Piglets vocally express the anticipation of (pseudo)-social contexts in their grunts

Emotions not only arise in reaction to an eliciting event but also while anticipating it, making this context a way to assess the emotional value of events. Anticipatory studies have poorly considered vocalisations whereas they carry information about the emotional state. We studied the grunts of piglets that anticipated two (pseudo)social events known to elicit positive emotions more or less intense: arrival of a familiar conspecifics and arrival of a familiar human. Both time and spectral features of the vocal expression of piglets differed according to the emotional context. Piglets produced low-frequency grunts at a higher rate when anticipating conspecifics compared to human. Spectral noise increased when piglets expected conspecifics, whereas the duration and frequency range increased when expecting a human. When the arrival of conspecifics was delayed grunts duration increased, while when the arrival of the human was delayed spectral parameters were comparable to those during isolation. This shows that vocal expressions in piglets during anticipation are specific to the expected reward and to the time duration between signal and the delivery of the reward. Vocal expression (time and spectral features) is thus a good way to explore emotional state of piglets during anticipation of challenging events.