Understanding Tail-Biting in Pigs through Social Network Analysis

Today, we are sharing a publication on pig welfare by our colleagues in the Outreach and Extension center at the University of Minnesota, Drs. Li, Zhang, Johnston and Martin. More specifically, the researchers focused their study on the effect of social network on tail-biting in pigs. The full-text of the article is available in open-access on the website of the journal Animals.

We know that pigs are social animals and that they naturally form social structures to maintain a cohesive group. However, we have little understanding of how those group dynamics affect deleterious behavior like tail-biting.  To answer the question of the association between social structure and incidence of tail-biting in pigs, the researchers created 18 groups of 8 pigs.

  • 6  groups were Littermates: all the 8 pigs were born from and nursed by the same sow.
  • 6 groups were Half-group of littermates: 4 pigs were born from the same sow whereas the 4 others came from the litter of another sow.
  • 6 groups were Non-littermates: all 8 pigs were born from a different sow.

Each group was housed in a nursery pen after weaning where the pigs stayed for the duration of the study until they reached 10 weeks of age. Researchers analyzed growth performances, tail injuries, and behavior.

Growth performances did not differ among groups in this study. However, littermates showed a higher incidence of tail-biting with 15% of the pigs showing chewing or puncture wounds with visible blood but no infection.

tailbiting and social networkBehavior was analyzed by videotaping the pigs 2 weeks after they were placed into their pens, 1 week later when each group was moved together to a new pen and 1 week after the move. The video recordings were viewed by a trained researcher to determine association interactions among pigs. Pigs were considered associated with each other if they were lying together frequently and with more than 50% or more of their bodies in contact with each other. For each pig (white circle in the figure above), researchers measured the direct association between each individual pig and its penmates (1) as well as the peripheral association among the penmates (2).

At the individual pig level, littermates had lower direct association than non-littermates and half-group of littermates, suggesting that littermates might be less socially connected directly among themselves. However, the indirect association among penmates did not vary.

Another interesting observation, although statistically insignificant, is that littermates appeared to spend less time in the lying posture than other groups.

Overall, littermates had a lower strength of social connections, more absent ties, and fewer weak ties, compared to non-littermates and half-group of littermates. Less social connection with pen-mates might predispose pigs in littermate pens to development of tail-biting. Regardless of litter origin, most pigs appeared connected by weak social ties and few pigs formed strong social ties with their pen mates.

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.

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