Does parental heart rate affect embryonic heart rate during incubation? An experiment in Common Terns Sterna hirundo

Embryonic heart rate is a strong determinant of metabolic rate and the rate of embryonic development in oviparous species. In humans, embryonic heart rate is positively correlated with the heart rate of the mother. However, human embryos do not develop autonomously from the maternal circulatory system, making it impossible to separate maternal control from an independent embryonic adjustment of its heart rate to that of its mother. In birds, embryonic development does occur autonomously from the mother, while the embryo can auditively perceive the heart rate of the incubating parent. In this study, we used Common Terns ( Sterna hirundo ) to experimentally test whether the embryonic heart rate is affected by parental heart rate during incubation. We artificially incubated freshly laid eggs under standardized conditions and continuously exposed them to the sound of either a slow or fast parental heart rate throughout the first 18 days of embryonic development. At day 18 of embryonic development (a few days before hatching) there was no significant difference in the embryonic heart rate between both treatments. There was also no effect of treatment on either total duration of embryonic development or hatching success. We suggest that embryonic heart rate and development rate may not be affected by the heart rate of the parent because natural parental heart rates are very sensitive to external conditions that may fluctuate drastically, while embryos may need a relatively stable heart rate for optimal development.