Understanding Tail-Biting in Pigs through Social Network Analysis

SIMPLE SUMMARY: Pigs are social animals that form social structures to maintain group stability. Grouping pigs without consideration of their social preferences may result in the development of abnormal behaviors such as tail-biting. This study was conducted to evaluate the association between social structure and incidence of tail-biting in pigs. Pigs were grouped according to their relatedness: (1) all pigs in a pen were related (born and raised by the same mother pig and called littermates); (2) all pigs in a pen were not related (born and raised by different mother pigs—non-littermates); and (3) half of the pigs in a pen were related (all pigs in a pen were born and raised by two mother pigs with equal number of pigs from each mother—half-group of littermates). Results indicate that littermates were less socially connected among themselves within a pen by spending less time lying together with their pen-mates than non-littermates. Littermates had a higher incidence of tail-biting compared to non-littermates. These results suggest that less social connection with pen-mates might predispose littermate pigs to the development of tail-biting. This pilot study demonstrates that social network analysis may provide us new insights into development of tail-biting. ABSTRACT: The objective of this study was to investigate the association between social structure and incidence of tail-biting in pigs. Pigs (n = 144, initial weight = 7.2 ± 1.57 kg, 4 weeks of age) were grouped based on their litter origin: littermates, non-littermates, and half-group of littermates. Six pens (8 pigs/pen) of each litter origin were studied for 6 weeks. Incidence of tail injury and growth performance were monitored. Behavior of pigs was video recorded for 6 h at 6 and 8 weeks of age. Video recordings were scanned at 10 min intervals to register pigs that were lying together (1) or not (0) in binary matrices. Half weight association index was used for social network construction. Social network analysis was performed using the UCINET software. Littermates had lower network density (0.119 vs. 0.174; p < 0.05), more absent social ties (20 vs. 12; p < 0.05), and fewer weak social ties (6 vs. 14, p < 0.05) than non-littermates, indicating that littermates might be less socially connected. Fifteen percent of littermates were identified as victimized pigs by tail-biting, and no victimized pigs were observed in other treatment groups. These results suggest that littermates might be less socially connected among themselves which may predispose them to development of tail-biting.