Development of endothermic metabolic response in embryos and hatchlings of the emu (Dromaius novaehollandiae)

During hatching, there is a maturation of the mechanisms controlling the respiratory physiology involved in endotherm in precocial avian species. Here we examined the timing of the development of an endothermic response of oxygen uptake (M˙O2) to an alteration of ambient temperature (Ta) in a model precocial species, the preterm and hatching emu (Dromaius novaehollandiae  ). Late stage pre-pipped and pipped embryos and hatchlings were measured for responses of M˙O2 and shell or skin temperature (Ts) to altered Ta (ΔTa). M˙O2 remained unchanged in pre-pipped and internally pipped (IP) embryos at the end of 1.5 h exposure to ΔTa of ±10 °C. Externally pipped (EP) embryos responded to a cooling and a warming exposure with marked increase and decrease in M˙O2, as hatchlings responded to ΔTa with an endothermic change in M˙O2. The demonstration of the endothermic inverse metabolic response first appearing in EP embryos suggests that pre-EP embryos may also possess the ability to produce the endothermic inverse metabolic response, but they are restricted by the eggshell gas conductance. Late pre-pipped and IP embryos were measured again for responses of M˙O2 to ΔTa in air and then in a 40% O2 environment. The metabolic response of pre-pipped embryos at 90% of incubation was partially altered by switching from air to hyperoxia. IP embryos responded to ΔTa in 40% O2 with apparent inverse changes in M˙O2. The late stage emu embryo possesses the ability to produce an endothermic metabolic response at an earlier stage of development than in chickens, but this response is limited by the eggshell gas conductance.