Spontaneous helping in pigs is mediated by helper’s social attention and distress signals of individuals in need

Abstract

Helping behaviour is of special interest for the study of prosociality because it appears to be motivated by the needs of others. We developed a novel paradigm to investigate helping behaviour in pigs (Sus scrofa domesticus) and tested n = 75 individuals in eight groups in their home pens. During familiarization, two identical compartments were attached to the home pen, equipped with a mesh window and a door that could be opened from the outside by lifting a handle. During testing, a pig was briefly isolated from its group and then placed in one of the compartments. Each pig was trapped once, during two daily test sessions per group. Pigs spontaneously learned to open doors, and in 85% of cases, pigs released a trapped group member within 20 minutes (median latency = 2.2 minutes). Pigs who were more proficient at door-opening during familiarization, and more attentive to the trapped pig were more likely to help. Trapped pigs exhibited behavioural signals of distress and increases in cortisol. Distress signals by the trapped pig increased its chances of being helped. Results are consistent with several criteria for identifying targeted helping in animals, but we also discuss alternative explanations based on selfish motivations.