The protective effect of social support: Can humans reassure pigs during stressful challenges through social learning?

Abstract

Farm animals can socially learn to trust a human after observing that human handling a familiar conspecific gently, even if they never positively interacted with the human themselves. This study aimed to evaluate whether pigs are reassured by a human during a stressful challenge, having previously acquired a positive perception of the human by witnessing the gentle handling of a high or low socially ranked conspecific. Additionally, the effectiveness of reducing pigs' stress response based on the degree of human familiarity was examined. Seventy-five pigs (21 days old, 5.6 ± 0.2 kg) housed in 15 nursery pens (5 pigs/pen) were randomly assigned to one of three treatment groups (5 pens/treatment): Dominant Demonstrator Group (DDG), Subordinate Demonstrator Group (SDG), and Control Group (CG). Pigs from DDG and SDG observed a high or low socially ranked conspecific ("demonstrator"), respectively, while the demonstrator received gentle handling consisting of stroking for 10 minutes and a sucrose solution twice a day for 5 weeks. The CG received minimal human contact required for feeding, cleaning, and health examination. Following the treatment period, pig behavior was evaluated in response to a familiar (handlers involved in treatments) or an unfamiliar person (unknown human) in a social support test, consisting of physical restraint in a cage for 3 minutes while the person provided stroking, followed by the release of the animal into a test pen for 1 minute. Data were analyzed using a mixed linear model in R Software. Post-hoc comparisons were conducted using the Tukey test. Both DDG (P = 0.009) and SDG pigs (P = 0.005) showed a greater frequency of behavioral reactions reflecting restlessness when tested with an unfamiliar human compared to a familiar human, whereas CG pigs exhibited no differences (P = 0.74). Moreover, when tested with their familiar human, DDG (P = 0.0001) and SDG pigs (P = 0.001) remained calm for longer compared to CG. When pigs were released from the cage by their familiar human, SDG (P = 0.0006) and DDG pigs (P = 0.001) contacted the human sooner compared to CG. Subordinate demonstrator group (P = 0.0007) and DDG (P < 0.0001) pigs also spent more time investigating the familiar human in comparison to CG. No differences in behavioral reactions based on the demonstrator's social rank were observed. In conclusion, the positive perception of humans acquired by pigs through social learning persists in aversive situations and can lead to positive reassurance by familiar humans during stressful events.