Longitudinal observational studies were made of social play (wrestling) in laboratory-housed Rattus norveguus, between 21–60 days of age. Particular attention was paid to the temporal features of this behaviour. Data are presented on the duration of play bouts, the percentage of time spent by each participant in the “on-top” position, and the number of role reversals occurring during play.

Play bouts were found to be very brief events, in which very little reciprocity was shown; the initiator of a bout tended to remain on top of his partner for most of the bout. Males initiated more play than did females, but there were relatively few age or sex differences in the temporal measures of play. However, bouts involving females did tend to be shorter than those between males. Females initiating mixed-sex play were less likely to remain on top for prolonged periods than was the case in play bouts initiated by males with females or in bouts between same-sex pairs.

The duration of play bouts appeared to be randomly determined. This fact, together with the absence of age-changes in bout duration, is held to represent evidence against the hypothesis that play's primary function is to provide physical training for the participants, at least for social play in this particular species.

The implications of these results for the use of “reciprocity” as a defining characteristic of play are discussed.