Qualitative aspects of grunts vary with pigs' mental states

Studying human-animal interactions in domestic species and how they affect the establishment of a positive Human-Animal Relationship (HAR) may help us improve animal welfare and better understand the evolution of interspecific interactions associated with the domestication process. Understanding and describing the quality of an HAR requires information on several aspects of the animal biology and emotional states (social, spatial and postural behaviours, physiological and cognitive states). Growing evidence shows that acoustic features of animal vocalisations may be indicators of emotional states. Here, we tested the hypothesis that the quality of vocal expression may indicate the quality of HAR. At weaning, 30 piglets were positively handled by an experimenter who talked to and physically interacted with them three times a day, while 30 other piglets only received the contact necessary for proper husbandry. After two weeks, we recorded the behaviours and vocalisations produced in the presence of the static experimenter for five minutes. We repeated this test two weeks later, after a conditioning period during which human presence with additional positive contact was used as a reward for all piglets. We hypothesized this conditioning period would lead to a positive human-piglet relationship for all piglets. As expected, piglets that were positively handled at weaning expressed a higher attraction toward the experimenter, and, after the conditioning, piglets that were not positively handled at weaning expressed a similar level of attraction than the positively handled ones. Piglets positively handled at weaning generally produced shorter grunts than the other ones. However the latter expressed more flexibility in call structure when vocalising close to a human, with a decrease of grunt duration and an increase in pitch, frequency range and noisiness in their grunt. This differential effect of proximity between groups of piglets was attenuated after the conditioning during a standard reunion with a static human but remained over time when the human was providing additional positive contacts. Results suggest that first, changes in vocal structure are consistent with indicators of positive states in the presence of a human. Second, increasing familiarity and proximity between a human and a pig may induce changes in the acoustic structure of its grunts. Third, a human providing additional positive contacts triggers more changes in vocalisation structure than by their presence only. We show that vocalisation structure may allow us to assess the quality of human-pig relationship.