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The immune system can influence social motivation with potentially dire consequences for group-housed production animals, such as pigs. The aim of this study was to test the effect of a controlled immune activation in group-housed pigs, through an injection with lipopolysaccharide (LPS) and an intervention with ketoprofen on centrality parameters at the individual level. In addition, we wanted to test the effect of time relative to the injection on general network parameters in order to get a better understanding of changes in social network structures at the group level. 52 female pigs (11-12 weeks) were allocated to four treatments, comprising two injections: ketoprofen-LPS (KL), ketoprofen-saline (KS), saline-LPS (SL) and saline-saline (SS). Social behaviour with a focus on damaging behaviour was observed continuously in 10 x 15 min bouts between 0800 am and 1700 pm 1 day before (baseline) and two subsequent days after injection. Activity was scan-sampled every 5 min for 6 h after the last injection in the pen. Saliva samples were taken for cortisol analysis at baseline and at 4, 24, 48, 72 h after the injections. A controlled immune activation affected centrality parameters for ear manipulation networks at the individual level. Lipopolysaccharide-injected pigs had a lower in-degree centrality, thus, received less interactions, 2 days after the challenge. Treatment effects on tail manipulation and fighting networks were not observed at the individual level. For networks of manipulation of other body parts, in-degree centrality was positively correlated with cortisol response at 4 h and lying behaviour in the first 6 h after the challenge in LPS-injected pigs. Thus, the stronger the pigs reacted to the LPS, the more interactions they received in the subsequent days. The time in relation to injection affected general network parameters for ear manipulation and fighting networks at the group level. For ear manipulation networks, in -degree centralisation was higher on the days following injection, thus, certain individuals in the pen received more interactions than the rest of the group compared to baseline. For fighting networks, betweenness decreased on the first day after injection compared to baseline, indicating that network connectivity increased after the challenge. Networks of tail manipulation and manipulation of other body parts did not change on the days after injection at the group level. Social network analysis is a method that can potentially provide important insights into the effects of sickness on social behaviour in group-housed pigs. (c) 2021 The Authors. Published by Elsevier B.V. on behalf of The Animal Consortium. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Social network analysis (SNA) has provided novel traits that describe the role of individual pigs in aggression. The objectives were to (1) estimate the genetic parameters for these SNA traits, (2) quantify the genetic association between SNA and skin lesion traits, and (3) investigate the possible response to selection for SNA traits on skin lesion traits. Pigs were video recorded for 24 h post-mixing. The observed fight and bullying behaviour of each animal was used as input for the SNA. Skin lesions were counted on different body parts at 24 h (SL24h) and 3 weeks (SL3wk) post-mixing. A Bayesian approach estimated the genetic parameters of SNA traits and their association with skin lesions. SNA traits were heritable (h2 = 0.09 to 0.26) and strongly genetically correlated (rg > 0.88). Positive genetic correlations were observed between all SNA traits and anterior SL24h, except for clustering coefficient. Our results suggest that selection for an index that combines the eigenvector centrality and clustering coefficient could potentially decrease SL24h and SL3wk compared to selection for each trait separately. This study provides a first step towards potential integration of SNA traits into a multi-trait selection index for improving pigs’ welfare.

Early play fighting mimics later aggression in many species, and may, therefore, be expected to reduce costs from later aggressive interactions. Using social network analysis (SNA) the effect of a central play fighting network position on later skin lesions from aggression was assessed in domestic pigs. Piglets (n = 263) were kept in litter groups or socialised pre-weaning with another litter to enhance play fighting experience. Play fighting was recorded for 1.5 h per day over 6 days pre-weaning. Play fighting network centrality was quantified using measures of individual network position and entire network structure (degree, eigenvector, betweenness, clustering coefficient). Skin lesions from aggression were counted after a dyadic contest and at 24 h and 3 weeks following group mixing. Pigs with play fighting interactions with many partners experienced fewer lesions from the dyadic contest (in-degree, p = 0.01) and tended to received fewer lesions 3 weeks after group mixing (degree, p = 0.088) but no other play fighting centrality measures affected the number of lesions at any point. The benefits of play fighting were therefore limited to specific aggressive social contexts. The tendency of socialised piglets to play fight with non-littermates did not affect subsequent lesions. We advocate the use of SNA over approaches that only consider dyadic interactions to further our understanding of the influence of early social group interactions on later life experience.

The effects of acute stress on sow reproductive performance are well established, but we know less about the implications of chronic stress for sow performance. This study investigated associations between total skin lesion counts 24hr and 3 weeks post-mixing, hair cortisol concentrations at the end of pregnancy, and reproductive performance of sows. Sows (n = 264; parity 1-5) were artificially inseminated and locked into individual feeding stalls within 11 fully slatted gestation pens, immediately after service. Sows were released from the stalls at approximately 25 days post-service, allowed to mix, and thereafter had free access to the stalls. Skin lesions were counted 24hr post-mixing (i.e. one day after release from the stalls), and 3 weeks post-mixing on the anterior (head, neck, shoulders and front legs), middle (flanks and back), and posterior (rump, hind legs and tail). The sum of counts across all sites yielded a total skin lesion count for each sow. Back fat depth measurement and hair collection were carried out one week prior to farrowing. Sow reproductive performance measures included the number of piglets born alive, born dead, mummified, and total born. Piglets from 75 sows were tagged, weighed and scored for vitality (0 = least vital, to 4 = perfect) and intra-uterine growth retardation (IUGR; 0 = none, to 3 = severe). There was a positive association between total skin lesion counts 3 weeks post-mixing and both the number of mummified piglets (P = 0.045), and IUGR scores (P = 0.018). There was no correlation between total skin lesion count either 24hr (Rho = 0.10; P > 0.05) or 3 weeks post-mixing (Rho = -0.02; P > 0.05) and hair cortisol concentrations. There was also no association between hair cortisol concentrations and measures of sow reproductive performance (P > 0.05). Higher skin lesion counts 3 weeks post-mixing were associated with aspects of sow reproductive performance. This suggests that chronic stress caused by sustained aggression had a negative impact on the reproductive system. Nevertheless, given the lack of significant associations between hair cortisol, skin lesion counts, and measures of reproductive performance, this was not supported by the findings for hair cortisol.