Listening for Bats

Susan Ray Smith
January 01, 2002
blue sketch of bat flying facing left

The eight students and I have been sitting in the graveyard for an hour, facing an innocent-looking white church. We stare at a two-foot by two-foot opening in the side of the wall and glare at it like it's a television. Below it, as if someone had driven dozens of random nails into the side of the old building, are dark specs of guano—bat droppings. In the church's attic is the largest known maternal colony of little brown bats east of the Mississippi River and one of the largest in the United States—about 20,000 bats.

Dusk approaches at Canoe Creek State Park in Altoona, Pennsylvania. We hear fluttering in the attic. Suddenly, a dark creature emerges from the hole in the wall—but it flies back in almost as quickly as it flew out. One or two bats do this repeatedly, then more appear.

They dive from the hole, speeding downwards, falling fast, until they decide to fly. They swoop so close to some of our heads that we think we will be hit. A group of three students stands up to get out of the path of the bats, but other bats swoop close to them as well. With each pass, I could feel them just inches above my head.

It saves bats energy to fly like this, says Michael Gannon, the students' biology professor. Don't worry, he assures us, bats are exceptionally good at avoiding objects. So good they can even avoid the mist nets that he hopes to set up tonight.

Catching bats in mist nets, explained Gannon, an associate professor at Penn State's Altoona campus, is the standard method for researchers to study bats. The lightweight, nylon nets measure 32 feet long and Gannon most often hangs them approximately 10 feet from the ground. Catching bats with the mist nets allows researchers to band them and record their weight and sex, to check them for parasites and to see whether a female is pregnant.

But it's rare to catch the same bat more than twice. The first time a bat comes across a net, it knows there is something there, but can't identify it. After being trapped, it knows to avoid them in the future. To know a bat's lifespan, researchers need to be able to catch the same bat repeatedly. Also, nets can identify what species are present, but cannot say what species are not present. The endangered Indiana bat, Gannon explained, tends to not be caught as often as other Pennsylvania species of bats simply because the Indiana bat forages for insects out of the nets' range. Researchers might strongly suspect Indiana bats are in an area, but by using only mist nets, they cannot say for certain without capturing one.

Gannon and graduate student Tim Blackburn are working on a new way to study bats by identifying their sonar calls. Bats are not blind, but it is really their ears that enable them to fly so accurately. A bat calls up to 40 times per second, emitting a high-frequency sound from 16,000 hertz (just within the human range of hearing, which stops at 20,000 hertz) to 120,000 hertz. The sound waves reflect off an object, like a bug or a student, and back to the bat's ears. The bat can judge how far away the object is by how long it takes for the call to be reflected back, a process known as echolocation or biosonar. Just talking and listening in the darkness allows bats to find the moths and mosquitoes they eat.

blue sketch of bat flying facing right

Each bat has its own "voice." Yet, just like birds, each bat species has a unique "song," within which are these individual variations. As Gannon explained bats have many different types of calls, such as an alarm call to warn other bats of danger or social calls bats use when interacting with each other. When hunting, a bat will use a call known as the search phase almost exclusively, making it the type of call a bat is most likely to be using when just flying around, Gannon said.

To hear bat calls, scientists have devised electronic bat detectors that bring the high-frequency calls of the bat within the range of human hearing. When magnified, the calls can sound like a series of clicks or sometimes chirps. On a computer screen, a sonogram is generated from a recorded call as a pattern of dots on a graph of frequency versus time. The call of each bat species has a different shape on the graph. A researcher could look at this sonogram and identify the species. However, it takes more than a trained eye (and ear) to separate the calls of different species. The problem is that different researchers have different opinions about the identity of the same unknown bat call. "With some calls you can identify them immediately because they are so distinct, but many of them you can't," Gannon said. "We're trying to devise a system that leaves the decision to the computer and not the human operator."

Gannon and Blackburn are working on a new methodology that combines mist nets and bat detectors. A two-year study Blackburn conducted between 1997 and 1999 found that neither netting nor ultrasonic detection alone identified all the species at his 71 study sites in Pennsylvania and West Virginia. "The hope is that a combined method using nets and detectors will provide more accurate elucidation of bat populations," he said, "and that the detectors can be used to help pinpoint where to concentrate effort."

