Tickling does not increase play in young rats

There is abundant evidence that tickling can be used successfully to induce positive affect in rats. Play behaviour has been identified as one of the most promising indicators of positive welfare. However recently, Ahloy-Dallaire et al. (2018) have suggested there is a lack of evidence to support that play increases with positive affect. In this work we tested the hypothesis that tickling would lead to an increase in rat play assumed to be mediated by an increase in positive affect. We also expected that tickled rats would show more anticipatory play in the home cage and the testing arena as a reflection of being motivated to receive tickling. Male Wistar rats (n=24; 45 days old) were housed in pairs and assigned by cage to receive tickling or control handling for 5 days. Home cage behaviour was recorded for 15 minutes before and after handling to allow investigation of the effect of tickling on spontaneous play. Rats were placed individually into the experimental arena and given a 30-s free-roam opportunity to express anticipatory or exploratory behaviour. Rats then experienced tickling or control conditions (hand moving on the side of the arena in the same 15 sec intervals as tickling) for a total of 2 minutes, followed by another 30-s free-roam opportunity. Within the arena, we recorded ultrasonic vocalisation (USV) production and scored behaviour during the free-roam tests. During the free-roam tests, tickled rats produced more positive USVs than control rats (p = 0.049) and made more frequent investigations towards the static hand of the experimenter (p = 0.038). There was no effect of treatment on home cage play (solitary p = 0.6; social p = 0.4). Play in the home cage only occurred before handling (p > 0.001) and increased across test days (p = 0.002) in all rats regardless of handling regime. Rats became inactive after handling (p > 0.001). Thus, tickling did not induce play in rats despite some evidence that tickling enhanced positive affective state. However, predictable exposure to the arena and handling (either tickled or control) may have been sufficient enrichment to promote play. As such, the relationship between play and positive affect requires further exploration.