Listening to BATS
Scientists keep learning about the animal that “sees” with its ears.
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 from on the screen. “This one’s a mastiff bat,” says Miller, pointing to a sequence of parallel slashes.
Using a type of electronic “eat” 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-dispersers 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.
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.”
Bats aren’t the only animals that use sound to navigate and hunt for food. Other well-known echolocators include dolphins, toothed whales, seals, small mammals like shrews and rats, and some birds.
Like bats, these animals emit pulsing sounds, with intervals of silence to listen for returning echoes. The time it takes for a signal to echo back helps animals calculate the distance of an object. The “shape” of this echoed sound wave provides information about a possible meal’s size.
As hunters close in on their prey, signals increase from I to 5 pulses per second to as many as 200 per second in bats. Killer whales can send 500 pulses per second!
Animals build up a kind of acoustic “map” of their environment, says Dean Waters, a biologist at the University of Leeds in England. This is vital for animals who use a sightless navigation system such as bats, or birds like swiftlets who live in dark caves.
Different animals produce sounds at various frequencies (the number of vibrations per second, which is measured in hertz). Bat cries are ultrasonic, above the range of human hearing, or about 15 kilohertz (one kilohertz equals 1,000 hertz, or 1,000 cycles per second). Whale and dolphin calls span a range from 250 Hz to around 220,000 Hz.
Scientists don’t yet know how young animals learn their repertoire of echolocation sounds and frequencies. Dolphins use a natural sonar (acronym for “Sound Navigation Ranging”) system through a series of chirps, squeaks, and clicks–unique to each individual dolphin. The sounds are emitted through the melon, a waxy, lens-shaped organ in the forehead that directs sounds forward.
Dolphins hear returning echoes with the slitted ears on the sides of their heads, but they also use their lower jaws to direct sound to the inner ear and nerve centers. Researchers have found that eyen when blindfolded, dolphins can detect minute differences in shape, size, and thickness of objects. Killer whales can distinguish between cod and salmon, and some whales can perceive an object up to 2,000 miles away!
BAT WORLD CLASS: Mammal
VARIETY: Almost 1,000 species (almost a quarter of all mammal species)
HABITAT: Roosts include large caves, trees, and attics in every continent except Antarctica.
OFFSPRING: Pups (baby bats) are usually born in June.
LIFE-SPAN: The little brown bat of North America holds the record for longest life: 33 years.
DIET: Includes fruit, insects, flowers, and frogs. Some South and Central American bats eat fish, while vampire bats lap up blood from sleeping horses and cattle.
LARGEST BAT: The giant flying fox of Southeast Asia weighs 1 kilogram (2.2 pounds), and has a whopping 1.8 meter (6 foot)-wide wingspan!
SMALLEST BAT: The tiny Kitti’s hog-nosed bat of Thailand weighs just 2 grams (0.07 ounce), and its spread wings measure only 16 centimeters (6.3 inches).
LARGEST COLONY: In the first half of the 20th century, up to 50 million bats lived in Eagle Creek Cave in Arizona. By the 1960s human disturbance had cut their numbers to about 30,000.
ACTIVITY: While tropical bats are active year-round because of abundant food supplies, bats from colder regions either migrate to warmer regions or hibernate. A bat that hibernates spends the summer eating a lot of food to store energy for the winter. During hibernation, the bat goes into torpor–a resting phase where body temperature drops to near that of its environment.
USING SOUND TO “SEE”
Bats use echolocation to hunt and to avoid obstacles as they fly at night. They emit high-frequency sounds that echo back; the elapsed time helps them calculate distance.
Bats ears are highly developed. How about yours?
YOU NEED: song * tape or CD player
1. Set volume of music player at medium.
2. Play song.
3 Stand 1 meter (3.3 feet) in front of player.
4. Turn your left ear toward the player. Note noise level of the song.
5. Turn your back toward the player. Note noise level of the song.
6. Cup your left hand and place over your left ear (like an enlarged outer ear) and repeat steps 4 and 5.
7. Compare your findings from steps 4, 5, and 6.
How different are the noise levels? Why?
Ecology: Have students study the environmental importance of bats. How can we better protect bats?
Did you Know?
* The scientific name for bats is Chiroptera, meaning “hand-wing.”
* Fossil records show bats used echolocation as long as 50 million years ago.
* About 1790, Italian scientist Lazarro Spallanzani proposed that bats could “see with their ears.” The scientific community didn’t accept his hypothesis until the 1930s.
* Out of almost 1,000 species of bats, only three are considered bloodsuckers, or vampire bats.
National Science Education Standards
Grade 5-8: transfer of energy * diversity and adaptation of organisms * structure and function in living systems * abilities of technological design
Grades 9-12: interactions of energy and matter * interdependence of organisms * matter, energy, and organization in living systems * behavior of organisms
Bats in Question, by Don E. Wilson (Smithsonian Institution Press, 1997)
Bats, by M. Brock Fenton (Facts on File Books, 1992)
Bats: Did you Know: www.cccoe.k12.ca.us/bats
Bat Conservation International: activities and Information: www.batcon.org for Bats magazine: www.batcon.org/topbats.html
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COPYRIGHT 2008 Gale, Cengage Learning