In the course of his research, Blackburn has analyzed more than 40,000 bat calls recorded with the Anabat II bat detector. "These are not calls from known individuals," he said, "but a percentage of the calls recorded from unknown bats during sampling." In the methodology he is developing, an unknown call is statistically compared to known calls in a "call library," or database that includes Pennsylvania's nine widespread bat species. The call library includes 40 to 400 calls for every species, each of these a clean, high quality recording verified by netting the bat and identifying it.

Unknown calls are compared to the approximately 1,500 calls currently in the call library, Gannon explained. The computer program itself decides which attributes of a call should be compared against the library. For example, calls of some species have a distinct end frequency, while others have noticeably higher average frequencies. If an unknown call matches 85 percent of the characteristics for a particular species in the call library, it is considered a match. The system can identify most calls with 90 percent accuracy, Gannon said.

With this technique, the results are quantifiable and repeatable. Researchers no longer look at images on a computer screen to give a best guess as to the species. "The problem with using the bat detector in the past is it's visual," Blackburn said. You look at it and say 'this is this.' The problem is anything in science that is not quantifiable is questioned."

Now, in Canoe Creek State Park, the bats swoop in and out of the church attic, never quite convinced the weather is good enough to stay out long. Blackburn and the call library are still back at the lab, but Gannon has brought mist nets to capture bats so the students and I can look at them more closely. Contrary to the image that all bats are black, little brown bats vary from pale tan to reddish brown to dark brown, I've read. They range from 3 to 3.7 inches long. Each ear, so essential to echolocation, is the same size as their heads. Bats' wings are nothing like birds' wings. The wings of bats are actually made of skin stretched across their elongated fingers. In humans, it would be like having fingers that dragged on the ground, with skin that ran almost to the fingertips. Cold? Just wrap them around you. Dropped something? No need to bend over. However, flying comes at a price; many bat species cannot walk.

But we will have to forget about a face-to-face meeting tonight. The weather is not cooperating and there are not enough bats out to warrant attempting the nets. It is raining and well below 60 degrees. Bats do not thermoregulate well when they are wet, Gannon explains, meaning it is difficult for them to maintain their body temperature. The students, too, are miserable in the damp cold, even with coats and hats and gloves.

And the bats flying just a foot above us make us nervous. Because we cannot hear the bats, we have no warning when they will swoop toward us. Gannon stands facing us, almost unaware of the small bats flying just to the left or right of him. He tells us about the little-known benefits of bats. Most people are afraid of bats, but bats do us a great service. They act as an all-natural pesticide. A single little brown bat can catch 300 to 3,000 insects per night, and a nursing mother little brown bat eats more than her body weight each night—up to 4,500 insects. Colonies of 1,000 little brown bats, which wouldn't be an uncommon size in the attic of a house, eat 2,600 pounds of insects in a summer. Do the math, Blackburn had said. The average mosquito weighs 5 or 6 milligrams. So, 1,000 mosquitoes weigh 6 grams. There are 454 grams in a pound. You're talking millions of insects being consumed by one colony.

blue sketch of bat with wings extended

"Whoa," a student says. He'd felt the breeze in his hair from the bat that flew just inches above it. Gannon continues, noting that the bats can hear him speak, but are not afraid because people are watching this church all the time. Bats are a keystone species, he says. A keystone is the stone at the top of an arch that holds the structure together. Remove just that one stone, and the arch will collapse.

"In ecology, a keystone species supports a lot of other things in the environment," he says. "If you remove the keystone species, the ecosystem changes dramatically.

"From talking to local farmers we know that they use less pesticides than other farmers growing the same amount and type of crop in other areas of the state," he says. "The bats eat millions and billions of insects over the summer. Now, think what happens when you tear this building down. Then those insects can multiply and damage crops. Think of the economic impact." Loss of habitat is the major threat to bats. If they had lost the church, they might have moved to human homes, where they would most likely have been exterminated.

A maternal colony, like the one in the church, is a group of mothers and pups. Bats only have one or two offspring each year. The pups remain in the cave—or in this case the attic—while the mothers go out to feed. Bats born in May are weaned in the late summer. By now, late September, they can fly on their own.

Before it was known that this attic held one of the largest maternal colonies of little brown bats in the United States, there was talk of tearing the old church down. For 15 years, it had been used for only storage. Then 11 years ago a park official saw bats entering the attic. The state of Pennsylvania bought the church using the Wild Resource Conservation Fund raised from selling conservation license plates.

