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 (MO2) to an alteration of ambient temperature (T(a)) 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 MO2 and shell or skin temperature (T(s)) to altered T(a) (DeltaT(a)). MO2 remained unchanged in pre-pipped and internally pipped (IP) embryos at the end of 1.5h exposure to DeltaT(a) of +/-10 degrees C. Externally pipped (EP) embryos responded to a cooling and a warming exposure with marked increase and decrease in MO2, as hatchlings responded to DeltaT(a) with an endothermic change in MO2. 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 [Formula: see text] to DeltaT(a) in air and then in a 40% O(2) 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 DeltaT(a) in 40% O(2) with apparent inverse changes in MO2. 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.

Allometric relationships between embryonic heart rate and fresh egg mass in birds.

Previously, we have measured daily changes (developmental patterns) in embryonic heart rate (fh) in altricial and semi-altricial (ASA) birds (range of mean fresh egg mass approximately 1-20 g), semi-precocial seabirds (egg mass approximately 38-288 g) and precocial birds (egg mass approximately 6-1400 g). An allometric relationship between embryonic fh at 80 % of incubation duration (ID) and fresh egg mass (M) has been derived for six species of precocial bird (fh at 80 % ID=429M(-0.118)). In the present study, additional measurements of embryonic fh in three ASA species, the barn owl Tyto alba, the cattle egret Bubulcus ibis and the lanner falcon Falco biarmicus, were made to extend the egg mass range (20-41 g), and the allometric relationships of embryonic fh for these ASA birds and the precocial and semi-precocial (PSP) groups were investigated from published data. The developmental patterns of embryonic fh in three relatively large ASA species did not show a significant increase prior to the pipping period, unlike those in small ASA birds, but tended to be constant, with a subsequent increase during pipping. The allometric relationship derived for ASA birds was fh at 80 % ID=371M(-0.121) (r=-0.846, P<0.001, N=20) and that for PSP birds was fh at 80 % ID=433M(-0.121) (r=-0.963, P<0.001, N=13). The slopes were parallel, but fh of ASA embryos was low compared with that of PSP embryos with the same egg mass. In ASA birds, embyronic fh was maximal during the pipping (perinatal) period, and the maximum fh (fh(max)) was significantly related to fresh egg mass: fh(max)=440M(-0.127) (r=-0.840, P<0.001, N=20). The allometric relationships for fh at 80 % ID in PSP and fh(max) in ASA embryos were statistically identical. Accordingly, embryonic fh at 80 % ID in PSP birds and fh(max) during pipping in ASA birds can be expressed by a single allometric equation: fh=437M(-0.123) (r=-0.948, P<0.001, N=33).