Castration-induced vocalisation in domestic piglets, Sus scrofa: Complex and specific alterations of the vocal quality

The surgical castration of male young piglets, Sus scrofa, is a routinely applied procedure in domestic pig production. The various handling and surgery procedures during the castration process evoke a considerable amount of vocalisation thought to be indicators of experienced pain and stress. We analysed complex and single measures of the vocalisation of 19 two-week-old male piglets during three different periods in the castration procedure: a presurgical handling period, the surgical period, and a postsurgical handling period. Piglets predominantly responded with high-frequency calls (>1000 Hz) at a maximum level (“all-or-none” strategy). These calls were reliably detected (96.6%) by an automatic stress call monitoring system (STREMODO) using vocal cues of sound production (e.g. formant-like structures) which are thought to be involved in “honest” signalling. Subsequent statistical discrimination analysis based on these vocal cues achieved the significantly best call assignment to the respective handling or surgical periods at the surgical part of the castration procedure (85.2%) meaning that the stress characteristic of the calls was most pronounced in this period. Except for the number of calls all other single vocal measures of the high-frequency vocalisation (call duration, peak frequency, pureness and entropy of the sound) were significantly affected by castration. The surgical period was significantly different from the postsurgical handling period in all these measures, but compared to the presurgical handling period it differed only in the pureness and the entropy of the sound. Taken together, the observed changes of acoustical parameters during the surgical period can be interpreted as vocal indicators for experienced pain and suffering. We conclude that a careful analysis of the vocal behaviour of animals may help to gain deeper knowledge of pain, stress and discomfort that an animal perceives. These results deliver further facts for a critical re-evaluation of the current practice of non-anaesthetised castration of piglets.