Do T3 levels in incubating eiders reflect the cost of incubation among clutch sizes?

Complete development of avian eggs requires external heat, inducing in most species an energetic cost of incubation for the parents. Triiodothyronine (T(3)) has been implicated in the control of the metabolic rate and is decreased during fasting in most bird species. This raises the question of the regulation of T(3) during reproduction when incubation (thus heat production) is associated with fasting (and energy sparing). In this study, plasma concentrations of T(3) were studied for different clutch sizes in incubating, as well as in nonincubating, fasting female eiders. Our results show that the T(3) levels decrease during fasting in nonincubating birds, whereas they were maintained during the incubation fast. T(3) levels increased in female eiders at hatching. The plasma T(3) level did not vary among natural clutch sizes in eiders but did so when manipulated. T(3) levels increased when eggs were added (to a maximum of six eggs, i.e., the biggest natural clutch size) or removed (to two eggs, i.e., the smallest natural clutch size). Our results suggest that (1) high T(3) levels during incubation may participate to a threshold of heat production and incubation metabolic rate in eiders despite the fact that they are fasting; (2) since T(3) is associated with the energy expenditure in birds, incubating an enlarged or reduced clutch size may lead to a higher energetic cost of incubation in eiders; and (3) the energy demand of the ducklings at hatching is probably important, as the female T(3) concentrations are then at their highest levels. Thus, any modification of the natural clutch size leads to a rise in the T(3) level of the incubating female, suggesting an additional cost of incubation. Knowing that there is no variation of T(3) levels among natural clutch sizes, this study suggests that a female eider produces a number of eggs corresponding to the energy she can invest in incubation.