Emotional states and emotional contagion in pigs after exposure to a positive and negative treatment

After-effects of events that elicit an emotional state on both the animals that experienced these events and on their group members have only scarcely been studied. We investigated effects of a positive vs. negative treatment on the behaviour and emotional state of pigs and their naive pen mates afterwards. Behaviour of 96 pigs was observed in the home pen for 5. min on two different days (day 2 and 18), directly after two pigs per pen (N = 16) had been subjected to a positive or negative treatment in a test room. On day 2, treated pigs lay down more (30.78. ±. 4.07 vs. 15.25. ±. 3.74% of time, P = 0.01), walked less (17.91. ±. 2.82 vs. 26.87. ±. 2.32% of time, P = 0.02) and explored the pen less (12.30. ±. 1.34 vs. 18.29. ±. 1.71% of time, P = 0.01) after the negative compared to the positive treatment. Naive pigs simultaneously also lay more (45.67. ±. 6.00 vs. 18.79. ±. 5.88% of time, P = 0.003), walked less (6.33. ±. 0.80 vs. 12.83. ±. 1.74% of time, P. <. 0.001) and explored the pen less (6.80. ±. 1.23 vs. 13.47. ±. 2.34% of time, P = 0.02) after their pen mates' negative treatment. After their pen mates' positive treatment, in contrast, naive pigs showed more nosing behaviour, nose-nose (0.83. ±. 0.14 vs. 0.40. ±. 0.06 freq./min, P = 0.004) and nose-body contact (0.73. ±. 0.10 vs. 0.47. ±. 0.06 freq./min, P = 0.02), and tended to play more (0.10. ±. 0.03 vs. 0.01. ±. 0.01 freq./min, P = 0.09). On day 18, treated pigs were only found to eat longer after the negative than the positive treatment (10.75. ±. 3.73 vs. 0.96. ±. 0.79% of time, P = 0.02), whereas their naive pen mates, similar to day 2, lay more (45.01. ±. 5.16 vs. 22.59. ±. 5.52% of time, P = 0.006), stood (40.73. ±. 3.84 vs. 57.32. ±. 4.29% of time, P = 0.007) and walked less (7.00. ±. 1.21 vs. 10.88. ±. 1.04% of time, P = 0.01). After their pen mates' positive treatment, at day 18, they still nosed the nose (0.52. ±. 0.06 vs. 0.21. ±. 0.04 freq./min, P. <. 0.001) and body of their pen mates more (0.68. ±. 0.06 vs. 0.29. ±. 0.05 freq./min, P = 0.002) than after their pen mates' negative treatment, and they tended to wag their tails more (2.30. ±. 0.95 vs. 0.68. ±. 0.41% of time, P = 0.08). Thus, pigs still appeared to be in a negative emotional state for some time after the negative treatment had ended. Furthermore, their pen mates also seemed to be (emotionally) affected even though they were not subjected to the treatment themselves. Negative and positive events may thus have consequences that extend beyond the duration of these events, for both the welfare of the exposed animals and their group members.