Abstract
Reproduction and moult are amongst the most costly seasonal events encountered by birds and it has been suggested that trade-offs exist between these events and immune function. The underlying physiological mechanisms of such potential trade-offs are still unclear, but they could be mediated by changes in hormone concentrations and/or fuel utilization. In the present study, our primary aim was to assess variations in immunoglobulin levels (maintenance of the humoral immune function) of free-living king penguins (Aptenodytes patagonicus) during their moult and breeding fasts in relation to the associated endocrine and metabolic states. A second objective was to determine the effect of critical energy depletion on immune function in fasting captive penguins. Throughout moult, there was a progressive increase in plasma immunoglobulins (45%), accompanied by a 120% increase in plasma triiodothyronine and the maintenance of high thyroxin levels. Breeding fast was characterized by immunoglobulin levels 30% lower in courting than incubating males, which coincided with a peak in plasma testosterone during courting. Finally, intense fat and protein depletion under controlled conditions resulted in a 40% decrease of immunoglobulin levels, while plasma concentrations of corticosterone were high. Variations in plasma immunoglobulin levels are further discussed in relation to different hormonal and metabolic states.
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