Developmental patterns of heart rate in altricial avian embryos and hatchlings

The aims of this study were to determine the patterns of development of heart rate (fH) in altricial avian embryos and hatchlings, and then to examine how fH is regulated to meet metabolic requirements in altricial embryos. Embryonic mean heart rate (fH̄) in 12 altricial species (Passeriformes and Psittaciformes) increased during pre-pipping incubation in all species except the cockatiel (Nymphicus hollandicus), in which fH̄ tended to decrease prior to pipping. The rate of increase in fH̄ tripled during the pipping phase in all species, and fH̄ was significantly higher during the pipping period and in hatchlings than during pre-pipping development. The O₂ pulse (O₂ consumed per cardiac beat) of altricial embryos increased in direct proportion to embryo mass (log(e)/log(e) base), although fH̄ was often low prior to pipping, implying that stroke volume increases in the second half of incubation. We conclude that fH̄ contributes more than other factors towards supplying the metabolic demands of the embryo during the middle of incubation and the final pipping phase, but less during the intervening period of late incubation.