Physiological stress in ecology: lessons from biomedical research

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Abstract

Increasingly, levels of the ‘stress hormones’ cortisol and corticosterone are being used by ecologists as indicators of physiological stress in wild vertebrates. The amplitude of hormonal response is assumed to correlate with the overall health of an animal and, by extension, the health of the population. However, much of what is known about the physiology of stress has been elucidated by the biomedical research community. I summarize five physiological mechanisms that regulate hormone release during stress that should be useful to ecologists and conservationists. Incorporating these physiological mechanisms into the design and interpretation of ecological studies will make these increasingly popular studies of stress in ecological settings more rigorous.

Section snippets

The psychological context of stress

One lesson from psychological studies is that the context of a stressor is important in predicting how an animal will respond to that stressor. A classic example is the study by Weiss and colleagues [5], who gave two rats similar electric shocks. One rat was given a lever to press, which terminated the shock for both rats. The rat that controlled the lever had a dramatically lower GC response than did the second rat. Therefore, the degree of control over the stressor (i.e. the context)

Multiple GC receptors

All interpretations of GC concentrations are complicated by the fact that these hormones have two receptors, which when bound to the GCs have different physiological and behavioral consequences [11] (Box 2). The poor understanding of these dynamics resulted in many of the original controversies surrounding GC physiology in the biomedical literature.

Selye, a founder of the field of stress physiology, originally conceived of GCs as having a direct stimulatory role in successfully coping with an

Terminating stress responses

One underappreciated aspect of GC physiology is that the duration and the magnitude of GC release are often both as important. The total amount of GCs released in response to a stressor (the integrated response) is the important variable, not the maximum amount released [16]. This is because the biological effect results from hormone–receptor interactions that occur over the entire course of the stress response, not just at the peak of GC release. The total amount released is, at its simplest,

Neonatal experiences

A thriving area of biomedical research is the impact that early neonatal experiences have on stress responses once the animal has grown. Work has followed two paths, both of which produce similar results. In the first approach, mammalian mothers are stressed during pregnancy and the GC responses of their adult offspring are examined 31, 32. Stressors applied to non-mammalian mothers might also be important because many non-mammalian species give birth to live young. Determining whether maternal

Physiological acclimation and facilitation to stress

One perplexing aspect of GC physiology is how the HPA axis changes after chronic or repeated mild stress. In general, the more intense the stressor, the greater the amount of GC released [43]. Once the stimulus ends, GCs return to baseline concentrations. If the stressor persists or occurs at regular frequent intervals, however, the entire functioning of the HPA axis changes. Laboratory studies demonstrate that multiple stressors interact in unpredictable ways to alter GC release [21]. Two

Conclusions

Here, I have introduced some important topics on stress physiology from the biomedical literature. Each topic is much richer than is possible to present here, and all are active areas of biomedical research. The value of these topics to ecological projects should be clear. Many field studies would be much stronger if they were designed and interpreted within the context of the issues raised here. I have not, however, attempted to give prescriptions for how to deal with these issues. Partially,

Acknowledgements

I thank J.M. Reed and M. Wikelski for invaluable comments about this article. This work was supported by the U.S. National Science Foundation, grant #IBN-9975502.

Glossary

Glossary

Acclimation:
the concept that after repeated or chronic exposure to a stressor, an animal no longer considers the stressor to be noxious and reduces its GC response.
Context of a stressor:
the physical and psychological conditions present when a stressor appears.
Facilitation:
the concept that acclimation to one stressor enhances subsequent responses to novel stressors.
Receptor:
a protein that recognizes and binds to a hormone. The receptor then converts the hormonal signal into a biological action.

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