Stress and Physiological Indicators of Animal Welfare

Placed in difficult and demanding situations, we have become accustomed to using the word “stress” to describe how we feel. Personal familiarity with the word leads us to assume that it has a clear definition, and when faced with the necessity of assessing the welfare of animals, it seems logical to try to determine if they too are “stressed”. Scientists even claim to have physiological indicators of stress, apparently making the concept concrete. Unfortunately, the easy use of the word in everyday conversation obscures the semantic tangle that awaits those who venture in too deeply. In this chapter, we discuss the concept of stress as a justification for the various physiological measures that have been proposed as indicators of animal welfare. For a detailed historical exposition of the various concepts or models of stress, and a comprehensive review of the physiological systems involved, we refer the reader to Toates (1995), Sapolsky (1998), Moberg and Mench (2000), and Tsigos and Chrousos (2002). Lane (2006) and Mormède et al. (2007) present very good discussions of the relationship between animal welfare and the physiological responses to stress.

In this chapter, we discuss some of the potential and difficulties in using physiological measures to assess animal welfare. We focus primarily on the issues associated with validating such measures as indicators of animal welfare, and the complexities and difficulties associated with their measurement. Since our subject is animal welfare, we limit our discussion to aspects of the stress response that are the most clearly linked to animal welfare; space constraints prevent us from discussing other topics, such as the effects of stress on reproduction or meat quality. Although such effects can be of considerable economic importance, they are not directly relevant to animal welfare.