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

Previously, we have measured daily changes (developmental patterns) in embr yonic heart rate (fH) in altricial and semi-altricial (ASA) birds (range of mean fresh egg mass approximately 1-20 g), semi-precocial seabirds (egg ma ss approximately 38-288 g) and precocial birds (egg mass approximately 6-14 00 g), An allometric relationship between embryonic fH at 80 % of incubatio n 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, addition al measurements of embryonic fH in three ASA species, the barn owl Tyto alb a, the cattle egret Bubulcus ibis and the lanner falcon Falco biarmicus, we re made to extend the egg mass range (20-41 g), and the allometric relation ships of embryonic fH for these ASA birds and the precocial and semi-precoc ial (PSP) groups were investigated from published data. The developmental p atterns of embryonic fn in three relatively large ASA species did not show a significant increase prior to the pipping period, unlike those in small A SA birds, but tended to be constant, with a subsequent increase during pipp ing. The allometric relationship derived for ASA birds was fH at 80 % ID=37 1M(-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 e gg mass: fH(max)=440M(-0.127) (r=-0.840, P<0.001, N=20). The allometric rel ationships for fH at 80 % ID in PSP and fH(max) in ASA embryos were statist ically identical. Accordingly, embryonic fH at 80 % ID in PSP birds and fHm ax during pipping in ASA birds can be expressed by a single allometric equa tion: fH=437M(-0.123) (r=-0.948, P<0.001, N=33).