Discrimination of conspecifics by juvenile domestic pigs, Sus scrofa

Social recognition is important for the formation and maintenance of dominance hierarchies that are based on conspecific assessment. Pigs are assumed to recognize individuals because they react aggressively towards unfamiliar conspecifics, but we know little about the extent of their discriminatory abilities or the influential variables. We investigated the ability of 6-week-old female pigs to discriminate between pairs of female conspecifics in a Y-maze. Members of the pair were either unrelated or littermates and were either familiar or unfamiliar to the test pig. Olfactory, auditory and visual cues from both stimulus pigs were available to the test pig. We rewarded approaches to a predetermined correct stimulus pig with food in a series of sessions, each of 10 consecutive trials. Stimulus pigs also received a food reward to avert frustration. Each pig completed 6-12 sessions. At least one-third of the pigs successfully discriminated between stimulus pigs in each of three consecutive sessions, showing rapid task acquisition. Our findings indicate that pigs' ability to discriminate conspecifics is not reliant on familiarity or differences in familiarity. Furthermore, closely related animals are distinguishable. We also found a significant effect of reward procedure on learning. Test subjects learned faster when stimulus pigs were rewarded after each correct choice (i.e. at the same time as the test pig) than when the stimuli were rewarded after each incorrect choice, indicating subtle effects of experimental procedure on subject performance and live stimulus behaviour.