Differential vocal responses to physical and mental stressors in domestic pigs (Sus scrofa)

Vocalisation can indicate an animal's emotional and/or physiological state. Thus, the detailed analysis of acoustic signals might provide a useful non-invasive method of assessing the welfare status of animals. Focussing on vocal expressions of distress, the present study examined vocal responses of 10-week-old domestic pigs during a classical conditioning paradigm, where a neutral light stimulus (conditioned stimulus) was repeatedly followed by a short electric shock (unconditioned stimulus). In contrast to the physical distress elicited by the unconditioned stimulus, the anticipation of this aversive event constitutes fear, i.e., mental distress. Accordingly, three stages characterised by distinct stressor qualities were defined: (1) non-anticipated aversive stimulus; (2) anticipated aversive stimulus; (3) anticipation of the aversive stimulus. Vocal responses comprised both high-frequency elements and low-frequency elements. High-frequency elements were analysed using linear prediction coding (LPC) to describe frequency structure. Calls were compared using MANOVA and subsequently tested for their classifiability regarding stage using a non-parametric discriminant function analysis. The calls differed significantly between stages (p < 0.001), mainly based on differences in the first resonance frequency. Hence, perceived stressor quality did indeed have an effect on vocal response, and furthermore predictability of the aversive stimulus influenced the responses towards it. Additionally, classifiabilty was high, with the median percentage of correct classification of the calls above 80% for all three stages. Therefore, we suggest that vocalisation elicited during stressful situations can serve as a differentiated indicator of stressor quality and can be used for the instantaneous assessment of the internal states of pigs.