Personality types and environmental enrichment affect aggression at weaning in pigs

We investigated the effects of different environmental treatments and personality types on aggression at mixing of newly weaned domestic piglets. From birth to weaning, 16 litters were housed with their dams in either substrate-impoverished (I) or larger, substrate-enriched (E) environments. At 14 days old, piglets were classified as „high¿ (HR) or „low resistant¿ (LR) in a manual restraint test (back-test, after Bolhuis et al., 2003, Physiology & Behavior, 79), which is thought to identify proactive (HR) and reactive (LR) stress coping strategies as part of different personality types. At 30 days old, 128 piglets were weaned and mixed into 32 pens of 2 HR and 2 LR unfamiliar pigs. 8 I and 8 E groups changed environment whereas the others remained in the same environment. Fighting was scored for 5 h post-mixing and for 6 h the following day. Data were analyzed using generalized linear mixed models with pre- and post-weaning housing, back-test classification and interactions as fixed effects and pen as random effect. Pre-weaning E pigs fought for longer after mixing and on the next day than I pigs. Post-weaning E did not affect fighting at mixing but reduced the time spent fighting the next day. Subtle and environment-dependent effects of personality on fighting were found. In E groups, for instance, HR pigs won more fights than LR pigs. Also, HR pigs bullied (i.e. chased surrendering pigs) more often and their fighting behaviour was less affected by body weight than that of LR pigs. Overall, pre-weaning E increased aggression after weaning whereas post-weaning E reduced it, and coping strategy related to some aspects of fighting behaviour.