Although most people have an intuitive understanding of what ’stress’ is, it has been difficult for biologists to develop a widely accepted, precise definition (Moberg and Mench 2000; Romero and Wingfield 2015; Bradshaw 2017).
Adding further confusion is the fact that stressors can vary considerably in their duration and/or magnitude, leading to drastically different physiological outcomes. Short-term activation of a stress response helps an individual cope with unexpected or demanding situations (Moberg and Mench 2000; Mostl and Palme 2002; Romero and Wingfield 2015).
Such acute stress responses are considered adaptive and represent a normal, healthy response to unpredictable stimuli (Moberg and Mench 2000; Romero and Wingfield 2015). As such, acute stressors do not generally pose a threat to animal health and are of little clinical significance. Short-term, or acute stress, is also referred to as ‘good stress’ or ‘eustress’ (NRC 2008).However, long-term exposure to stressors generally has negative consequences for an animal. Chronic stress can lead to reproductive suppression, impaired immune function (thus increased susceptibility to infectious disease), compromised growth and repair processes, altered behaviour, muscle atrophy and the formation of gastric ulcers (Moberg and Mench 2000; Romero 2004). Chronic stress is also referred to as ‘bad stress’ or ‘distress’. In this chapter the terms ‘acute’ and ‘chronic’ will be used.
Despite the drastically different physiological outcomes of these two types of stress, a clear way to distinguish between them based on the characteristics of the physiological stress response has yet to be identified (Moberg and Mench 2000; NRC 2008). The ability to differentiate between acute and chronic stress would be valuable for monitoring the health and welfare of both free-ranging animals and those in human care.
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