FREQUENCY FLYERS: Vampire bats (Desmodus rotundus), pictured here, use laryngeal echolocation to find food. Biologists and geneticists alike are using new technology to peer into this ability. Image: BROCK FENTON
A handful of animals rely on specially tuned sound—and hearing—to detect objects around them. Bats and whales have some of the best biosonar, and some birds and shrews can also "see" sonically.
Dolphins (technically, a cetacean, or type of whale) and some bats produce particular pitches that they use to find prey in the space beyond their field of vision. They process the sound as it is deflected—or echoed—back to them off a moving morsel or other object, thereby gaining a sonic sense of their physical and food landscape. Less precise forms of echolocation used by other so-called tongue-clicking bats as well as echolocating birds employ the ability to navigate dark surroundings rather than precision-hunt meals.
But human researchers are still in the dark about many of the precise biological and evolutionary mechanisms that make this skill possible. Two new studies, however, have taken advantage of improving technology to help home in on what makes echolocation tick (or click or squeak).
Boning up on bat echolocation
One group of researchers at the University of Western Ontario in Canada is using microcomputed tomography (micro-CT) scanning to create precise images of the inner ear of echolocating and non-echolocating bats in hopes of understanding some of the differences among them.
By examining the detailed 3-D images of individual bats representing 26 species, the researchers discovered that those that used the common form of echolocation that relies on sound produced by the larynx had a unique feature in their skeletal structures. In their case, a common mammalian bone called the stylohyal bone physically connects the larynx to the ear (tympanic) bone, the researchers reported in a study published online January 24 in Nature. (Scientific American is part of Nature Publishing Group.)
The findings were "unexpected," says Brock Fenton, a biology professor at Western Ontario, and senior study author. The only other work on this area of bat anatomy was done in the 1930s and '40s, he says, and at that time the researchers depended on careful dissection and were not yet aware of echolocation.
The micro-CT scanner is similar to larger CTs commonly used in hospitals but provides "many times the resolution," says David Holdsworth, an imaging scientist at Robarts Research Institute and co-author of a report about the work. Although this type of imaging was originally designed for medical research, he says, it worked quite well for this basic biological investigation. To study these tiny physiological details in the past has often required physical dissection—a process many researchers and museum collection managers are wary of because it can damage or destroy the specimens. With the noninvasive technology, the researchers were able to study dozens of bat specimens on loan from the Royal Ontario Museum in Toronto.
Although exactly why the stylohyal bone connects only in these echolocating bats remains unknown, the researchers suggested that it might help both in perceiving the outgoing signal and in dampening the vibrations to prevent the bat from deafening itself with the sound it produces, which can be more than 100 times louder than the reflected echoes. By having a physical connection, Fenton says, it would allow the bats to have a "completely crisp and close-to-the-source representation of the original signal," which is crucial for comparison with incoming signals.
The new data also "reopen basic questions about the timing and the origin of flight and echolocation in the early evolution of bats," the researchers wrote in the published study. "This means if you find a good fossil, you can actually say, 'well, the fossil has this, therefore it could echolocate,'" Fenton says. The oldest known fossilized bat, Onychonycteris finneyi, had been presumed incapable of echolocation, suggesting that flight developed before biosonar did. Like the laryngeally echolocating bats of today, however, its stylohyal bones appear to have connected to its tympanic bones (although the main fossil is flattened—or as Fenton calls it, "a pancake fossil"—making it difficult to confirm the connection), hinting that echolocation and flight might have evolved at the same time, after all. Developing such a hunting strategy would have meant early bats were "exploding a new niche," because they would have been able to hunt bugs at night that birds and other creatures could not track, Fenton says.