Context-dependent stress responses and their connections to fitness in a landscape of fear

The acute glucocorticoid stress response is presumed to facilitate escape from life-threatening situations such as predation and thus it is assumed to be linked to fitness. However, the fitness effects of glucocorticoid reactivity remain controversial, as these effects may be context-dependent. Individuals differing in their emphasis on current versus future reproduction may differ in their risk-taking under threat of predation; this variation in risk-taking may be mediated by variations in stress reactivity. We set out to test whether predation risk (island- and year-specific proportion of depredated nests) modified the relationships between stress responsiveness and current reproductive investment (clutch weight) and between stress responsiveness and reproductive success (viable proportion of the clutch) in the long-lived female eider Somateria mollissima. This study system shows large spatial and annual fluctuations in predation risk, indexed by the annual island-specific proportion of depredated nests. The capture stress-related corticosterone output was attenuated with increasing clutch weight under low predation pressure but elevated under severe predation pressure, and females in well-concealed nests had lower stress responsiveness. The viable proportion of the clutch decreased with increasing corticosterone reactivity under low to moderate predation pressure, but slightly increased under severe predation pressure. The acute stress response may thus mediate adaptive plasticity; dampened stress reactivity may ensure successful reproduction under low predation threat or in nest sites reducing detection by visual predators, whereas preparing for potential attacks may be favoured under elevated predation risk.