| Summary: | Understanding the causes and consequences of life-history variation is one of the fundamental goals in evolutionary biology. A fairly recent discovery is that even individuals of the same species may differ in their life-histories strategies. However, we still have only a rudimentary understanding of how state and the environment jointly shape such individual life-history strategies within species. Glucocorticoid stress hormones (GCs) may serve as key endocrine modulators integrating information about the internal and external environment. However, we are still lacking detailed knowledge about how GCs may be modulating individually variable life-histories. In this thesis I set out to explore proximate and ultimate causes of intraspecific variation in life-history strategies, with a particular emphasis on GCs as potential drivers of life-history trade-offs. I have conducted this project on long-lived eider (Somateria mollissima) females and their ducklings. Female eiders invest heavily into reproduction which is linked to decreased investment into self-maintenance and survival. Moreover, eiders show repeatable individual differences in stress responsiveness and behavioural strategies, thus serving as a suitable system for studying life-histories in the wild. The results showed that eiders trade-off self-maintenance for reproduction, but that the magnitude of this trade-off depends on the state (e.g., telomere length, age, body condition) of the individual. The state of the individual is also important in making reproductive decisions in the face of predation risk. In light of my findings, GCs may serve as the link between reproductive investment decisions and reproductive success in the face of danger in eider females. Rather than having uniformly negative effects on reproductive decisions and reproductive success in female eiders, the effects of GCs are context-dependent. The effects of elevated GCs are advantageous in areas of high predation risk by allowing to better prepare for predatory attacks or to shorten ...
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