Vocal expression of emotional valence in pigs across multiple call types and contexts

Emotions, unlike mood, are short-lived reactions associated with specific events. They can be characterized by two main dimensions, their arousal (bodily activation) and valence (negative versus positive). Knowledge of the valence of emotions experienced by domestic and captive animals is crucial for assessing and improving their welfare, as it enables us to minimize the negative emotions that they might experience and to promote positive ones. Emotions can affect vocalizations directly or indirectly through the brain, lungs, larynx or vocal tract. As a result, vocal expression of emotions has been observed across species, and could serve as a non-invasive and potentially very reliable tool to assess animal emotions. In pigs (Sus scrofa), vocal expression of emotions has been relatively well studied. However, it is not known if the vocal indicators revealed in previous studies are valid across call types and contexts. To find this out, we conducted a meta-analysis of the effects of emotional valence on pig vocalizations, including calls recorded in the most common emotional situations encountered by pigs throughout their lives, from birth to slaughter. Our analyses revealed that pigs produced calls characterized by a higher center of gravity, a shorter duration, less noise (lower Wiener entropy), lower formants (measured using the formant dispersion) and LPC coefficients in positive compared to negative contexts. Overall, these vocal parameters could be very useful for developing automated methods to monitor pig welfare on-farm.