Limits to using HPA axis activity as an indication of animal welfare.

HPA axis activity is often measured by corticosteroid release as a means to evaluate stress and well-being in animals. While the analysis of corticosteroid levels can provide useful information in some circumstances, a variety of methodological and technical problems make them difficult to accurately interpret. Furthermore, there is considerable evidence that is inconsistent with the widespread notion that high levels of stress result in a large amount of corticosteroid release, and in some cases the converse is true. This review highlights the strengths and weaknesses of the techniques used to measure corticosteroids, describes a number of studies that failed to find a positive correlation between stress and corticosteroid levels, and delineates ancillary behavioral and cognitive tests that provide insight into an animal's well-being. We conclude by emphasizing that the most holistic account of animal welfare is provided by utilizing a combination of physiological and psychological methods.