Social influence and attention bias in free-ranging domestic pigs: Effects of demonstrator rank and friendship

Pigs, as highly social animals, form stable hierarchies and different relationships to other members of their group. Pigs, mostly of very young ages, have been shown to use social learning to gather information about food locations and even extractive foraging techniques from various types of sources. The present study aims to investigate whether also adult pigs, living in a long-term stable kin-based sounder, use information of their group mates and, more specifically, whether this is based on their mates' rank or their relationship with them. The dominance order of 20 female semi-naturally housed Kune Kune pigs (aged between five to seven years) was established with dyadic hierarchy tests, where two pigs competed against each other over a monopolizable food source. Their affiliative relationships (i.e. friendships) towards each other were assessed based on observations during their daily lives. After finding suitable demonstrator dyads, differing in rank and/or relationship quality to the respective observer, six rounds of two-choice tests were conducted. Prior to each two-choice test, two demonstrators were positioned on each side of the observer and were allowed to feed from a trough with an assigned olfactory cue that provided additional information in the subsequent test. After two minutes, the observer pig was allowed to enter a test room and was given the choice between two food bowls, which were marked with the olfactory cues connected to the respective demonstrators. Results indicate that sows, irrespective of their own rank, preferred to pay attention to the relatively lower-ranking demonstrator during the exposure phase, possibly due to avoidance of the relatively higher-ranking demonstrator. Furthermore, the observers with higher sociability preferred to pay more attention to the demonstrators with whom they had better friendships. In the two-choice test, we found no significant effect of demonstrator rank or friendship. However, a left side bias in more sociable observers may point towards at least some social effects. Overall, the study provided evidence that not only dominance, but also socio-positive relationships between individuals affect the behaviour of domestic pigs.