Playful pigs: early life play-fighting experience influences later life contest dynamics

Abstract

Animal contests are costly and predicted to be won by the individual with the higher resource-holding potential (RHP). Weight is often used a proxy for RHP; however, victory does not always go to the heaviest competitor, indicating that other traits may also contribute to RHP. Here we investigated the effect of preweaning play-fighting experience on postweaning contest behaviour in the domestic pig, Sus scrofa. We predicted that individuals that played more would win contests later in life. Play-fighting experience was also predicted to influence contest escalation behaviour, on both an individual and a dyadic level. Lastly, a pre-established framework and eight contest cost measures were used to establish whether competing individuals gather/use information regarding play-fighting experience as part of an assessment strategy. Play-fighting experience was recorded for socialized and control litters before weaning and contests were staged between unfamiliar pigs of the same treatment after weaning. Controlling for competitor weight difference revealed that increased play-fighting experience was linked to contest success in females, while the opposite pattern was found in males. Play-fighting experience did not influence which individual within the dyad escalated contest behaviour, but dyads containing more experienced losers were more likely to perform a stage of nondamaging aggression. When we used skin lesions in losers as a measure of contest cost, we found evidence for the role of play-fighting experience in a novel mutual assessment strategy in socialized dyads, whereas control dyads performed opponent-only assessment. We suggest that while assessments of RHP can be made using a physical correlate of play-fighting experience (such as skilfulness), early life socialization is required for individuals to gain an understanding of their own RHP.

The influence of early life socialisation on cognition in the domestic pig (Sus scrofa domestica)

“…It should be noted that due to the large number of statistical tests used in the analysis, the occurrence of Type 1 error was possible, and these results should therefore be interpreted with some caution [see 87 ]. However, previous studies exploring the influence of early life social environment on pig behaviour have shown such effects can often be very subtle and may be missed when research is focussed on overall outcomes, rather than individual components of behaviour 84 . Additionally, Nakagawa 88 has reported that behavioural research is more likely to suffer from Type 2 errors than other fields, due to the practical and ethical limitations of testing large numbers of animals.…”

“…Alternatively, control pigs may have been better able to assess the RHP of the stimulus pigs than those experiencing socialisation. A recent study suggested that control individuals performed a form of opponent-only assessment, engaging less with larger opponents, regardless of their own RHP 84 . In contrast, pigs experiencing socialisation pre-weaning were observed to perform a novel form of assessment in which they gather information regarding both the RHP of themselves and of their opponents 84,85 .…”

“…A recent study suggested that control individuals performed a form of opponent-only assessment, engaging less with larger opponents, regardless of their own RHP 84 . In contrast, pigs experiencing socialisation pre-weaning were observed to perform a novel form of assessment in which they gather information regarding both the RHP of themselves and of their opponents 84,85 . Socialised individuals were more prepared to further escalate contests when the RHP difference between themselves and their opponent was large and in the opponents' favour, which may explain the observed preference of socialised pigs for the large side of the T-maze.…”

The Association Between Play Fighting and Information Gathering during Subsequent Contests

“…Contests were also ended immediately if fighting was deemed to be too severe (as determined by skin lesions and risk of lameness), if either individual displayed repeated fear behaviours (such as sustained vocalization or escape attempts), or if one pig was repeatedly mounted. Food was not present during the contest as pigs typically engage in contest behaviour upon initial introduction to a non-familiar conspecific in order to establish a dominance hierarchy even in the absence of resources 26,49,55 . It is this hierarchy that later established priority of access to tangible resources.…”

“…However, direct testing of this hypothesis has been limited and the results yielded have often been contradictory 24 . While Sharpe 25 observed no effect of juvenile social play on contest success in meerkats (Suricata suricatta), a recent study in the domestic pig (Sus scrofa) found evidence to suggest that pre-weaning play fighting frequency increased contest success in females, and decreased it in males 26 . Furthermore, the dominance ranks calculated for juvenile and yearling yellow-bellied marmots using the directional outcomes of their playful interactions were found to correlate with their later life dominance ranks as calculated from agonistic encounters 27 .…”