Identifying hormonal habituation in field studies of stress.

Habituation is a term commonly used to explain a decrement in response intensity to a repeated stimulus or set of stimuli. In the stress literature, hormonal habituation is often used to describe a situation where an individual has learned to perceive a repeated stressor as innocuous, and thus the intensity of the release of hormonal stress mediators reduces over time. Consequently, a habituated individual is not considered stressed. There are, however, situations where an individual may be chronically stressed despite a reduction in the response intensity of hormonal stress mediators to a repeated stimulus. These alternative explanations are rarely considered in field studies even though a false conclusion that an individual has habituated (i.e., is not stressed) may lead to false conclusions regarding the animal's overall physiology and health. The present paper provides four alternative explanations for an observed attenuation in the response of hormonal stress mediators to a repeated stimulus or set of stimuli which lead to six criteria that define habituation in a field context. Furthermore, we propose four diagnostic tests to help distinguish hormonal habituation from these alternative explanations in field studies. These tests will help identify hormonal habituation in free-living animals and prevent potential problems of falsely describing an individual or population of individuals as habituated.