Development and regulation of heart rate in embryos and hatchlings of gulls (Larus schistisagus and Larus crassirostris) in relation to growth

We compared the developmental patterns of mean heart rate in Larus crassiro stris and L. schistisagus embryos and chicks with other avian species of di fferent hatchling developmental modes. We proposed that, since mean heart r ate is inversely related to fresh egg mass in all birds, larger species rea ched a higher fraction of their hatchling mean heart rate by the end of the early phase of incubation and that heart rate contributions to supplying t he increasing metabolic demands during mid and late incubation phases were less important than in smaller avian species. Mean heart rate was essential ly independent of age throughout the mid-incubation phase (33% of normalise d incubation until pipping), but increased with time during early (L. schis tisagus only investigated) and late-incubation phases in both species. The O-2 pulse of L. schistisagus embryos and chicks increased linearly with yol k-free body mass (log-log) after the early-phase of incubation until shortl y before pipping, but was independent of body mass in the periods before an d after. We conclude that a high heart rate in this first period is probabl y more important for increasing the circulation of nutrients to the embryo at a stage when extra-embryonic circulation to the yolk sac is limited by t he size of the growing area vaculosa. Furthermore, large increases in mean heart rate during the late-incubation phase are probably important for incr easing the cardiac output to hatching levels with onset of endothermy. Howe ver, mean heart rate is stable over the mid-incubation while O-2 pulse incr eases, suggesting that increases in stroke volume and other circulatory adj ustments may be entirely responsible for the largest increases in O-2 trans port during incubation of large avian species.