Bats are perhaps best known for their sophisticated use of sound: Like a ship’s sonar, the flying mammals make high-pitched noises and listen for returning echoes to navigate and hunt, an ability known as echolocation. But one family—the fruit bats—doesn’t use this sort of advanced tracking. Now, a new study suggests that all bats were once able to echolocate in this fashion, providing new evidence in a decades-long debate and shedding light on the origins of bat sonar.
Evolutionary biologists have long been divided over how bats developed their sonar. Fruit bats are closely related to a group of bats that are expert echolocators. Some say this means that advanced echolocation evolved once; an ancient bat developed the ability and passed it on to successive bat species, but fruit bats lost it along the way. Others argue that advanced echolocation evolved twice—once in an ancient ancestor bat, and again in the close relatives of fruit bats—and that fruit bats never had it.
Scientists have tried settling the question by looking for hard-to-find fossils of ancient bats, and by examining the genes of modern bats for clues about their past lifestyle. But Emma Teeling, an evolutionary biologist at University College Dublin, and colleagues from Shenyang Agricultural University in China, looked at a different window into the past: modern bat ears. Sonar-wielding bats have extra-large cochleae, coiled ear bones that they use to pick up tiny differences in the pitch of returning echoes. The cochleae of adult fruit bats, on the other hand, are much smaller—more like other mammals’ that don’t echolocate. But the team suspected that they might still show traces of their echolocating ancestry.
Using x-ray microradiographs, the researchers examined the developing fetuses of seven species of bat: two fruit bats, including the short-nosed fruit bat (Cynopterus sphinx), and five bats that use echolocation, including the great leaf-nosed bat (Hipposideros armiger). For comparison, they also looked at the cochleae of developing fetuses of five other mammals, including cats and rats.
The baby fruit bat’s cochleae were similar in size to those of echolocating bats, and they were about 65% larger than those of the other mammals, the team reports today in Nature Ecology & Evolution. That means the direct ancestors of fruit bats probably used echolocation—the large fetal cochleae are a sort of “living fossil” from an earlier time. And if it is true that fruit bats lost their ability to echolocate, then it’s likely that bat sonar evolved only once.
But not everyone is convinced. Rick Adams, an evolutionary ecologist at the University of Northern Colorado in Greeley, doesn’t agree with either side of the debate. He subscribes to a different version of the bat family tree, which has two main branches: one with all the bats that echolocate and one with all the bats that don’t. In his version, advanced echolocation also evolved only once, but only after the fruit bats split from the rest of the tree.
“As a preliminary study, it’s pretty interesting,” Adams says. But he adds that he would have chosen other mammals to compare with the fruit bats. Cats and rats are not that closely related to bats, Adams says; mammals like tree shrews or lemurs might be better, because it’s possible they could share fruit bats’ large cochleae.
But for many others, the discovery is a welcome one. “I really applaud the authors for taking us out of the wilderness,” says bat biologist Brock Fenton of Western University in London, Canada. “It was an endless argument.”