Day 16 to Day 27. In +A pairs following often led to playful fighting, while in -A pairs fighting was practically absent. The experiment demonstrated that differences between lines +A and -A appeared in those social contacts which required higher energy expenditure and which realized themselves in the larger space of the testing box.