Changes of the vocalization during the castration of the domestic pig are indicators of pain

The surgical castration of male young piglets without local anaesthetics is a routinely applied process in the pig industry. The different handling procedures and the surgical intervention evoke a considerable number of vocalizations, that seem to indicate pain and stress. In the present approach we examined various parameters of the vocalization of 19 male piglets during the three phases of the castration process (presurgical, surgical, postsurgical). The animals predominantly responded with high-frequency calls (> 1000 Hz). These calls were reliably detected (96.6%) by an automatic stress call monitoring system (STREMODO) using vocal cues of sound production which are thought to be involved in “honest” signalling. With a following statistical discrimination analysis based on these vocal cues we achieved the significantly best call assignment to the respective handling or surgical phase at the surgical part of the castration procedure (85.2%). Hence, the stress characteristic of the calls was most pronounced in this phase. Besides the number of calls per time unit also all other single call parameters (call duration, peak frequency, pureness and entropy of the sound) were significantly influenced by the surgical phase of the castration. Together, the observed changes of the acoustic parameters can be interpreted as vocal indicators for experienced pain of the piglets. We conclude that a careful analysis of the vocal behaviour of animals may help to gain deeper knowledge of pain and stress that an animal perceives. The results of this work deliver further causes for a further critical re-evaluation of the current practice of piglet castration without anaesthetics.

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