Rius, M. M., Pageat, P., Bienboire-Frosini, C. et al. 2018. Tail and ear movements as possible indicators of emotions in pigs. Applied Animal Behaviour Science 205, 14-18.

A better understanding of animal emotions is an important goal in disciplines ranging from neuroscience to animal welfare science, but few reliable tools exist for measuring these emotions. Play behaviour is generally recognized as a trigger of positive emotions in mammals, and previous studies have suggested that tail movement (a behaviour often seen during play) may indicate positive emotions in pigs, while ear movements may indicate negative emotions, or a decrease in positive emotions. This study investigated whether the provision of toys to solicit play behaviour in pigs was associated with tail and ear movement changes, with the goal of confirming the use of these movements as indicators of different emotions in pigs. Sixteen mini-pigs were divided into two identical rooms of 8 individuals. During 4 consecutive days, each animal participated in the study for a total of two days in two different situations: one Control session and one Play session. Each animal served as its own control. During Play sessions, pigs were placed in their own pens of two individuals, where two medium-sized dog toys were introduced. During Control sessions, animals were placed in their own pens, but no toy was provided. Behaviours were scored from video using continuous recording with an ethogram, which also allows to confirm that pigs play during Play sessions. Results showed a significant difference in tail movement duration between sessions (DF = 15; t = −3.40; p < 0.01; Student t-test for paired sample) with longer durations during the Play session than during the Control session. Tail movement frequency also varied significantly between Play and Control sessions (DF = 15; t = 2.96; p = .01; Student t-test for paired samples) with significantly higher frequencies during the Control session. A significant difference between sessions was also observed for ear movement frequency (DF = 15; t = 4.69; p < 0.01; Student t-test for paired samples), which was significantly higher during the Control session. In addition, 69% of the pigs (11 of 16) performed displacement behaviours during the Control session, while none of the pigs displayed this behaviour during the Play session. Finally, during the Play session, a negative correlation was found between tail movement duration and tail movement frequency (rho = −0.79; N = 16; p < 0.001). In conclusion, tail and ear movement changes are linked to play behaviour: in a play situation, tail movement duration increases, and ear movement frequency and displacement behaviours decrease, compared to a control situation (without enrichment). These results could be useful for improving the analysis of emotions in pigs and assessing animal welfare.