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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.
In the air or at sea, echolocation is in the genes
Other research groups are investigating echolocation's genetic roots. They have zeroed in on a mammalian gene called Prestin, which makes a protein that helps to amplify sounds via the outer hair cells in the cochlea (inner ear). In two studies published online January 25 in Current Biology researchers report that the genetic modifications in echolocating bats' Prestin and that of echolocating toothed whales are remarkably similar—much more so than that of their more closely related (non-echolocating) kin.
By using genetic sequences for the gene that were available in public databases, one team (based in part at the University of Michigan at Ann Arbor) resolved evolutionary trees for whales, bats and their close relations based only on that gene's modifications. Out of 25 mammal species, the toothed-whale (in this case, a bottle-nosed dolphin) was the only one to crop up in the echolocating bat branch. Jianzhi Zhang, a professor of statistical genetics at Michigan, and his colleagues found 11 sites on Prestin that had undergone parallel changes. "We think that those sites are probably important for high-frequency selection," he says.
Many species that use echolocation produce very-high-frequency sound. To receive this sonic data, the animals also have to have specially tuned receptors. Zhang proposes that in echolocating bats and dolphins, Prestin has been modified specifically to amplify those frequencies. Although the ways that bats and dolphins create the sounds they use for echolocation are complex and quite different (bats' pings travel short distances through air, whereas dolphins' squeaks traverse hundreds of meters through dense water), some morphological similarities in the outer ear hairs had already been noticed—namely that they are shorter and stiffer than in other mammals, thus better-tuned to higher frequencies.
To Zhang, the genetic findings suggest that "maybe there's only limited ways to accomplish this task, so you have to have a specific combination of the Prestin protein."
The other group of researchers, based in part at the School of Life Sciences at East China Normal University in Shanghai, sequenced the gene in various species of echolocating toothed whales (which includes dolphins) and non-echolocating baleen whales and came to the same conclusions: "Adaptive sequence convergence between two highly divergent groups that share a complex phenotype is unprecedented," the authors, led by Yang Liu of East China Normal University, wrote.
"Convergent evolution and parallel evolution are very common at the phenotypic level—birds can fly and bats can fly," says Zhang. Spots and tusks are also examples of similar features found in divergent species. "But at the molecular level and genetic level convergent evolution is rare," he says.
As genomic sequencing becomes more commonplace, though, these newfound similarities might prove to be more prevalent. "Convergence is apparently due to common selection," Zhang says. "The current thinking is that at the molecular level, most of the changes are neutral." But if it proves to be more frequent across many species, it might change the way scientists think about genetic selection—and evolutionary change—in general.
In the meantime, says Fenton, both his and the genetic studies are helping to close in on the details of echolocation, revealing more about how these boisterous animals use sound to see.
