Thermal Responses of Late Embryos and Hatchlings of the Sooty Tern

Abstract

Oxygen consumption (VO₂) and body temperature (T_b) of young Sooty Terns (Sterna fuscata) were measured during late incubation and in the first 24 hr after hatching at ambient temperatures (T_a) between 28° and 38°C and between 15° and 43°C, respectively. Evaporative cooling by hatchlings at T_a of 36°-43°C was also measured. Significant increases in embryonic metabolic level occurred between external and internal pipping and between internal pipping and establishment of a pip hole. However, despite the improving access to oxygen produced by these events, VO₂ and T_b both varied directly with T_a in an ectothermic pattern throughout the final stages of incubation. A capacity for sustained endothermy only became apparent in hatchlings. Their VO₂ varied inversely with T_a between 27.5° and 36°C to an extent indicating a modest capacity for regulatory thermogenesis. This served to maintain T_b above 35°C in this range of T_a. The apparent abruptness of the appearance of this regulatory capacity after hatching suggests that emergence from the physical confinement of the egg could be an important proximate factor in the establishment of endothermy in this semi-precocial species. Hatchling Sooty Terns underwent a progressive fall in T_b with declining T_a below 27.5°C. On the other hand, they appeared quite proficient at evaporative cooling with increasing T_a between 36° and 43°C.