Three experiments were performed attempting to establish the validity of vocal measures as an indicator of the immediate response to pain in domestic piglets. Vocalisations were measured while piglets were subjected to the routine farm practice of castration without anaesthetic, or restrained identically but not castrated (i.e., sham-castrated). In Experiment 1 we measured how calling changed during the different stages of the procedure, and in Experiments 2 and 3 we measured the effect of different restraint techniques. Piglets that were castrated produced significantly more high frequency calls ( >1000 Hz) than sham castrates in all three experiments. In Experiment 1, we found the greatest differences between the castrated and sham piglets during the severing of the spermatic cords (1.0 high calls/s vs. 0.3 calls/s, P < 0.01., and lesser differences when the scrotum was incised and the testicles extruded. In Experiments 2 and 3, castrates again produced high frequency calls at significantly faster rates than shams. The rate of low calls (frequency < 1000 Hz), the mean frequency of the high calls, and mean call duration did not vary consistently between sham and castrated pigs. These results suggest that the increased rate of high calls is a reliable indicator of the pain due to castration. In Experiments 2 and 3, calling varied in response to how the piglets were restrained. However, there was no significant interaction between castration and restraint method for the rate of high calls, suggesting that the way in which the piglets were restrained did not affect the pain caused by castration.
Weary, D. M., Braithwaite, L. A., & Fraser, D. (1998). Vocal response to pain in piglets. Applied Animal Behaviour Science, 56(2), 161-172.