Social isolation elicits deficits in the ability of newly weaned female piglets to recognise conspecifics

In pigs, weaning and social isolation can be highly stressful and may impair social recognition (SR). The goal of this study was to assess the ability of newly weaned female piglets to recognise familiar non-related conspecifics and to examine how this was affected by weaning age and/or acute social isolation. For that purpose, 48 female piglets were either weaned at day 11 (D11, n = 24) or at day 22 (D22, n = 24) of age and subjected to the familiarisation procedure during the first 2 days post-weaning. To examine whether social isolation affected SR, 24 piglets were isolated for 15 min after the last familiarisation procedure, while the control group (n = 24) remained with their littermates in the nursery pens. In the SR test, taking place immediately after social isolation, each subject was exposed to a familiar (n = 24) or an unfamiliar (n = 24) conspecific for a 3-min period. The animals were tested only once, 2 days after weaning, i.e. at 13 days of age for D11 piglets and 24 days of age for D22 piglets. The assessment of SR was based on the natural tendency of pigs to investigate unfamiliar conspecifics more intensely than familiar ones. Overall, during the SR test familiar piglets spent less time on social investigation (16.70 ± 2.86%) than unfamiliar ones (29.09 ± 3.71%). The results indicated a significant interaction between weaning age and social isolation among familiar pigs (F1,17 = 23.12, P < 0.001). Neither weaning age nor social isolation affected the duration of social investigation in piglets exposed to unfamiliar conspecifics (F1,17 = 0.24, P = 0.63). However, when isolated D11 piglets were exposed to familiar conspecifics they investigated more than non-isolated D11, isolated D22 and non-isolated D22 piglets. The amount of social investigation displayed by familiar isolated D11 piglets was similar to that displayed by unfamiliar animals. In conclusion, it may indicate that SR can be disrupted by 15 min of social isolation in piglets weaned at D11, but not in piglets weaned at D22.