Development of heart rate responses to acoustic stimuli in Muscovy duck embryos.

Heart rate (HR) of Muscovy duck embryos (Cairina moschata f. domestica) was continuously recorded from the 21st day of incubation (E21) until hatching (E35). During that period, embryos were exposed to different acoustic stimuli (species-specific maternal and duckling calls, music, rectangular and sine waves, white noise). Sudden HR changes occurred at the onset of acoustic stimulation (on-response), as well as spontaneously. From E27 onwards, the response rate was significantly higher than the rate of spontaneous HR changes. The on-response rate increased further until E30. Most responses were elicited by maternal calls and music, but rarely by duckling calls. On-responses could be classified into: HR increase (36.4%), HR decrease (37.9%) and an increase in instantaneous HR variability (23.2%). The increase in HR variability occurred only in response to sounds, but not spontaneously. HR increases were mainly observed when the baseline HR was lower than the long-term HR trend. On-response duration was no longer than 3 min in 90% of all observations. The hourly mean HR and standard deviation did not change, even during phonoperiods composed of several sound patterns and lasting several hours. We conclude that Muscovy duck embryos are able to perceive exogenous acoustic stimuli, and that the acousto-sensory-->cardiac axis is functional from E27.