Endogenous analgesia in the chicken (1995)

Gentle, M. J., Corr, S. A.

Abstract

When chickens were placed in pairs into pens containing a deep layer of wood shavings, they either showed complete analgesia or significantly less pain-related behavioral reactions to a joint inflammation produced by urate injection. When tested in the barren cage, it appeared that the whole of the birds attention was occupied in trying to reduce the pain as far as possible [one-legged standing, limping, sitting]. In the more stimulating pen, the birds attention was shifted from the pain to the social partner and the wood shavings. Gentle (2001) concluded from these findings that "These attentional shifts indicate a cognitive component of pain in the chicken and provide evidence of consciousness. The implications for the welfare of the bird is that the pain they experience may have some of the complex facets of pain normally only ascribed to pain in humans."