Heart rate in developing ostrich embryos

1. A non-invasive condenser microphone was used to detect cardiogenic, acoustic pressure changes (acoustocardiogram, ACG) over the eggshell in order to determine embryonic heart rate (HR) of ostriches in a commercial hatchery. 2. HR measured for 36 eggs at 36·3°C was maintained at about 185 bpm during the middle stage of development (days 19 to 23) and then decreased with embryonic development. 3. The daily changes in HR were not related to egg mass, but HR of high water vapour conductance (GspH₂O) eggs was found to decrease less during the last stages of incubation relative to low and medium GspH₂O groups. 4. The averaged HR at 80% of incubation period was close to the value predicted from the allometric equation determined previously for embryos of domesticated precocial birds.