Girls play more with toys than boys: sex differences in positive behaviour in domestic piglets, Sus scrofa domesticus

Abstract

Positive emotion represents one pillar of positive animal welfare, which can be partially measured by behaviour. Previous research has related play, exploration, and social affiliation to positive emotion in pigs, but their sex-based variation remains underexplored. This study thus aimed to investigate potential sex differences in positive behaviours of piglets, Sus scrofa domesticus. It was hypothesized that sex differences would be present, with females performing more locomotor and object play, and males more exploration and social affiliation.

Behavioural observations were conducted over two experimental rounds on 96 piglets (per round=49) from ~1-7 weeks of age. Piglets were kept in mixed sex natal groups (n=8 piglets per pen) in 12 pens per round. To increase behavioural diversity, pens were randomly allocated to enriched (EH) and conventional housing (CH), with three pens per housing condition per round. Live continuous focal sampling was used to score the occurrence of object play, locomotor play, exploration, and social affiliation behaviours. Generalized linear mixed models (GLMM) were used to evaluate the effects of housing, round, and sex on the observed number of positive behaviours, with pen included as a random factor. Results were interpreted from the ratio estimates (RE) of the mean observed instances of a positive behaviour of females over males, and their statistical significance was signified by the absence of 1.0 in the range of the 95% confidence intervals.

A significant sex effect for object play was found, whereby males exhibited less object play than females (RE: 0.53, 95%CI: 0.37-0.77). No significant sex effects were found for the total number of positive behaviours (RE: 0.96, 95%CI: 0.88-1.05), nor when analysed in more detail, as total play (RE: 0.99, 95%CI: 0.81-1.20), locomotor play, (RE: 1.13, 95%CI: 0.90-1.41), exploration (RE: 0.94; 95% CI: 0.85-1.03) and social affiliation (RE: 1.07, 95%CI: 0.92-1.25). While the experimental set up or sample size of this current experiment may not have allowed for the evaluation of sex differences in other behaviours, the observed effects indicate a female-biased sex difference for object play, as hypothesized. In practice, these results may have implications in generating sex-specific welfare practices, applicable in pig farming and veterinary care, as well as contribute to wider research on pig welfare, behaviour, and their evolution since agricultural domestication.