Hormones and immunoglobulin levels in king penguins during moulting and breeding fasts

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.