An Incubation Timer for Seabirds: Progesterone and Its Relationship to Hatching in Adélie Penguins

Abstract

The survival of newly hatched chicks of Adélie Penguins Pygoscelis adeliae, and those of other species of seabirds with long incubation spells, is dependent upon the foraging parent returning to the nest about the time of hatching. We found that both male and female AdClie Penguins curtail the duration of their foraging trips if hatching is imminent, suggesting that they possess an interval timer capable of measuring the incubation period. Plasma levels of testosterone and oestradiol dropped markedly after laying in males and females, respectively, and could potentially set the timer relative to the incubation period. Progesterone levels were high in the five days before hatching in both males and females that had just returned from foraging. We suggest that a high concentration of progesterone, if not itself precipitating the return of foraging penguins, may at least be associated with whatever triggers their return. The mechanism whereby progesterone secretion increases near the end of the incubation period is unknown, but we hypothesise that this sort of hormonally based interval timer is likely to be relevant to other seabirds where parents must complete long foraging trips before their chicks hatch.