Abstract
Predator-induced changes in the glucocorticoid responses of prey have been proposed to mediate indirect predator effects on prey demography. Ambiguities exist, however, as to whether differences in predation threat in the environment at large affect the mean glucocorticoid response in wild birds and mammals, and whether this is likely to affect reproduction. Most studies to date that have examined glucocorticoid responses to environmental variation in predation threat have evaluated just one of the several potential measures of the glucocorticoid response, and this may be the source of many ambiguities. We evaluated multiple measures of the glucocorticoid response [plasma total CORTicosterone, corticosteroid binding globulin (CBG) and free CORT] in male and female song sparrows (Melospiza melodia) sampled at locations differing in predation threat in the environment at large, where we have previously reported reproductive differences suggestive of indirect predator effects. Total CORT varied markedly with predation threat in males but not females whereas the opposite was true for CBG, and both sexes demonstrated the same moderately significant free CORT response. Considering all three indices, a glucocorticoid response to environmental variation in predation threat was evident in both sexes, whereas there were ambiguities considering each index singly. We conclude that collecting multiple physiological measures and conducting multivariate analyses may provide a preferable means of assessing glucocorticoid responses to environmental variation in predation threat, and so help clarify whether such glucocorticoid changes affect reproduction in wild birds and mammals.
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Acknowledgments
We thank C. de Ruyck, L. Erckman, J.C. Wingfield, A. Duncan-Rastogi, J. Malt, N. Goodenough, M. Travers, P. McDougall, A. Moore, L. Orr, and E. Knight for assistance in the field and lab. We also thank Parks Canada, the Saanich Municipality and the owners of Tortoise and Domville islands for access to the sites; and T. Golumbia and B. Clinchy for support. O. Olsson, B.P. Kotler, and M. Thaker all provided many helpful comments on earlier drafts. Funding was provided by the Natural Sciences and Engineering Research Council of Canada. This work was approved by the Animal Care Committee of the University of Western Ontario and conforms to the legal requirements of Canada.
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Communicated by Ola Olsson.
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Clinchy, M., Zanette, L., Charlier, T.D. et al. Multiple measures elucidate glucocorticoid responses to environmental variation in predation threat. Oecologia 166, 607–614 (2011). https://doi.org/10.1007/s00442-011-1915-2
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DOI: https://doi.org/10.1007/s00442-011-1915-2