Development of cardiac rhythms in altricial avian embryos

Mean heart rate (MHR) was determined during incubation and in hatchlings of 14 altricial avian species to investigate (1) if there is a common develop mental pattern of heart rate in altricial embryos and (2) if heart rate cha nges during incubation are correlated with changes in embryonic growth rate . On the basis of normalized incubation MHR increased approximately linearl y in 12 of 14 species from as early as 30-40% of incubation to that of pipp ed embryos. The MHR of hatchlings was equal to or higher than that of pippe d embryos in seven species. Passerine embryos and hatchlings maintained hig her MHR in comparison to parrots of similar egg mass, which may reflect phy logenetic differences in development. Embryonic MHR increased at a higher r ate while embryonic growth rates were highest during the first 40% of incub ation in tit, budgerigar and crow embryos than during subsequent developmen t when relative growth rates decreased. MHR became independent of yolk-free wet mass at a smaller fraction of hatchling mass in budgerigar and crow th an in the tit, suggesting that MHR is more likely to increase continuously after 40% of incubation in small altricial species than larger species. (C) 1999 Elsevier Science Inc. All rights reserved.