"It's a very good story for reclaiming things that were used by humans that are now used for wildlife," Gannon says. "The downstairs of the church is in good shape, but it's not used because we don't want to disturb the bats unduly."

As it grows dark, Gannon takes us over to see a hibernacula, a cave where bats spend the winter—in this case, another species, the Indiana bats. The Indiana bat is elusive. It eluded its very discovery in Pennsylvania until 1928. Researchers estimate there are only 500,000 Indiana bats left in existence. The U.S. Fish and Wildlife Service put the mammal on the endangered species list in 1967, but many of the winter caves of the Indiana bat have become unsuitable or destroyed since 1950. Now the bat is nearly extinct over most of its former range of Alabama through New England.

We drive slowly in four-by-four utility vehicles. In the sudden dark, Canoe Creek State Park looks as exotic as the places on the Caribbean Islands where Gannon has traveled to study bats. The cars skid and slip in the mud on the steep, sloping dirt roads.

The Indiana bats live in old limestone mines that are closed to people year round. Hibernation saves bats energy, and if they are awakened, it costs them precious fat reserves. If the bats are disturbed from hibernation just twice during the winter, there is a good chance they will die. Even researchers only enter the caves every other year. The Indiana bat is at particularly high risk because over 85 percent of the entire population hibernate in just a handful of caves. In one cave alone, 80,000 Indiana bats could be roommates for the winter. In tight clusters, up to 485 bats may hang per square foot of cave, dangling by their tiny feet in a giant, sleeping shadow on the stone.

"If just one of those colonies were wiped out," says Gannon, "that would be a significant loss in population."

No one knows where the little brown bats that reside in the church will hibernate. Some of them will share the limestone caves with the Indiana bats, but others travel elsewhere. Where exactly is part of the mystery that draws scientists like Gannon and Blackburn to study bats. While other mammals in Pennsylvania have been heavily researched, there have been relatively few studies on bats.

The thing you have to understand about bats, Blackburn had said, is that there's really no baseline data for many aspects of bat ecology. There are many questions remaining about bats and their habitats. How do bat populations differ between wooded areas and those with water? As altitude increases in Pennsylvania, plants change and so do insects. Do bats change as well? How do bats adapt to the insects in a particular habitat?

In the darkness, the students climb over a wire designed to keep humans away from the Indiana bats' cave. Signs shout ENDANGERED SPECIES and warn of the fine for disrupting the caves. Because Gannon works with the Game Commission, we are one of the few groups with permission to be so close to these caves. We climb up the steep, wet slope toward the mouth of the cave. We shine our flashlights inside, looking for dark spots as small as the palms of our hands.

The Indiana bat could be easily confused for a little brown bat, I'd read. At 2.9 to 3.7 inches long, they are similar in size, but have a few distinguishing features. The Indiana bat has pink lips, while the little brown bats' are dark-colored. The Indiana bats' fur is a dull chestnut that fades to gray, with underparts of pink to cinnamon. The hind feet are smaller than those of the little brown bat and have a distinguishing keel on their calcar (a spur).

I squint and glare, willing my eyes to just see a little further into the cave. The Indiana bat remains hidden.

Outside the caves, Gannon would normally be able to catch 30 bats in a few minutes in mist nets, he tells us. But, now, the bats are staying indoors to avoid the cold, wet conditions. Usually, the researchers might be catching bats for five hours. Not tonight. "If we were out here working, we'd be going home by now," Gannon tells us.

There are a few bats flying around overhead. We hear a flutter here and there, and flashlights act as spotlights on the flying bats overhead. Sometimes, for a few seconds, we manage a glimpse of the tiny, black shadows moving among the trees. Then, Gannon takes out a bat detector the size of a videotape. By adjusting the dials that say "sensitivity" and "division radio" and "volume," we can suddenly hear the series of clicks and chirps that make the bats' ears its eyes. I glance one more time into the dark sky, close my eyes and see them.

Michael Gannon, Ph.D., is an associate professor of biology and teaches Wildlife and Fisheries Science 408 (Mammology) at Penn State Altoona, Smith Building, Altoona, PA 16601-3760; 814-949-5210; Tim Blackburn graduated with a master's in ecology from Penn State. He is now working on his doctorate. His adviser is Michael Gannon. Their research in Pennsylvania is funded by the U.S. Department of Agriculture's Forest Service and the Wild Resource Conservation Fund.

Last Updated January 01, 2002