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Volume 45, No. 2
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On a Wing and an Ear
Nancy Simmons ’81 delves into the mysteries of how and when bats became the flying hunters of the night.

Story by Sally Ann Flecker / Photos by Piotr Redlinski, World Picture News

One of the great pleasures of summer is that sliver of evening suspended between twilight and dusk when the deepening sky seems in no hurry to yield to night. That’s the time the bat comes swooping over our roof. Look up, I tell my kids, and we watch, transfixed while it sails through the air, wings fluttering as fast as a baby’s heartbeat. It makes a few figure eights in long elliptical loops above our houses, darting here and there to snatch an insect in mid-flight. Then it’s gone, and we’re left, undeniably earthbound, with a feeling of wonder.

There’s a lot to marvel over when it comes to bats. I didn’t know that while our bat was putting on its air show, it was figuring out what was in the environment around it by producing high frequency sounds—much too high for human ears. That’s called echolocation. It’s one of two very specialized capabilities that give bats a big advantage in the mammal world. The other is flight. For years, evolutionary biologists have been curious to know which of the two came first. This past year, Nancy Simmons ’81, in an article in Nature, put the question to rest.

Simmons is curator-in-charge of mammalogy and chair of the vertebrate zoology division at the American Museum of Natural History in New York City, and an expert on bats. Simmons’ office, which you would never be able to find without a guide, is the kind of well broken-in, professorial study that would look right at home at many a college. Well, except for the trophy lion from the museum’s collection looking down from its perch above the fireplace mantel. And the flying fox—one of the larger bats, hovering with spread wings in the airspace next to Simmons’ computer monitor. And the pickle jars further down on her worktable, each holding a bat—one labeled India, May 1923, the other Belize 2002—preserved in a murky solution. Then there are the real gems: two recently discovered 52 million-year- old fossils of a kind of bat never before seen, collected at Wyoming’s Green River geological formation. One is on loan from a private collector, the other from the Royal Ontario Museum. Each is oddly beautiful and mysterious—delicate, petrified bones encased in a grainy slab of putty-colored stone that fits comfortably in Simmons’ hand. The extravagantly long hand and finger bones that once supported its wings dangle like streamers at the end of folded arms. Its rib cage seems swelled with air, and each bone in its clawed feet seems poised to push off for a flight that never came. The fossils were central to her work that landed on the cover of Nature. Simmons doesn’t know how much they’re worth. Actually, she clarifies, she’s been careful not to find out.

Bats weren’t something Simmons knew a lot about when she accepted a post-doctoral fellowship at the American Museum in 1989, although, she tells me, she had always thought they were “very cool.” She had spent years studying fossils—her graduate work focused on a group of Mesozoic mammals that have no living relatives. When she finished her Ph.D., she thought she would like to try working on an animal that had, as she puts it, living relatives and living diversity.

There’s no doubt about bats having living relatives. There are 1,100 known living species, which means that one in every five living mammal species is a bat. They live on every continent except Antarctica, with many congregating in the tropics. When Simmons did field work in French Guiana a few years back, she found 76 species of bats, all within a three-kilometer walk from camp. That may make it sound like it was a piece of cake to find so many. But Simmons and her team worked hard for that bounty, placing their traps and nets high and low. They crawled into wet, mucky roosts, which, in the rainforest, might be in a hollow log or in the shelter formed by the many buttress roots of a fallen tree—neither being particularly inviting unless you have a penchant for brushing up against creepy-crawlies.

That’s not to mention the conditions—nasty, rainy, muddy, with multitudes of mosquitoes, and having to work on little sleep. Bats are nocturnal. If you’re going to study bats in the field, you have to work at night. But in order to maximize your catch at night, you work most of the day, too. You move nets and process samples. Then you grab something to eat and run back out to spend most of the night picking bats out of nets.

The sheer diversity of the order Chiroptera—from the Greek words for hand and wing—fascinates Simmons. Give Simmons three minutes and she can paint a pretty good picture for you. Consider differences in size, for starters. The very smallest, she says, weighs about a gram—less than a penny—as an adult and when its wings are fully open, the span is around three inches. At the other end of the spectrum is a bat that has a wing span up to six feet.

Most of us think that bats fly around and catch bugs, and many of them do. There are bats that are specialized for feeding on beetles, she says, and bats that are specialized for feeding on moths, and bats specialized for feeding close to the ground, and still others that fly above the canopy. Some bats listen for the sounds their quarry makes when hitting a leaf, and then dive in to catch them. There are bats that snatch fish with their hind feet and bats that eat mice, lizards, small birds—sometimes even other bats.

