Prenatal sound perception is more widespread than previously thought, new research suggests
Over a decade ago, behavioral ecologist Diane Colombelli-Négrel was wiring superb fairy wrens’ nests to record the birds’ sounds when she noticed something odd. Mother fairy wrens sang while incubating their eggs, even though it would have made more sense to keep quiet to avoid attracting predators.
The discovery “was a bit of an accident,” says Colombelli-Négrel, of Flinders University in Adelaide, Australia. And it made her wonder: Could the baby birds be learning sounds, or perhaps even songs, even before they hatch?
Scientists have long wondered how early in development individuals learn to perceive distinct sounds. It’s known that human fetuses learn to recognize their mother’s voice. For birds such as superb fairy wrens (Malurus cyaneus) that perfect their songs with parental tutoring, it was thought that sound perception began well after hatching. But when it became obvious that mother birds were intentionally singing to their eggs, “we knew we were on to something,” says avian ecologist Sonia Kleindorfer of the University of Vienna.
Previous research by Colombelli-Négrel, Kleindorfer and colleagues showed that unhatched superb fairy wrens learn a vocal “password” from mom that helps mothers discriminate their own nestlings from those of pesky cuckoo invaders. What’s more, unhatched superb fairy wrens appear to distinguish between songs of their own species and others, the team reported in 2014.
That ability extends beyond superb fairy wrens, new research suggests. At least four additional types of birds recognize sounds specific to their species while still in their eggs, the researchers report in the Oct. 25 Philosophical Transactions of the Royal Society B.
The finding is a surprise to many birdsong scientists, says vocal learning neuroscientist Wan-chun Liu of Colgate University in Hamilton, N.Y., who wasn’t involved in the new research. “We used to think a lot of the learning happened after hatching, but now there seems to be more and more evidence suggesting, even in the embryonic stage … they are listening,” he says.
In birds and humans, a drop in embryonic heart rate is known to indicate attention to a stimulus. Colombelli-Négrel and colleagues’ earlier studies of unhatched fairy wrens showed a slowed heart rate in response to repeated sounds of their own species, but not others.
To investigate whether this phenomenon is widespread among birds, the team turned their attention to the embryonic heartbeats of captive Japanese quail (Coturnix japonica domestica), plus three more wild species: little penguins (Eudyptula minor), red-winged fairy wrens (Malurus elegans) and Darwin’s small ground finches (Geospiza fuliginosa).
The team temporarily removed 109 eggs from nests and measured the heart rates of unhatched chicks before, during and after exposure to playbacks of songs from their own species or others. And the researchers investigated whether 138 individual embryos became habituated to repeated sounds of unfamiliar individuals singing their species’ own songs, which would imply learning had occurred.
“We expected to find learning evidence in the songbirds but not in the quail and penguins,” Colombelli-Négrel says. That’s because penguins and quail are “vocal nonlearners” — birds thought to have calls that are genetically programmed from birth and not learned from a tutor.
To the researchers’ surprise, all of the embryos showed not only a slowed heart rate in response to repeated sounds of their own species, but also habituation. That finding suggests that these birds learn to perceive the sounds of their species-specific songs embryonically.
The scientists don’t know why the penguins and quail, which have their own calls genetically baked in, have the ability to distinguish their own species’ calls from those of other birds right from birth. Perhaps it’s useful for survival, the researchers speculate.
“Birds are like humans in that there is mother- or father-offspring communication even before birth,” says coauthor Mark Hauber, a neurobiologist at the University of Illinois at Urbana-Champaign. The team hopes to study prenatal sound perception in even more bird species to probe the advantages of this early egg-u-cation.