Forewarned is forearmed: Parents' calls to eggs prepare babies for heat

In a fast-changing world where strategy is often key to survival, exciting new Deakin University research reveals birds begin coaching their offspring even before birth on how to grow in a warming climate.

In a paper published today in Science, the researchers from Deakin’s Centre for Integrative Ecology, within the School of Life and Environmental Sciences, reveal how a small Australian desert bird calls to its embryos during incubation to warn them about the heat they will face upon hatching.

Lead researcher Dr Mylene Mariette said hearing this heat warning call before hatching allowed baby birds to become more productive as adults, and also impacted on their preference for hot temperatures.

The researchers argued that not only could this previously unknown ecological function for embryonic hearing abilities prove critical for thriving in a warming climate, but it also shows how much impact the prenatal acoustic environment has on development.

The Deakin research team leader, and Australian Research Council Future Fellow, Professor Kate Buchanan, said embryos´ capacity to hear and even learn external sounds had been known since the 1960s, in humans and animals alike, but how much impact this had both on parents strategy and offspring success was not apparent until this new study.

During the study, the researchers placed small microphones in nests of incubating zebra finches, breeding in outdoor aviaries at Deakin, which revealed that, in over 600 hours of audio recording, parents only called to their eggs when ambient temperatures rose above 26ºC.

“Moreover, parents did not call to freshly laid eggs. Calling only occurred within five days of hatching, once embryos had presumably developed hearing capacities,” Dr Mariette said.

“This suggests that parents are deliberately communicating with their embryos about the heat.” 

The researchers then exposed the eggs to acoustic playbacks in artificial incubators, discovering the embryos were paying attention to their parents´ calls.

“Nestlings hatched from eggs exposed to this heat-warning call reacted differently to temperature after hatching, compared to control nestlings that heard another type of parental call,” Dr Mariette said.

“Prenatal acoustic exposure changed both how nestlings solicited food to their parents, and how much weight they gained throughout their development, in relation to temperature.

“Astonishingly, the calls birds heard as embryos impacted on these individuals until adulthood, up to two years later.

“We found that adjusting their growth rate to ambient temperature allowed experimental birds to themselves produce more young as adults.”

The researchers found acoustic experience also changed bird thermal preferences at adulthood, with the birds exposed as embryos to parental incubation calls preferring to breed in hotter nests than control birds.

“Such profound and long lasting effects of prenatal acoustic experience had never been demonstrated before,” Dr Mariette said.

“Our work highlights that acoustic environment may have a much stronger impact on development than we currently realise.

“This is also the first evidence that parents can adjust the development of their offspring to ambient temperature in warm-blooded species.”

Dr Mariette said there was still a lot to do to fully understand this phenomenon.

“Our priority is to establish the physiological mechanisms underlying these effects and how widespread this strategy is in birds,” she said.

“We are also very excited about the implications of this study for song learning in birds, as sound clearly impacts development from a much earlier stage than we have assumed so far,” Professor Buchanan said.

Watch this video from AAAS Science to see how the parents talk to their chicks before birth.