Transfer of emotional states between pigs

Typical for humans is that we often ‘feel’ the emotional state of another. When a mother witness her baby crying, she feel also upset too, and when we watch a movie with a happy ending, we also empathize with the main characters. In psychology, experiencing of emotion in one individual upon witnessing it in another individual is called emotional contagion.1 Interestingly, we do not only share a conscious experience of feeling happy or sad, but other bodily responses such as changes in heart rate or even blushing, have also been proposed to to transfer from one person to another during a social interaction.3

Why is the transfer of emotions between individuals important? Emotional contagion is an automatic response which is considered to be the simplest for of empathy. We feel more empathic towards individuals we are familiar with and with whom we are socially closed than with strangers.5 The matching of our emotional state with that of another often makes us help them. Because the mother feels for her crying baby, she understand that the baby is upset and needs taking care of (e.g., changing a diaper). If we feel sorry for the situation of a homeless person, we will support them financially. Emotional contagion is therefore a building block for more complex social behaviors, and observed not only in humans, but also in non-human animals such as dogs and great apes.4 Recent evidence suggests that pigs can also experience each other’s emotions.

In one study, a pair of pigs were guided either to a rewarding or aversive chamber. In the rewarding chamber, there was plenty of straw, peat and hidden raisins which pigs could freely explore together. In contrary, the pair of pigs were separated before being guided to the aversive chambers where each of them were restrained to increase the unpleasantness from being separated. Before entering each of the chambers, the pigs spent some time in a so-called starting chamber in which different sounds and visual information were shown to the pigs so that they could learn to predict which chamber they subsequently enter.

In the rewarding chamber, the pigs were playing in the straw: they were running and pivoting with straw by shaking their heads, they were barking (check out how pigs bark here), and wagging their tails. All these behaviors are associated with positive emotions. Pigs in the aversive chamber, on the other hand, showed more freezing, high pitched vocalizations, attempts to escape, defecating and urinating. They were also grunting more (which are thought to be a sign of social contact calls), showed more ears back and their movement from front to back as well as more tail low (similarly to dogs). In pigs, a default tail position is a tail in curl from which other tail positions – wiggling tail or tail low – can emerge if they experience positive or negative emotions, as the authors imply.

Interestingly, when presented only with the cues predicting the rewarding and aversive events in the starting chamber, the pigs also displayed behavior suggestive of positive and negative emotions. They nosed the doors of the rewarding chamber but oriented their heads more towards the aversive chamber, and showed more ears back and their movement back and forth (vigilance) suggesting that they knew what was going to happen.

After demonstrating that pigs can indeed show (anticipatory) emotional responses to rewarding and aversive events, the authors were interested in whether these responses can be also observed in new pigs who joined them in the starting chamber. The new pigs have never experienced the above procedures before and if they show behaviors suggesting of negative or positive emotions, that would suggest emotional contagion.

As expected, during a rewarding cue and event, the new pigs showed more curly tail and more playful behavior suggesting the presence of positive emotions. In the contrary, the new pigs showed more tail low and more defecating compared to the rewarding event, indicating that they were stressed. The researchers remarked that these behaviors cannot be due to simply due to copying because low tail was not displayed by the initial pigs in the starting chamber.

This study suggest that pigs – similarly to dogs and primates with more complex social interactions – also show emotional contagion and are likely to experience positive or negative emotional states displayed by their companions. This implies that on animal farms – where pigs are often kept in large groups together, unpleasant or harmful procedures performed on individual pigs such as handling (catching pigs) or tail docking, are likely to cause stress in other present pigs.

1Hatfield, E., Cacioppo, J. T., & Rapson, R. L. (1993). Emotional contagion. Current Directions in Psychological Science: 2(3), 96–99. doi: 10.1111/1467-8721.ep10770953
2Reimert, I., Bolhuis, J.E., Kemp, B., Rodenburg, T.B. (2013). Indicators of positive and negative emotions and emotional contagion in pigs. Physiol Behav 109:42-50. doi: 10.1016/j.physbeh.2012.11.002.
3Prochazkova, E., Kret, M.E. (2017). Connecting minds and sharing emotions through mimicry: a neurocognitive model of emotional contagion. Neurosci Biobehav Rev 80: 99-114. doi: 10.1016/j.neubiorev.2017.05.013.
4Marino, L., & Colvin, C. M. (2015). Thinking pigs: A comparative review of cognition, emotion, and personality in Sus domesticus. International Journal of Comparative Psychology, 28, Article 23859.
5De Waal, F.B. (2008) Putting the Altruism Back into Altruism The Evolution of Empathy. Annual Review of Psychology: 59, 279-300. doi: 10.1146/annurev.psych.59.103006.093625