Plasma thyroid hormone pattern in king penguin chicks: a semi-altricial bird with an extended posthatching developmental period

Abstract

Plasma concentrations of thyroid hormones (TH) were investigated during the extended posthatching developmental period (∼11 months) of a semi-altricial bird species, the king penguin ( Aptenodytes patagonicus). The first period of growth in summer was marked by a progressive rise in plasma T 4 concentration that paralleled rapid increases in body mass and in structural and down growth. By contrast, plasma T 3 concentration had already reached adult levels in newly hatched chicks and did not change thereafter. Circulating TH of king penguin chicks thus follow an original pattern when comparing to altricial and precocial species. During the austral winter, the long period of undernutrition of king penguin chicks was characterized by a decrease in circulating TH that can be related to a seasonal stop in growth and energy saving mechanisms. Plasma TH concentrations increased again during the second growth phase in spring, and they reached their highest levels at the end of the fledging period, slightly before juveniles initiated their first foraging trip at sea. As expected, plasma T 4 levels were elevated when chicks moulted, developing a true-adult type waterproof plumage. The data also suggest that T 4 plays a major role in skeletal development and pectoral muscle maturation in anticipation of marine life. Plasma T 3 was at its highest during the period when juveniles improved resistance to cold waters by going back and forth to the sea, suggesting a role for circulating T 3 in cold acclimatization occurring at that time.