Cardiac rhythms in prenatal and perinatal emu embryos

Emu eggs weigh approximately 600 g and have an incubation duration (ID) of approximately 50 days. The egg mass is approximately 10-fold heavier than the chicken egg and the ID is approximately 2.5-fold longer. Daily changes in mean heart rate (MHR) of emu embryos were previously determined, but further measurement was needed to investigate the species-specific behavior of cardiac rhythm for comparison with other species. In the present study, we continuously measured the electrocardiogram of emu embryos while maintaining adequate gas exchange through the eggshell and determined instantaneous heart rate (IHR) during the last 2-7 days of incubation until hatching or death. The MHR over 1-min intervals was calculated from IHR data in order to present continuous developmental patterns of heart rate (HR) in a single graph and 24-h recordings of HR in a single panel, showing the HR trend over a prolonged period. However, neither circadian nor ultradian rhythms of HR were shown in these figures or by power spectrum analysis. The IHR distinctively fluctuated and the fluctuations were mainly comprised of three patterns of irregular HR accelerations in embryos that hatched. Respiratory sinus arrhythmia also occurred in perinatal embryos. During the final stages of the perinatal period, short-term, repeated, large accelerations of IHR appeared, which signaled imminent hatching and has been reported for chick embryos. IHR fluctuations in embryos that failed to hatch tended to become inactive towards death.