Context-dependent individual differences in playfulness in male rats

Play has been proposed as an indicator of positive emotions and welfare in higher vertebrates. This study investigated playfulness in male rats by exploring its consistency across motivational states (with/without prior short social isolation) and two age points at early and late adolescence. Twenty-four male Lister Hooded rats housed in cages of four underwent two play tests: conspecific Play-in-Pairs and Tickling by the experimenter, which were compared with play in the home cage and basal anxiety levels. Play-in-Pairs measures were consistent across age and motivational states, and were independent from anxiety. Positively valenced vocalizations in the Tickling test were also consistent across age, yet were negatively related to anxiety. Play-in-Pairs and Tickling play contexts, as well as social and solitary play types, were unrelated. Therefore, this study supports the existence of consistent individual differences in playfulness in rats, and suggests that different play contexts and types represent motivationally distinct systems.