Kaitlyn M. St. Charles ¹ Kimberly VanderWaal ² Jon E. Anderson ³ Lee Johnston ⁴ Yuzhi Z. Li ^4*

• ¹ Department of Animal Science, College of Food, Agriculture, and Natural Resource Sciences, University of Minnesota Twin Cities, St. Paul, United States
• ² Department of Veterinary Population Medicine, College of Veterinary Medicine, University of Minnesota Twin Cities, St. Paul, United States
• ³ University of Minnesota Morris, Morris, Minnesota, United States
• ⁴ West Central Research and Outreach Center, College of Food, Agricultural and Natural Resource Sciences, University of Minnesota, Morris, Illinois, United States

Tail biting is a multifactorial behavior that causes welfare and economic challenges in swine production. As of 2024, research exploring the influence of pig social structure on the development of tail biting is limited. The objective of this study was to explore whether social structures of pigs from different litter origins can impact tail biting and, ultimately, tail damage. Pigs (n = 96) were grouped (8 pigs/pen) based on their litter origin: non-littermates (NLM), half-littermates, and littermates (LM). Tail injury scores were assessed twice weekly from 10 to 24 weeks of age, with a maximal tail injury score (MTS) over the study period being used to evaluate victimization by tail biting. Pig behavior was video-recorded at 15, 19, and 23 weeks of age. Association networks based on lying behavior and tail biting interaction networks were evaluated at pen- and pig-levels using social network analysis. Pigs in LM pens experienced higher median MTS compared to pigs in NLM pens (Chi-square = 9.1, df = 2; p = 0.01). Within association networks, NLM pens had lower degree centralization measures than other pens at both 15 (Estimated marginal mean [EMM] = 0.07; 95% CI = 0.02 – 0.12; p = 0.003) and 23 weeks (EMM = 0.09; 95% CI = 0.04 – 0.14; p = 0.007) and pigs in NLM pens had higher degree centrality than those in other pens (EMM = 1.00; 95% CI = 0.90 – 1.11; p = 0.002), suggesting pigs in NLM pens had more uniform, stronger, and more connections with their pen-mates. In tail biting networks, increased weighted in-degree centrality was associated with increased odds of pigs receiving a more severe MTS (OR = 1.56; 95% CI = 1.08 – 2.27; p = 0.02). Pigs with increased weighted out-degree centrality tended to have increased odds of receiving a more severe MTS (OR = 1.19; 95% CI = 0.97 – 1.48; p = 0.09). These preliminary data suggest a potential relationship between social structures and tail biting in growing-finishing pigs.