Glucocorticoid Stress Hormones and the Effect of Predation Risk on Elk Reproduction

Editor’s note: A while back I wrote about a new study about how merely the presence of wolves is having a devastating effect on the sustainable reproduction of elk. Below is the complete report on that study.

Scott Creel1, John A. Winnie, Jr., and David Christianson
Department of Ecology, 310 Lewis Hall, Montana State University, Bozeman, MT 59717
Edited by Mark S. Boyce, University of Alberta, Edmonton, Canada, and accepted by the Editorial Board June 12, 2009 (received for review March 3, 2009)

Predators affect prey demography through direct predation and through the costs of antipredator behavioral responses, or risk effects. Experiments have shown that risk effects can comprise a substantial proportion of a predator’s total effect on prey dynamics, but we know little about their strength in wild populations, or the physiological mechanisms that mediate them. When wolves are present, elk alter their grouping patterns, vigilance, foraging behavior, habitat selection, and diet. These responses are associated with decreased progesterone levels, decreased calf production, and reduced population size [Creel S, Christianson D, Liley S, Winnie JA (2007) Science 315:960]. Two general mechanisms for the
effect of predation risk on reproduction have been proposed: the predation stress hypothesis and the predator-sensitive-food hypothesis. Here, we used enzyme immunoassay to measure fecal glucocorticoid metabolite concentrations for 1,205 samples collected from 4 elk populations over 4 winters to test the hypothesis that the effect of predation risk on elk reproduction is mediated by chronic stress. Across populations and years, fecal glucocorticoid concentrations were not related to predator-prey ratios, progesterone concentrations or calf-cow ratios. Overall, the effect of wolf presence on elk reproduction is better explained by changes in foraging patterns that carry nutritional costs than by changes in glucocorticoid concentrations.<<<Read the rest>>>

Tom Remington

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