Early-Life Socialization Enhances Social Competence and Alters Affiliative Preference in Piglets

Early-life socialization (comingling with other litters before weaning) may affect social behaviors in pigs, but whether the impacts are sex-dependent is unknown. We assessed the effects of early-life socialization on interactions and behavioral responses of female and male pigs in a controlled test setting. Piglets (N = 12 litters) were either comingled from 14 days of age till weaning (Socialized, SOC) or not (Control, CON). Pigs (n= 96) were weaned at 28 days of age and regrouped in mixed-sex groups of eight. At 40 days of age, pigs were individually placed in an arena with one pen mate and one unfamiliar pig of the same sex kept behind fences. Behaviors (exploring the environment, exploring the fences, social contact, aggression, etc.) were observed for 8 min. Generalized linear mixed models with treatment and sex as fixed and pen as random effects were used. Overall, males spent more time exploring the fences of conspecifics than females. Although aggression was displayed by only a small minority of pigs overall, it was more frequently directed towards unfamiliar pigs. Despite this low prevalence, SOC pigs (10.4%) were significantly less likely to show aggression towards strangers than CON pigs (27.7%) (<i>p</i> = 0.0217), with this effect observed irrespective of sex. Critically, a time-dependent treatment-by-sex interaction emerged for the time spent exploring the pen mate's fence. While the interaction was only a trend over the full 8 min (<i>p</i> = 0.0889), it became statistically significant during the latter 4 min of the test (<i>p</i> = 0.049). This interaction was observed, with SOC-males spending more time on this behavior than SOC-females and all CON pigs, potentially indicating a time-specific and possible shift in social preference. General exploratory and locomotor behaviors (including walking, standing, and general exploration of the environment) were not or only minimally influenced by treatment or sex. In conclusion, early socialization has a significant impact on reducing aggression towards unfamiliar conspecifics. However, the effects on social preference and interactions appear sex-dependent, possibly indicating an enhanced preference for familiar pen mates in males under specific testing conditions.