Family pigs’ and dogs’ reactions to human emotional vocalizations:a citizen science study

Human distress vocalizations elicit an increase in dogs' stress responses. This modulation of behaviour to match one's emotional state to that of another individual is often described as emotional contagion. Whether this phenomenon is promoted by the dogs' selection for cooperation with humans or is rooted more generally in the universal vocal signals of emotion is unclear. To test this, we compared the reactions of companion dogs, Canis familiaris, and companion pigs, Sus scrofa domesticus (which are popular companion animals but whose domestication history lacks selection for cooperation), to human sound playbacks of crying, a high-arousal, negatively valenced sound, and humming, a low-arousal, less emotionally valenced sound, in a citizen science study. Dogs exhibited higher levels of behaviours associated with increased arousal and negative emotional states and vocalized more in response to crying compared to humming. In contrast, pigs showed more negative and high-arousal behaviours in response to humming than to crying. The fact that dogs seemed to have been affected by and reacted accordingly to the emotional content of human vocal sounds is in line with previous works and the emotional contagion account. In contrast, pigs' elevated stress to the low-arousal humming sound compared to the negative and high-arousal crying sound, cannot be fully explained by emotional contagion but rather by the novelty of the sound (neophobia). Selection for cooperation with humans may thus be key for promoting human sound-induced emotional contagion in domestic mammals.