How a disappearing ear bone turned bats into masters of echolocation | Science

Bats use sound to hunt a dizzying array of prey. Some zero in on flowers to sip nectar, whereas others find cattle and suck their blood. Many nab insects midflight. One species of bat senses small fish beneath the water and snatches them as osprey do. Now, scientists have discovered an anatomical quirk in the ears of some bats that could help explain how they evolved so many hunting specialties.

“For me this is a huge revelation,” says Zhe-Xi Luo, a University of Chicago evolutionary biologist who has studied the origins of mammalian hearing and supervised the new research. “This is totally distinct and unique from all other hearing mammals.”

Most bats use their ears to “see” the world around them: After a bat chirps, its ears sense shapes and movement as sound waves bounce off objects, much as ships use sonar. Bats’ ears were long thought to be just a finely tuned version of the ears of nearly all mammals.

Then, in 2015, Benjamin Sulser, a University of Chicago biology student on the hunt for a thesis project, took detailed 3D images of the inner ear of a bat skull. But he couldn’t find a feature common in virtually all mammals—a bony tube that encases the nerve cells and connects the ear to the brain. Thinking he’d made a mistake, he and Luo imaged the skulls of two more related species using a computed tomography scanner, with similar results. The researchers realized they might have stumbled across an answer to a mystery that had bedeviled bat biologists for 2 decades—and an explanation for why some families of bats had such a diverse echolocation arsenal.

For years, bats were divided into two groups: big fruit bats, which don’t generally echolocate, and small bats that hunt by sound. But in 2000, a genetic analysis revealed bats had actually diverged into two different groups some 50 million years ago: one group that included the large fruit bats and some of the echolocating insect eaters (known as Yinpterochiroptera, or “yin”), and another group that included the rest of the small, sound-hunting bats (known as Yangochiroptera, or “yang”). The latter make up 82% of the 1250 known echolocating species of bats.

But until now, no one could find a physical difference linked to the genetic split between the two groups. After Sulser’s discovery, the duo spent 5 years scanning the skulls of 39 bat species. In 24 of 26 yang bats they scanned, the bony nerve channel was missing or contained large holes making it resemble lace, Sulser and Luo report today in Nature.

That feature sets the bats apart from their yin counterparts, both large and small—and almost all other mammals, Luo says. In a typical mammal’s ear, sound waves send vibrations down a fluid-filled tube in the skull, called a cochlea. Inside the tube, tiny, hairlike cells transmit the vibrations to neurons that feed directly to the brain though the auditory nerve. A bony channel winding around the cochlea encloses part of the nerve cells. Before now, the only known mammals whose auditory neurons weren’t encased in bone were monotremes, egg-laying animals including platypuses, which split from other mammals 180 million years ago.

Luo suspects that—because yang bats’ nerves aren’t as confined—the loss of the bony nerve channel unleashed new hearing capabilities. Large nerve channels in rodents have been linked to larger bundles of auditory neurons, and past studies have found yang bats tend to have more neurons in the auditory nerve than yin species. Removing parts of the canal wall could also create room for a more robust auditory nerve or different nerve configurations, Luo adds.

And indeed, compared with other bats, yang bats show greater diversity both in the anatomy of the inner ear and in their hunting methods, says Sulser, now a Ph.D. student at the American Museum of Natural History. That suggests the yang ear design enabled bats to adapt to a variety of niches, from nabbing frogs to feeding on flowers, he says.

Many yin bats with solid nerve canals use a single, sustained note in their call. That approach is well tuned to identifying the flutter of insect wings at longer distances, Luo says. By contrast, most bats known to have the missing or lacy nerve channel use a broader range of frequencies, making them better at picking out complex details at short distances, and potentially enabling them to specialize in catching distinctive prey in specific habitats.

The new study is “one of the neatest papers, ever” for revealing this overlooked anatomical feature, says Brock Fenton, a retired bat biologist and echolocation expert at the University of Western Ontario. The findings open questions about how variations in the nerve and its bony enclosure from one species to another might translate into subtle differences in how bats hunt.

The discovery also underscores the continuing power of studying anatomy at a time when many scientists are focused on genetics, says Liliana Dávalos, an evolutionary biologist at Stony Brook University. “I hope that this discovery will actually inspire other people to explore this anatomical diversity because it has a lot to teach us.”