Review
Stress and the evolution of condition-dependent signals

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Abstract

Stressful events are known to initiate a cascade of physiological mechanisms that are potentially costly for metabolic processes. Although these mechanisms are well understood, the long-term costs and the potential implications for individual condition and behaviour have been considered only recently. Combining information from physiological, ecological and behavioural studies can help us to understand the implications of stress for individual life history strategies. Furthermore, the concept of individual variation in stress tolerance has implications for the immunocompetence handicap hypothesis and the evolution of secondary sexual signals.

Section snippets

What do we mean by stress?

To examine the effects of physiological stress on the individual we need to understand what is meant by ‘stress’ (Box 1). For the individual, the stress response is typified by physiological changes to short- or long-term unpredictable alterations in environmental circumstances, which cause a redirection of resources to vital processes and impair, or threaten to impair, homeostasis1, 8. However, the stress response will vary enormously according to the nature of the stressor. A rapid-onset

Measuring stress

Individual stress levels can be assessed in a variety of ways. For example, invertebrate stress has been assessed extensively using levels of heat shock proteins (HSPs) (Box 2). By contrast, the most common method in vertebrate studies is to measure glucocorticoid production in relation to a standardized stressor (Box 3). Glucocorticoid production varies in relation to the intensity of the stressor and it is essential to focus upon biologically relevant levels of stress when examining the

Stress and immune function

The effects of stress upon the immune system are without doubt complex, variable and extremely controversial. The stress response is multifaceted in nature and, therefore, has the potential to affect immunocompetence through a variety of routes. Furthermore, immunocompetence involves allocation between different aspects of the immune system, which might be affected differentially.

This assessment is complicated further by the fact that the effects of glucocorticoids on the immune system differ

Stress and behavioural strategies

Individual differences in stress responses and the consequences for immune function open the possibility for a range of optimal behavioural strategies. Establishing and maintaining dominance rank involves potentially dangerous interactions, as social defeat can have important effects on individual physiology, which can last for periods of up to several days14. A long-term study of olive baboons (Papio anubis) has demonstrated differences in basal levels of glucocorticoids in relation to both an

Stress and condition-dependent signals?

According to the immunocompetence handicap hypothesis, immunosuppression by sex hormones mediates the cost of development of many secondary sexual signals30. The hypothesis predicts that levels of circulating androgens, necessary for the development of many sexual signals and behaviours, should covary with immune function31. Therefore, elaborate secondary sexual traits honestly reflect male quality by indicating the level of immune suppression an individual can tolerate. Although these

Prospects

Although the integration of physiological understanding with behavioural ecology is necessary if we are to understand the mechanisms underlying basic evolutionary processes, it must be done with care. Furthermore, it is necessary to be pragmatic about the difficulties of demonstrating the role of stress. Researchers should be aware of the potential pitfalls of generalizations concerning the physiology underlying exposure to stressful events. The stress response is complex and specific both to

Unlinked list

7, 9, 12, 27

Acknowledgements

I would like to thank Matthew Evans, Arthur Goldsmith, Kirsty Park, Ken Wilson, John Wingfield and two anonymous referees for their helpful comments on this article. K.L.B. is supported by NERC grant GR3/11426.

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