Listening to Bats
As light dims in the falling dusk, sounds in the Central American jungle of Belize become a screeching concert of insect, tree frog, and bird calls. Giant ceiba trees with huge, snaky roots tower over a tangle of foliage, thickly woven with vines. A sliver of sky is visible above a narrow horse trail, and biologist Bruce Miller has blocked the trail in two places, erecting tall traps to capture and study bats.
He turns a powerful headlamp skyward to search for the wild flutter of bat wings and slowly waves a small yellow and white box through the air. The box is linked to a laptop computer that sits on a camp table; the screen glows an eerie blue in the looming darkness.
Suddenly, as a bat swoops by, loud clicks, chirps, and beeps erupt from the laptop's speakers. A corresponding series of squiggles, dots, and lines form on the screen. "This one's a mastiff bat," says Miller, pointing to a sequence of parallel slashes.
Using a type of electronic "ear" called the Anabat detector, Miller listens to bats and records as computer files their ultrasonic (beyond human hearing range at over 15 kilohertz) cries. Bats emit high-pitched squeals in a type of animal sonar called echolocation sound waves that bounce off objects and echo back at them. Thanks to advanced computer technology, researchers are now using sophisticated detectors to identify bats and other animals by their voices. Each bat species has its own "vocal signature" translated on a computer screen in a distinct visual pattern and shape. Armed with this data, Miller can pinpoint habitats needed to protect threatened bat species. And there's a vital reason to protect them.
Bats are seed-dispensers responsible for reforestation (regrowing cut forests); species like the short-tailed fruit bat drop between 40,000 and 50,000 seeds per night. Like bees and hummingbirds, bats also pollinate flowers. And insectivorous (insect-eating) bats help control insect pests: they devour up to 600 mosquito-sized insects in an hour. "If we lose bats in the tropics, we're going to have serious problems," Miller says. "There's a whole batch of tropical plants and trees that simply wouldn't survive without them."
Fly by Night
Bats are the only mammals (warm-blooded animals that nurse their young with milk) that actually fly. (A few other mammals, such as the flying squirrel, can glide.) And don't confuse bats with birds. Bats are covered with fur rather than feathers, and their wings are actually very long, bony fingers covered by thin skin; bird wings are braced by light upper arm and forearm bones. Whereas birds rely heavily on "bird's eye" vision to navigate and forage for food, bats use their ears and echolocation to pinpoint prey. However, the expression "blind as a bat" is a myth: All bats can see, but researchers don't yet understand the extent of batty vision.
Like dolphins and whales, bats use ultrasonic pulsing sounds to locate obstacles in their path, identify the size and shape of prey, and to communicate with one another. Their echolocation is so refined that bats, which are nocturnal (active at night), can detect insect food as thin as a human hair in total darkness. As a bat zeroes in on its prey, echo pulses increase and the hunter captures its meal.
Biologists Bruce and Carolyn Miller examine a sword-nosed bat caught in a harp trap's strings. They must capture bats to confirm their species, and to weigh, measure, and study them.
Echolocation is a two-part process: transmitting high-frequency sounds, and receiving and analyzing them. Bats produce their cries in the larynx, or voice box, and "speak" through their mouth or nose; some bats also click their tongues. Bats' ultrasonic cries span a frequency between 20 and 100 kilohertz, compared with 1 kHz for the human voice. Their cries travel nearly 340 meters (1,115 feet) per second, and by calculating the time it takes for its call to rebound, a bat can determine the distance of a meal. Scientists still don't know how animals that use echolocation compute such complex mathematics in their brains.
Until recently the only way for biologists to study bats was to trap them. Then researchers often attached tiny radio transmitters to bats' backs to track where they fly, where they roost during the day, and where they hang at night to digest their evening meal.
But trapping bats couldn't provide accurate data on the population of bat colonies. For the last five years, Miller has used the Anabat to study bat populations throughout Belize. So far, he's identified about 80 percent of the 74 known bat species in the country by their voices. He can detect bats flying 30.5 meters (100 feet) above the forest canopy-as well as those that have traditionally eluded capture. "Acoustic identification is another leap in the development of science to learn about little-known bat species," he says.
But Miller has been unable to identify certain "very quiet" bat families with Anabat. One such family, the leaf nosed bat, is so soft-spoken they're known as "whispering bats." "We're still pioneering the technique," Miller explains.
Still, he's made some startling discoveries, beginning with the shaggy bat: "The shaggy bat was considered one of the rarest bats in the New World tropics. Now, acoustically, we find they're everywhere in a healthy forest. It's causing us to reassess the concept of rarity."
Certain species are only abundant in relatively unspoiled forest, and tallying these populations helps biologists diagnose forest health. Bats are particularly sensitive to habitat changes because they bear just one pup each year. Because of their low reproductive rate, Miller explains, it takes years for threatened bat populations to rebound. That's one reason bats provide a good indicator of habitat quality "If we survey a tropical forest and find the most prevalent species are rare or absent," he says, "we can surmise that something's out of balance."