Development of cardiac rhythms in birds.

Heart rate (HR) in avian embryos developing inside an eggshell has been measured by various means while maintaining adequate gas exchange through the eggshell. This is an important requirement in order to avoid adverse effects of impeding gas exchange on the cardiac rhythms of developing embryos. The present report is a review of our ontogenetic study on embryonic HR, which was measured with fulfillment of the above requirement and also hatchling HR measured non-invasively. Firstly, we reviewed measurements of daily changes (developmental patterns) in embryonic mean heart rate (MHR), which were determined from a short-term measurement of HR once a day, in 34 species of altricial and precocial birds. The allometric relationship between the MHR during pipping in altricial birds and their fresh egg masses was the same as that between the MHR at 80% of incubation duration and fresh egg masses in pre-cocial birds. Secondly, we presented the developmental patterns of MHR in chick embryos and hatchlings, which were determined from long-term, continuous measurement of HR before, during and after hatching. The ultradian and circadian rhythms of HR were clearly shown in embryos and hatchlings, respectively. Thirdly, we summarized instantaneous HR fluctuations: HR variability and HR irregularities, in chick embryos and hatchlings. The distinctive patterns were shown in pre-pipped and pipped embryos and newly hatched chicks, individually, which were partly related to autonomic nervous functions and physiological functions.