Zebra finches sing to their eggs, but only when it's hot. The offspring serenaded in this way grow up smaller and better suited to a hot environment, and have more young ones of their own. Despite a host of unanswered questions, the finding represents a significant step forward in understanding embryonic development and the way desert birds relate to their environment.
Dr Mylene Mariette of Deakin University, Australia, was studying communication between zebra finch partners when she noticed something odd. “I was seeing how they sing to each other to coordinate parental care when I noticed that sometimes when a bird was alone in the nest it would make totally different calls,” she told IFLScience. “I wondered if they were making them for the egg.”
Mariette was familiar with research on fairywrens, where adults call to their eggs prior to hatching, but this was to help parents distinguish their own offspring from cuckoos. Since zebra finches are not subject to cuckoos invading their nests, another explanation was needed. Curious, Mariette placed a microphone in nests and soon noticed the calling only occurred in hot weather. Moreover, it was restricted to the five days before hatching, demonstrating it was not an automatic response to heated conditions.
In Science, Mariette has reported on controlled experiments she ran. She incubated eggs, raised nestlings at very different temperatures, and played recordings of the parental calls to some eggs and not others.
Eggs that were exposed to the recordings hatched birds that showed different food-begging behavior and grew more slowly in hot conditions. Undersized, high-temperature females stayed smaller than their peers for the two years Mariette tracked them, while males raised in this way caught up after a year.
Those birds exposed to the calls while in the egg and raised in hot conditions went on to produce more offspring, demonstrating an evolutionary advantage to the behavior. Most animals evolve to be smaller in hot environments, since a greater surface-to-volume ratio makes heat loss easier.
How the songs induce reduced growth remains unclear. “We can only speculate on the mechanism,” Mariette told IFLScience. “But there are studies that suggest music and noise can affect hormones, and this may influence the way the embryos thermoregulate.”
Birds who had been exposed to calls in the egg showed different begging behavior while young. Andy TD Bennett
Zebra finches (Taeniopygia guttata) are desert dwellers. They breed when big rains have assured a good food supply for future months. This can happen in any season, so young can be born into a wide range of temperatures, unlike temperate species that reliably breed in spring.
So far, temperature-dependent singing has not been observed in any other species, and Mariette has no idea how widespread the phenomenon is. Mariette said it is “quite far to stretch” her work to use it to support playing music to unborn children, as some advocate, since “human fetuses are in a very sound-rich environment. You would have to play them a lot of music to get any difference at all,” she said. Sadly, singing to your kids probably won't prepare them for global warming.