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 1-20g), semi-precocial seabirds (egg mass approximately 38-288g) and precocial birds (egg mass approximately 6-1400g). 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).