Evidence for residence-induced enhancement of aggressiveness in the non-territorial pig

Wild pigs (Sus scrofa) are only territorial during the peri-parturient period and when young compete for access to teats. In other species, territorial defence is affected by contextual factors including resource quality. Territoriality could be stimulated in species such as pigs by artificial mixing and intensive housing. Test pigs were introduced to a novel opponent for 5 min on consecutive days (days 1 and 2) in two locations (home pen of the test pig (P) or a neutral arena (A)) in a randomised block design. On day 1, animals tested in their home pen were more likely to initiate an escalated attack (proportion of pigs 0.43 vs 0.19 in the pen and neutral arena, P < 0.01). A shorter latency to attack on day 2 in the PP (pen on both days) and PA (pen then arena) treatments suggested that a previous encounter in the home pen affected later aggressiveness. The time spent engaged in non-escalated aggressive behaviours (parallel walking, pushing, knocking and biting) with the opponent was similar in the two environments. Some capacity for territorial defence through escalated aggression and the enhancement of subsequent aggressiveness appears to occur in pigs when in their home pen. The suppression of escalated aggression in the neutral arena by handling and environmental exploration cannot be discounted, but appears not to have affected the occurrence of non-escalated aggression. The mixing of pigs in an occupied pen is likely to enhance escalated aggression from the occupants without reducing that from the intruders and should be avoided.