Does the tickling of adolescent male rats (Rattus norvegius) share the rebound and contagion properties as play?

The tickling paradigm was developed to mimic social play in rats. This study investigated whether tickling shares properties of conspecific play, namely rebound and emotional contagion. 64 adolescent (28 days) male Wistar rats, split into two batches, were housed in pairs and assigned to treatment (tickling) or control (neutral handling). One animal within each cage (n = 32, 16 per batch) was handled. During handling, frequency modulated ultrasonic vocalisation (FM USV) production and hand-following (HF) was measured.  To study rebound, animals were given a two-day break from handling. To study contagion, we measured the responses of handled animals and their paired cage mate in the elevated plus maze and during home-cage anticipatory play. Cage mates were also subjected to a handling test on the final day. If contagion was present, a relationship between handled animal and cage mates would be expected. There were significant batch effects in FM USV response but not in HF response. In batch 1 (B1), tickled animals produced more FM USVs (F_1,89.1 = 36.18, p < 0.001) and showed increased HF compared with controls (F_1,88.7 = 16.46, p < 0.001), both measures indicating that tickled animals were in a more positive emotional state. In batch 2 (B2), treatment had no effect on FM USVs (F_1,94= 0.44, p = 0.508). However, treatment had the same effect on HF (F_1,188 = 0, p = 0.948). Irrespective of batch effects, there was no significant rebound effect in USVs or HF, as neither measure significantly increased after the two-day non-handling period (USVS; F_1,188 = 1.08, p = 0.303; HF; F_1,188 = 0.98, p = 0.325). There was evidence of contagion on HF as overall, cage-mates of tickled animals hand-followed for longer (F_1,9 = 11.59, p = 0.002).  This study suggests that tickling induced a behavioural change in the cage-mate although this was independent of changes in anxiety and of other changes in positive affect.