Frédéric Dulude-de Broin, Sandra Hamel, Gabriela F. Mastromonaco, and Steeve D. Côté

Picture yourself living in a cabin in the woods. You know there are bears around, but you rarely encounter any near your house. Then, imagine unexpectedly finding multiple signs of bears roaming on your property, every week, for a full month: broken door, fresh scats on the ground, intrusion in the shed, ripped garbage bags. Feeling at risk, you could move away to find a more secure living environment, or you could stay and might then experience stress: a suite of physiological changes that help you quickly react in the event of unexpected threatening encounters.

Likewise, predation risk can vary in the wild, and prey populations exposed to higher risk may react by moving to safer habitats or stay in the threatening environment, which sometimes leads to stress. While useful to avoid being killed, both strategies entail costs. Safer habitats might for instance offer less food resources, while stress, when sustained or chronic, may severely impair health. In the long run, these costs can alter reproduction and long-term survival of prey species. The reason why some species experience costs through reduced access to food or through stress is unclear, but it was recently suggested that stress would evolve preferentially in species that are unable to reduce their exposure to risk (e.g. by moving away).

We tested the hypothesis that costs of predation risk are mediated by stress in the rocky mountain goat, a species for which moving away is likely not possible because it is confined to mountain tops. We measured stress hormones (glucocorticoids) in faeces and hair sampled from a wild population for 23 years and assessed the relationships between predation risk, stress and reproduction. High predation risk directly increased stress, which in turn reduced the proportion of reproductive females, providing robust evidence that stress mediates the costs of predation risk in this species. Our study reinforces the idea that predation stress is likely to evolve in prey that cannot move to safer habitats and reduce their exposure to predators. This could have dramatic demographic consequences for small isolated populations of long-lived ungulates.

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