Playful handling as social enrichment for individually- and group-housed laboratory rats

Abstract

Social housing is recommended for laboratory rats because they are highly social mammals but research constraints or medical issues often demand individual housing and, when social housing is practiced, it typically involves housing with only one or two conspecifics. We hypothesized that playful social contact with humans (i.e. tickling), mimicking the dorsal contacts and pins characteristic of rat rough-and-tumble play, could act as a partial substitute for, or supplement to, conspecific social contact in situations when laboratory rats are housed individually or in pairs or triplets. Furthermore, we hypothesized that the beneficial effects of regular tickling when young would persist following discontinuation of tickling. Accordingly, we investigated the responses of juvenile male rats to handling conditions (minimally handled vs. tickled) and group size (singletons, pairs, triplets). We measured (a) production of 22- and 50-kHz ultrasonic vocalizations (USVs) during a 1-min period before handling (interpreted as evidence for negative and positive affective states, respectively), (b) corticosterone levels from faecal pellets collected in the home cage, and pre- and post-treatment body weight (as measures of physiological stress), and (c) behaviour in an Open Field test (to assess anxiety). After 3 weeks of tickling for 2min/day, individually-housed rats produced more 50-kHz USVs in anticipation of handling than their minimally-handled counterparts (P<0.0001). This effect persisted for at least 4 weeks after discontinuation of the tickling programme (P<0.0001), when all rats were transferred to individual housing. Tickling experience also reduced anxiety-related behaviour of individually-housed rats in the Open Field test (P<0.05). Faecal corticosterone levels and body weight were not affected by tickling experience or group size, although both corticosterone levels and anxiety-related behaviour were elevated following re-housing (P<0.05). No significant differences were detected between rats housed as pairs or triplets for any of the measures investigated (P>0.05). Both the Open Field and USV data affirm that tickling is beneficial for rats, especially when housed individually, and provide evidence that specific experiences derived from playful contact contribute to the well-being of group-housed animals. We conclude that tickling is an appropriate cross-species social enrichment manoeuver that can promote the well-being of laboratory rats and rats kept as companion animals.