Relationships between play and responses to tickling in male juvenile rats

Play is a putatively positive experience and of key interest to the study of affective state in animals. Rats produce 50kHz ultrasonic vocalisation (USVs) during positive experiences, including social play and tickling. The tickling paradigm is intended to mimic social play resulting in positively valanced ultrasonic vocalisation (USV) production. We tested two hypotheses on the relationship between tickling and play: that tickling would increase play behaviour or that play behaviour would increase in anticipation of tickling, and that tickling would share some specific properties of play(rebound and emotional contagion of unexposed cage mates). Male Wistar rats (N=64, with 32 rats/cohort) of 28 days of age were housed in pairs with one rat assigned to be tickled and one as the non-tickled control. Production of 50kHz USVs and hand-following behaviour was measured. Prior to handling, solitary and social play was recorded for 5 minutes in the home cage. A two-day break in tickling was used to assess a potential rebound increase in responses to tickling. Only one rat within each cage was handled to assess emotional contagion through changes in the behaviour of the cage-mate. Solitary but not social play increased prior to tickling relative to controls (p = 0.01).There were marked differences between cohorts; tickled rats in C2 produced less 50kHz USVs than those in C1(p = 0.04) and overall, C2 rats played less than rats in C1 (social p= 0.04 and solitary p< 0.001) and had a lighter start weight on arrival (p= 0.009) compared with cohort 1 (C1).In C1, there was evidence of rebound in USV production (p< 0.001) and a contagious effect of tickling reflected by increased hand-following in cage mates (p = 0.02). We found a positive relationship between start weight and USV responses to tickling (Rs= 0.43, p < 0.001), suggesting that the divergence in USV production may be due to developmental differences between cohorts. The results suggest that the relationship between tickling and play is complex in that tickling only affected solitary and not social play, and that tickling responses showed rebound and contagion effects on cage-mates which were specific to cohort responses to tickling.