Fruit Bats Have Sonar Too (But It’s Not Very Good)

One in every five species of mammal is a bat. This incredibly successful group splits into two major camps. The so-called microbats include vampires, horseshoes and some 1,000 other species, all of which use sonar to navigate through the dark. They make high-pitched clicks and they use the rebounding echoes to map the world, just like a submarine.

The other group—the megabats or fruit bats—has fewer than 200 species. They tend to be bigger and, with one exception, they don’t use echolocation. They have neither the specialised body parts needed to produce the necessary clicks, nor the genetic signatures that are common to sonar users. Instead, they rely on their large eyes to see at night.

Or, at least, that’s what everyone thought.

Arjan Boonman from Tel Aviv University has put a spanner in this long-held idea, by showing that three species of fruit bats all use a form of echolocation. They have sonar. Okay, it’s crap and inefficient sonar, but sonar nonetheless. Weirder still, the bats produce it with their wings.

There were hints of this before. In the 1980s, Edwin Gould found that the cave nectar bat of southeast Asia makes clicking noises as it flies. Gould thought that it was slapping its wings together with every beat, but couldn’t work out whether the clicks had a purpose. Boonman wanted to find out more.

Together with Sara Bumrungsri and Yossi Yovel, he studied the cave nectar bat, as well as the lesser short-nosed fruit bat and the long-tongued fruit bat. He found that as the animals flew in a pitch-black tunnel, they all made audible clicks. The clicks aren’t accidents of flight. The team showed that the bats can adjust the rate of these sounds, and they click more furiously when flying in the dark than in dim light. Perhaps they actually use these noises to find their way around.

To test this idea, the team released the bats into a room containing a dozen inch-thick hanging cables. This kind of obstacle course is a classic of bat research, and echolocating species can easily weave their way around the cables. The fruit bats could not. Despite their clicks, they crashed often.

But the team didn’t give up. They trained a dozen cave nectar and short-nosed bats to discriminate between two big metre-wide boards: a harder one that’s great at reflecting echoes, and a cloth-covered one that absorbs more sound. Visually, the boards were similar; acoustically, worlds apart. And the bats could tell. They quickly learned to land on the right target and did so 7 times out of 10.

They weren’t exactly graceful about it, though. As the team wrote, “Despite the target being very large, our bats mostly required several attempts in order to land, often crashing into the target in an uncontrolled manner.” Unlike microbats, some of which can snatch spiders from their webs without getting entangled, these big species can barely gauge the distance of a whopping big board. Their sonar is rather unsophisticated, which explains why this behaviour has never been discovered before. People were trying to get the bats to do tasks that were well outside their limited abilities.

Boonman’s team also found that the fruit bats make their sonar clicks in a weird way. Microbats use their voice boxes, in the same way that you might speak or sing. The Egyptian fruit bat—until now, the only megabat known to use sonar—has a different technique: it clicks with its tongue. But Boonman’s three fruit bats shut their mouths when they fly. They click nonetheless, and sealing their mouths with tape does nothing to stop them.

Instead, they seem to use their wings. The clicks are perfectly synchronised with their wingbeats, and can be stopped by weighing one wing down with tape. The bats could be slapping their wings together as Gould suggested, or slapping their wings against some other body part, or even clicking bones within the wings as you or I might crack our knuckles.

“There’s obviously something unusual going on because it doesn’t seem to be linked to the wingbeat frequency,” says Gareth Jones from the University of Bristol. That is, the bats don’t seemto flap any differently in bright light or pitch blackness. “If it was a simple wing-slapping thing, there should be a 1:1 relationship between wingbeats and clicks, but there isn’t. Something odd’s going on there.”

“The discovery changes the discussion of how bats may have evolved sophisticated echolocation,” says Aaron Corcoran from Wake Forest University. “The results are too preliminary to answer that difficult question, but they will make bat biologists rethink the possibilities.”

Bat researchers are divided over how many times echolocation evolved. Some think that it evolved once in the common ancestor of all bats, and was then lost in the fruit bats. Others suggest that it evolved twice in different lineages of microbats, and fruit bats never had it. In both scenarios, the Egyptian fruit bat evolved its tongue-clicking technique independently. And the wing-clicks of the other fruit bats might represent yet another origin. “It’s a major discovery in echolocation research,” says Marc Holderied from the University of Bristol. “It’s a third independent evolution of echolocation, which is truly exciting.”

The wing-clicks might even give clues about how the superior sonar of other bats first evolved. You don’t need any special adaptations to use a crude form of echolocation. Even blind humans can do it with enough training. Some people have suggested that the bats initially used echolocation to avoid large obstacles like cave walls, before they honed the technique for finer navigation.

To be clear, fruit bat sonar isn’t a direct predecessor of microbat sonar. Think of it more as a historical painting—a reconstruction of a possible past. As Boonman writes, “We believe that fruit bats are behavioral fossils, presenting an ancient sensory behavior that (even if recently evolved) allows a rare glimpse at the evolution of a sensory system.”

Reference: Boonman, Bumrungsri & Yovel. 2014. Nonecholocating Fruit Bats Produce Biosonar Clicks with Their Wings. Current Biology http://dx.doi.org/10.1016/j.cub.2014.10.077

More on bats and echolocation:

• Echolocation in bats and whales based on same changes to same gene
• Bats Jam Each Other’s Sonar
• Ninja bat whispers to sneak up on moths
• Tiny wing hairs allow bats to pull off hair-raising manoeuvres
• How bats find water and why metal confuses them
• The brain on sonar – how blind people find their way around with echoes