Long-term social preferences in a group of sub-adult female pigs

Abstract

Social relationships in farm animals, including pigs, have become a focus of research, yet long-term studies are scarce. Pigs, being highly social animals, offer an excellent model to explore social preferences over time. This study aimed to investigate social preferences in female pigs across life stages by observing a group of ten gilts over one year, with each season serving as an assessment point. Social interactions (allogrooming, snout-body, and snout-head contact) were recorded through live and video observations, totaling 396 h per animal. Social Network Analysis (SNA) assessed group cohesion using measures of density, reciprocity, and degree centralization. Monte Carlo simulations, half-weight association index (HWI) and the Quadratic Assignment Procedure (QAP) were used to evaluate social preferences and their recurrence across seasons. Results showed high density (0.95) and reciprocity, with weak centralization (in-degree 0.19, out-degree 0.27), indicating uniform distribution of social interactions. On average across the four seasons, 5.8 % of connections were strong, 35.7 % weak, and 58.5 % non-preferential. Social preferences correlated modestly between autumn and winter, but not with summer and spring. This study confirms previous findings that only a small proportion of pigs form non-random associations within a group. Social preferences lasted for a maximum of two seasons, likely influenced by pregnancy and transitions from gilt to adult sow, which resulted in temporary withdrawal from the group.