“We always sort of suspected there were two things that were key to the evolution of bats that made it possible for them to diversify the way they did. One of those being flight, obviously, because it allows them to take advantage of a whole different part of the eco-world,” says Simmons. “The second is echolocation because that allows them to access all of the flying insects of the air, and it also allows them to navigate at night. They can be nocturnal, flying, aerial insectivores, and take advantage of this vast set of riches of food stuff available.”

Understanding how—and when—these two bat “superpowers” evolved has eluded evolutionary biologists until now. Simmons’ primary research interest is evolutionary biology and the anatomy of organisms—how they are built, how they got that way, how they are related to one another, and how you can tell. Most of her work is done with living species of bats. But because of her training in paleontology, she is also interested in fossils, which is why the ancient bat fossils—in her hand now as she shows them to me—were brought to her attention.

Here’s how echolocation works. The bat beams out a high-pitched call through the mouth, and in some species, through the nose. When the sound encounters something, it bounces back. The bat’s specialized hearing allows it not only to detect the sound, but also process the amount of time it took to bounce back. What that gives them is kind of a sonogram on the fly, a map of what’s around.

But here’s a twist—not all bats echolocate. One family out of the 19 living families of bats uses vision rather than echolocation. That’s the Old World fruit bat or flying fox, like the one hanging at Simmons’ desk. They don’t eat insects; they eat fruit and flowers for nectar. Interestingly, they are less diverse than the other 80 percent of bats who use echolocation. “How did these features evolve? How did bats become flying mammals? How did they become echolocating mammals?” Simmons asks. “And, one of the most basic questions is, did these two features evolve together or did one come first?”

Until recently, there were three theories about bat evolution. There’s a flight first hypothesis that suggests bats had a gliding ancestor. They evolved flight as a means to get around in their environment, and later developed echolocation to help them better detect the insects they need to eat in abundance.

According to the second hypothesis, which argues for echolocation first, bats were a scrambling, tree-dwelling animal that used echolocation to detect food sources, and subsequently evolved flight to get around better and to be able to chase after the insects they detected through echolocation into the air.

In the 1990s, a new discovery showed that the high-intensity echolocation calls take a lot of energy to produce in a stationary bat. When the bat flies, though, the flight muscles pump the lungs, so there is a savings in energy. This suggested a third theory of tandem development: If there is no cost for echolocation when the bat is flying, maybe flight and echolocation evolved at the same time.

All three theories were plausible, says Simmons, and virtually untestable—but here’s where the 52 million-year-old fossils she is studying come into play. The new bat, which Simmons and her team named Onychonycteris, has characteristics that place it early in bat evolution. For instance, it has claws on all five fingers, just as its terrestrial ancestors did. And it has the anatomical features, the exceptionally long fingers, for instance, needed for flight.

Bats that echolocate share three anatomical features. The first is an enlarged cochlea—the spiral-shaped cavity of the inner ear. Next is the stylohyal element that connects the base of the skull to the bones in the throat that support the voicebox muscle. In bats that echolocate, the stylohyal is long and skinny, with a paddle at the end to help it attach more firmly to the base of the skull. Finally, the malleus, the ear ossicle that contacts the eardrum, has a knob on the end that affects how it vibrates. It’s clear from the specimen that Onychonycteris had none of these features. Simmons could see there was no way it could echolocate. Mystery solved—it’s flight first.

Before I leave her office, Simmons shows me a few other bats from the museum’s collection, which includes around 60,000 bat specimens representing almost every species from around the world. She shows me fossils, skulls, bones, molars, full skeletons, skins. She takes a bat out of a display drawer that looks like a little stuffed animal. This one has been prepared so that the banding patterns on its fur can be studied. As she talks, she strokes its head gently the way you would a pet. “From the mammal’s basic biological plan—we’re warm-blooded, we have hair, we give milk, we have a skeleton inside, we have a basic set of internal organs—you have things as diverse as bats and whales and horses and cats and shrews. I just find that amazing,” she says. “I want to understand as much as I can about the evolution of diversity. And as we collect more genetic data, more morphological data, and we actually start to solve some of these problems, it’s just going to get better.”

In the scientific world, answers beget more questions. Same at my house. It’s winter now, and our bat is either hibernating or has gone some place warmer. “Bats are blind,” my six-yearold tells me with great authority, adding the definitive, “Dad said.” He thinks about it for a minute then asks, “If bats are blind, how can they fly?” His brother picks up the rubbery bat I brought back for him from the museum’s gift shop. He is five, so, of course, he flies it like an airplane around the room. Come spring, when we welcome back our bat, we’re going to find a lot to talk about.
 

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