Trade-offs between proximity and physical contact during group integration in pigs (Sus scrofa domesticus)

During behavioural trade-offs, individuals have to decide whether to express a behaviour which may lead to a reward or potential costs when engaging in a risky situation. Social integration forces animals to make such trade-offs. We hypothesized that animals predominantly demonstrate non-tactile behaviour and hence less tactile behaviour in a high-risk context such as during social integration, while using tactile behaviour more than non-tactile in less risky situations such as under social stability. Pigs (Sus scrofa domesticus) typically are in close physical contact to each other, but physical contact also relates to increased aggression. We investigated 18 groups (142 pigs) across different phases of social stability, thereby observing snout proximity and snout contact. Additionally, aggression (reflecting costs) and growth performance (reflecting benefits) were measured. Data were analysed using mixed models while accounting for group stability. Snout proximity was indeed most frequent during social instability and reduced as stability increased, while snout contact remained more constant. The high occurrence of snout proximity during social instability suggests conflict avoidance and thus risk aversion. Animals that showed more frequent snout proximity grew slower, while initiators and recipients of frequent snout contact had a better growth performance. The causality of these effects cannot be ascertained but it is possible that slower growing, and thus weaker individuals may have made a behavioural trade-off by choosing proximity rather than contact during social instability. The results further emphasize the importance of distinguishing between nuances in behaviour.