Ear Wiggles Open Up New Worlds In Bat Echolocation

Greater Horseshoe Bat Ears (Photo: Mittu Pannala)

by Richard Conniff/Scientific American

One big problem with putting autonomous drones to work delivering packages—or flying search-and-rescue missions—is that the sky is complicated and unpredictable. Trees, utility wires and spiraling footballs can turn up almost anywhere in the flight path. A new strategy for dodging these obstacles could come from an unexpected source: the way bats wiggle their ears.

The idea first occurred to Rolf Mueller, a Virginia Tech mechanical engineering professor, a few years ago while looking at bat photographs. He noticed that the ears of some species often looked blurry, because the animals were continually making rapid ear movements. But why?

Mueller studies bat behaviors, including their adaptations to the Doppler effect or Doppler shift. Both terms refer to the way sound waves from a fast-moving object such as a train or an ambulance get compressed—and therefore higher in pitch—as the object approaches a listener. Then the sound waves lengthen out again and become lower in pitch as the vehicle moves away. Even when the train or ambulance is out of sight, a person can tell roughly where it is at any moment from these changes in sound. Bats use the Doppler shift to locate objects in much the same way, but far more precisely.

Scientists have known since the 1930s that insect-hunting bats produce bursts of sound as they bob and weave through the night. They use the reflected sound waves to identify obstacles and target prey, an ability called echolocation or biosonar. Research in the 1960s showed that bats also interpret Doppler shifts, in sounds bounced off of flying insects, to zero in on a meal with high precision—even while maneuvering at breakneck speed through

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Why Obscure Species Matter

Without bats, a lot more corn would look like this. (Photo: Flickr)

Odd bits of recent wildlife news, mostly about very small and obscure species, have left me thinking lately about a game called Jenga. If you happen never to have played it, here’s how it works: The game consists of small wooden blocks, and you start by assembling them into a tower, with each level consisting of three blocks laid horizontally, and the layers arranged crisscross to one another. On each turn, a contestant removes one block and places it on top, the point being to remove as many blocks as possible without causing the whole thing to collapse.

Believe it or not, something called the “Jenga hypothesis” has become an alternative model for understanding how ecosystems really work. In the conventional paradigm, keystone species are typically predators at the top of a food chain, and plenty of evidence has demonstrated that removing them can cause a “trophic cascade” of unexpected and sometimes catastrophic changes down the entire food chain. It’s like removing the “keystone,” the central stone that holds up an architectural archway, and watching everything fall down.   Take sea otters out of the coastal Pacific, for instance, and the sea urchins, abalone, and other invertebrates they feed on predictably boom; the sea urchins and friends in turn demolish the kelp forests, and so on down the ecosystem.

But according to the “Jenga Hypothesis,” first proposed in the journal Science in 2005, the keystone analogy is too static. It misses

Why Bats Are So Prone to Pathogens

A pallid bat holds a meal of a katydid. (Photo: Merlin D. Tuttle/Bat Conservation International)

I’ve written here before about why bats are the source of so many deadly diseases–including MERS, SARS, Nipah virus, Hendra virus, Lassa and Marburg hemorrhagic fevers, and of course Ebola.

Why bats? It’s partly because they are such a diverse group, with 1,250 species, comprising about 20 percent of all mammals, says Jon Epstein, a veterinary disease ecologist with EcoHealth Alliance in New York. Some researchers theorize that immune systems or other physiological differences might make bats more likely to carry viruses. But so far that’s only a theory. The bat lifestyle of roosting together in dense colonies may also encourage viruses. These colonies often occur in and around human habitations, and the ability of bats to fly means any virus can get dispersed across a wide geographic area.

But when you see an emerging disease come from wildlife, says Epstein, “it’s generally triggered by something people have done to manipulate the environment,” meaning agricultural expansion or intensification, or urbanization, coupled with the modern tendency to move plants, animals, and people all over the world.  “It’s really human activities that are driving spillover.”

In today’s New York Times, Natalie Angier suggests that the proneness to pathogens is all about the immune system:

Yet bats appear largely immune to the many viruses they carry and rarely show signs of the diseases that will rapidly overwhelm any human, monkey, horse, pig or other mammalian host the microbes manage to infiltrate.

Scientists have also learned that

There’s Something Fishy About These Bats

The long-fingered bat. (Photo: Antton Alberdi, UPV/EHU)

For researchers on the Mediterranean coast of Spain, near Valencia, the mystery began when bat excrement on the floor of a cave turned out to be loaded with scales, suggesting that an insect-eating bat species had instead turned to fishing.

It wasn’t a complete surprise. Bats are amazingly diverse, with 1,240 species (that’s 20 percent of all mammals), and they’ve had 50 million years to develop a multitude of quirky behaviors. Different bat species are known to eat almost anything—insects, of course, but also fruit, leaves, flowers, nectar, pollen, and, yes, blood. It might be genuinely surprising if somebody said bats catch and eat songbirds, except that researchers caught bats doing just that in 2007.

