| Summary: | International audience Abstract: Available resources being limited, life-history theory predicts that natural selection favours the evolution of physiological mechanisms that ensure their optimal allocation between competing activities. Accordingly, to maximize their selective value, long-lived species face a trade-off between survival and reproduction. Immunity is hypothesized to share limited resources with other physiological functions and this may partly account for the fitness costs of reproduction. However, both ultimate and proximate factors underlying the observed trade-off between reproductive effort and immunocompetence remain poorly documented. Using female common eiders (Somateria mollissima) as a model, it was earlier shown that acquired immunity is negatively affected during the incubation fast, while its activation has a negative impact on females' fitness. The current paper reports data about hormone (corticosterone and triiodothyronine) manipulations we designed to shed more light onto both ultimate and proximate mechanisms contributing to explain immunosuppression in breeding female eiders. Corticosterone does not seem to be the main proximate factor for immunosuppression. Interestingly, the immunosuppressive effects of the tested hormones seem to be mediated by their negative effects on body mass. This lends support to the proposed link between the immune system and body fat reserves and is also in agreement with the resource-limitation hypothesis. It appears that the immnunopathology-avoidance hypothesis and the resource-limitation hypothesis are not mutually exclusive in breeding eiders. Further experiments are required to determine the relationship between body condition and immunocompetence during the incubation fast in long-lived birds.
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