Do juvenile rats use specific ultrasonic calls to coordinate their social play?

Play fighting in juvenile rats is associated with a high occurrence of 50 kHz vocalizations. These calls are varied in form, ranging from long, flat calls to short, frequency-modulated ones. We hypothesize that at least some types of calls serve as play signals to facilitate play. In the present study, pairs of juvenile male rats that were unfamiliar with one another were paired in a neutral test enclosure to which they had been habituated. Video and audio records were made of the encounters. Pairs were of two types: both pairs could vocalize or only one partner could do so. There were some differences between the play of pairs containing a devocalized partner, but overall, the pattern of play, the frequency and types of calls were similar between the two types of pairs. We used a Monte Carlo shuffling technique to analyse the correlations between the playful actions performed and the types and frequencies of various 50 kHz calls that were produced. The analyses revealed that there were strong associations between types of calls and types of social contact: an approach followed by playful nape contact was associated with calls, but an approach followed by nonplayful contact (e.g. anogenital sniffing) was not. Similarly, different calls were associated with different actions, such as nape contact, evade and wrestling, with most of these calls being uttered by the initiator of the action, not the recipient. However, coordinating calls reciprocally with complementary calls uttered by participants as they engaged in complementary actions (e.g. attacking, being attacked) appeared to be a way in which calls could potentially be used as play signals to influence the ongoing cooperation needed to sustain play fights. (C) 2018 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.