Frogs too. Early this year, researchers in Central and South America reported on how the male túngara frog’s love song produces a widening pattern of ripples on the surface of the water. Bats have learned to use that pattern as a flight path to cruise in and pluck up the unfortunate Lothario for dinner.

Still, Ostaizka Aizpurua-Arrieta and her coauthors on a new study in the journal PLOS One wanted to find out how Spain’s long-fingered bat learned to fish. “It was a special challenge for me because we didn’t think fishing was among the habits of the long-fingered bat,” says Aizpurua-Arrieta. These bats use echolocation to hunt down their main food source, immature midges, at the surface of the water. But the sonic pulses the bats emit for echolocation can’t penetrate below the surface, to where fish live. The long-fingered bat also weighs no more than a third of an ounce, which is “why it is difficult to imagine it fishing.”

And yet,

Signs of Recovery in the White-Nose Scourge

The first time white-nose syndrome really hit Aeolus Cave, on the side of a mountain in Dorset, Vt., the carcasses littered the entrance like a carpet. “You couldn’t step on the ground without stepping on dead bats,” says Scott Darling, a biologist with the Vermont Fish and Wildlife Department. “Nowhere else have people seen that level of mortality in a cave.”

Bats afflicted with the fungus during their hibernation usually “fly out and die on the landscape,” unnoticed. But “in Aeolus Cave, for whatever reason, many, many died right there in Guano Hall,” the steeply sloped tunnel-like cave entrance. Statewide, Vermont lost 90 percent of its bats during the first few years of the epidemic, from 2008 to 2010. The other 19 states and five Canadian provinces where white-nose occurs have suffered a similar rate of loss. It may be the most precipitous decline in any animal group ever, with an astonishing 5.7 million bats now thought to have died. But Aeolus, the site of decades of pioneering research on bats, became the poster child of the epidemic.

Now, Darling sees the first tentative signs of a possible recovery there.

In an experiment at Aeolus Cave in April 2012, researchers placed identifying bands on bats emerging from hibernation. When they went back to count them last April,

The Magic of Bats: Not Just for Halloween

A Rodrigues fruit bat just hanging out. (Photo: Arnd Wiegmann/Reuters)

 

My latest, for TakePart:

When it comes to bats, a lot of people go a step beyond Ronald Reagan’s notorious remark about trees. They think that if they’ve seen one bat, they’ve seen one too many. And let’s admit up front that some bats aren’t much to look at it, unless you are partial to hairy carbuncles, scrunched-up noses, and needle-sharp teeth. Bats tend not to get much public love, except around this time of year, and then only as one of the little horrors of Halloween.

But the recent discovery that bats use a kind of natural megaphone to amplify their voices reminds me that amazing and often ingenious behaviors seem to be a bat specialty. This probably shouldn’t be too surprising. The 1240 bat species described so far represent about a fifth of all known mammals, meaning plenty of diversity. Their fossil record also dates back more than 50 million years, meaning lots of evolutionary time to develop strange behaviors.

Let’s start, for instance, with that megaphone behavior. Naturalists already knew that in New World rain forests, Spix’s disc-winged bats like to make their homes in the furled leaves of Heliconia and Calathea plants. These bats get their name from the little suction cups on their wrists and ankles, which enable them to stick to the leaves (and also to your fingernails, a juice glass, etc., says one bat biologist who has studied them).

The leaves have an annoying habit of unfurling, with the result that the bats must frequently relocate. When they arrive in a new home,

Shape Shifter

 

(Photo: Cally Harper)

Tongues can do delightful and astonishing things. I am thinking of the way a frog fires its sticky tongue halfway across the universe to snag a passing insect (see below). Or how an alligator snapping turtle wriggles its tongue like a worm as a dinner invitation to fish. And now the Pallas’s long-tongued bat (Glossophaga sorcina) joins this elite club of astonishing animal tongue artists.

These bats, found from Argentina to northern Mexico, and sometimes into Arizona and New Mexico, have the fastest metabolism ever recorded in a mammal, says a 2007 study in the journal Nature. They burn half their body fat each day, and have to make up for it at night by consuming as much as 150 percent of their body weight in nectar from flowers. And of course, they have to do it on the wing. According to a study published today in the Proceedings of the National Academy of Science, the secret to its success is a remarkable ability to change the shape of its tongue into a hemodynamic—or blood-swollen—“nectar mop.”

When lead author Cally Harper, a doctoral candidate in biomechanics at Brown University in Rhode Island, began her study, specialists already knew that bats of this species have an unusual fringe of hair-like structures around the tip of the tongue.  They assumed these were useful for collecting nectar—but passively, like raking icing off a cake using your fingernails. Biologists also knew that these bats have enlarged blood vessels in their tongues. But they didn’t know what to make of them. Harper had a hunch that the two features might be connected, especially since …  Read the full article here.