More than 50 years ago, researchers found that chickens start to learn the sounds of their mothers’ calls even before hatching. The findings, published in Science in 1967, demonstrated that chicken embryos were somehow listening to the outside world, interpreting and storing that information.
And it’s not just chicken embryos. A wide range of animal embryos pick up on external sounds and vibrations while inside eggs or the womb, according to a new review paper.
“What was really surprising is how common [embryos] using sound is,” says Mylene Mariette, lead study author and behavioral ecologist at Deakin University in Geelong, Australia. “Embryos are not as isolated from the external world as we tend to see them.”
The new research, published in the journal Trends in Ecology and Evolution, looks at studies from across the animal kingdom, from insects to amphibians to mammals, that seem to pick up on sounds or vibrations “prenatally” (before birth or hatching) and respond in some way. One common response to sound is to strategically coordinate, delay, or speed up egg hatching, says Mariette. “All animals that lay eggs seem to be doing that.”
For instance, stink bugs use the sounds of their siblings emerging from their eggs to coordinate simultaneous hatching and tree frogs hatch early in response to sounds from an approaching predator.
But the example that first got Mariette interested in the topic came from her own research on zebra finches. The small, social birds are adapted to life in their desert home of central Australia. To cool down in their hot environment, the finches pant like dogs, producing an audible “heat call”—the hotter it is, the more frequent the panting.