Hi-Tech Bat Detector Sheds Light on Shadowy Species
|March 13, 2002|
As evening falls in northwestern Belize, Bruce Miller stands in the
forest, waving a small white box through the air. When a bat swoops
overhead, Miller's laptop beeps and clicks, and corresponding lines and
squiggles appear on the screen. He points to a succession of slashes.
This one's a mastiff bat," says Miller, a zoologist with the Wildlife
The white box is an electronic "ear" called
the AnaBat. Hooked to a computer, it enables scientists to "hear" the
inaudible calls of bats and determine the species of the "speaker." The
device allows scientists to survey bat populations with unprecedented
accuracy. These surveys, in turn, are helping conservationists determine
which species are truly threatened and where protection is most needed.
Bats in Peril
"Bats are exceptionally vulnerable to extinction, in part because they are the slowest reproducing mammals on earth for their size, most producing only one pup annually," says Bob Benson, a spokesperson for Bat Conservation International. "More than half of the United States' 45 species are now either endangered or in severe decline. Losses are occurring at alarming rates worldwide."
William Gannon, a biologist at the University of New Mexico, adds, "In the U.S. and around the world, bats face serious threats, from habitat loss and pesticide poisoning to widespread extermination." He continues, "One of the big problems is that not enough is known about bat behavior because they're nocturnal and they fly."
Miller and Gannon are among those who combine acoustic technology with traditional study methodsnetting, trapping, and hand-examining bats to fill these knowledge gaps.
In order to protect these elusive mammals, scientists must determine which species are where and which are in trouble. They also must locate critical roosting and feeding areas.
Identifying Bats by Voice
Recent advances have transformed bat detectors from mere listening devices into "voice recognition" systems for echolocation calls. These cries, which are beyond the range of human hearing, bounce off objects and echo back, guiding bats as they hunt and fly.
Each bat species has a distinct vocal signature, which AnaBat translates into a unique shape and pattern on a computer screen.
Miller first tried out the AnaBat on his front porch in Belize six years ago. Within five minutes he identified four different species buzzing the house. "Then it became a question of capturing the bats to see who they were. It became a biological detective story," he remembers.
Miller and his biologist wife, Carolyn, began cataloging calls in the field. At the same time they trapped bats and used AnaBat to definitively match species to voices. "Once you have reference calls in an acoustic archive, then you can just ID them by voice in the field," says Miller.
Traditional bat-surveying methods, such as trapping, often over- or under-represent populations, says Bruce Miller. Some bats seem to either fly too high to be trapped often or have supersensitive echolocation skills that help them avoid capture. Others seem to readily fly into nets.
The old way is slower too. "In one or two nights work with the acoustic equipment, you can get data that would take months or years to get with trapsor couldn't get at all," says Miller.
With the voice-recognition technology, scientists can quickly amass data, and for the first time they can detect bats winging above the forest or across open fieldsall without laying a hand on the bats, an important factor when studying at-risk species.
Using the AnaBat, Miller is making some surprising discoveries that call into question decades of research. His data on the prevalence of some species is radically different from traditional data from trapping. Some bat species thought to be in trouble are seemingly ubiquitous. And others that seemed to be everywhere may really be quite rare.
Miller has established a database combining his and his colleagues data with records on bat distribution from published sources and museum data.
Miller is also compiling a library of bat calls for the tropics of the Americas. His current archive includes the vast majority of echolocating bats in Belize. (Some are too "soft-spoken" to monitor with AnaBat.) Regional bat-call libraries are also under way in the U.S. (at the University of New Mexico) and Australia (at Charles Sturt University in New South Wales).
Computer-generated bar graphs, GIS (geographic information system) maps, statistics, and photographs produced from these databases provide a gold mine of evidence for conservationists lobbying to protect certain species or habitats.
"If We Lose the Bats, We May Lose the Tropical Forests"
Why are these winged mammals important? "Bats are the primary seed dispersers," says Miller. "Certain bats, like the short-tailed fruit bat, drop guano containing 40,000 to 50,000 seeds in a night. They pollinate many tropical flowers, [as do] hummingbirds and moths." And myriad insect-eating bats help control pests.
"If we lose the bats, we may lose the tropical forests," predicts Miller. Many trees and plants would not survive without them. Using state-of-the-art technology, Bruce Miller and his colleagues are trying to make sure that doesn't happen.
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