Wildlife Spy: The scream you can't hear

TIP: Watch how bats fly to see them catch who they really terrorize: insects.

The air is full of screams ... you just can’t hear them. From spring through early fall, bats fly over Southeast Alaska, using high-pitched sounds to track down their insect prey. With their pug-noses and gargoyle-like ears bats may look like a Halloween joke, but don’t fall for the trick. They are experts at aerial acrobatics and masters at snatching insects with deadly accuracy.

 

Bats are some of the loudest creatures you’ll never hear. They hunt at dusk and in low light, so they rely heavily on echolocation, using reflected sound to sense the world around them. Out of about 1,000 species of bats worldwide, roughly 70 percent rely on echolocation; six species live in Southeast Alaska. Luckily for us, all of Alaska’s bats eat insects. A single little brown bat, the only species that ranges as far as Interior and Western Alaska, can catch more than 600 mosquitoes an hour! Birds can’t hear in the high-frequency range that bats do, so echolocation has allowed bats to reign as aerial hunters of the night.

Bats have different echolocation strategies: the screamers and the chatterboxes. Some bats use volume, anywhere from 110 to 130 decibels — a rock concert is about 115 decibels! The sneaky part is bats use frequencies too high for most insects, and humans, to hear. These bats clench muscles in their ears to block sound when they call to prevent ear damage. The bat gives a loud click or chirp, then relaxes its ears to hear the returning echoes as sound waves bounce off objects. The time it takes for the bat to hear the echo allows it to estimate an object’s distance.

Another echolocation method involves calling constantly. These bats hear best within a certain range of frequencies, but make sounds outside that range to avoid deafening themselves. Sounds change depending on if they are moving toward or away from you, a phenomenon called the Doppler shift. When you hear the noise of a car change pitch as it approaches and then zips past, you’re experiencing the way a bat uses this type of echolocation to determine the direction and speed of prey.

It’s thought bats use a combination of vision and echolocation when hunting. Echolocation is extraordinarily accurate for close objects, such as tasty mosquitoes or spiders, that are difficult to spy in low light. Because bats have to hear the sound waves that bounce back to use echolocation, beyond a few hundred feet it is useless. Even in low light, however, they can use visual cues for navigation and orientation beyond the range of their echolocation.

Echolocation goes hand-in-wing with flying skills. For many years, it was assumed bats flew like birds, but closer examination shows they have their own style.

In birds, the entire wing powers flight while wings and tail provide steering. Birds have hollow, light bones that are strong for their weight. One of the ways this works is that some bones have fused to develop a skeleton that can withstand the strain of flight, most notably in bird wings. Most of the wrist bones are one unit, and the “finger” bones have mostly merged to support the outer portion of the wing. Birds have fine control of the long primary flight feathers at the tip of the wings, able to shift them independently of each other. Feathers create a smooth cover for the wing, growing from the front to the back of the wing. Although feathers may shift and flex, the wing itself only bends at the shoulder, elbow, and wrist joints.

Bats, the only mammals capable of flapping flight, developed a different structure than birds. Bats belong to the order Chiroptera, which means “hand-wing.” Unlike birds, the bones of the hand lengthened to form the structure of bat wings, giving them many more joints and much finer control of the wings than birds. Recent studies have found that the bones of the bat’s fingers have lower concentrations of calcium and other minerals than the upper wing. This means the bones are more pliant and flexible in the outer part of the wing, where the bat generates the most power. This may help in their acrobatics.

Instead of feathers, bats wings have a thin membrane of skin and muscle, called the patagium, stretched across the elongated “hand” and shortened arm. The hand portion of the wing provides power for flight. Although bat wings aren’t furry, specialized hairs sprinkled across the wing surface connect with nerves similar to those in your fingertips, allowing the bat to sense air moving across its wings.

A membrane also connects a bat’s back legs. Many insect-hunting bats capture flying prey by scooping it up with the membrane between the back legs and bringing to where the bat can eat its meal. These bats can also sense the stretching of the skin to determine if they caught their insect prey.

When most birds flap their wings — hummingbirds are an exception — they generate lift on the downstroke of the wing. By rotating the wing on the upstroke and spreading the flight feathers, the bird minimizes air resistance until the wing returns downward. Bats, from the extraordinary flexibility of their wing structure, can bend and twist their wings as they flap so they can generate lift on the upstroke as well, making them efficient flyers. By bending different parts of the wing, bats are extremely maneuverable, all the better to pursue insects.

Bats of Southeast Alaska still hold many secrets. The first step is simply to find where the bats are and when. This past summer, Michael Kohan, of the Alaska Department of Fish and Game, as part of the Alaska Bat Monitoring Program, had citizen science volunteers in Haines and Gustavus using a gadget Batman would envy: a bat detector. She described the device as about the size of a soy milk container. It uses a microphone and then converts bat calls into a range people can hear and plays the sounds on a speaker. This year, volunteers mounted the microphones on the outside of their cars and drove 30-mile survey routes, listening for bats to give themselves away.

The bat detectors record any bat sounds, which are later analyzed individually to determine the species and activity of the bats.

“You can hear the bats in real time,” Kohan said. “It sounds like rapid clicking, with different buzzes and beeps for different activities, such as foraging versus flying.”

Although there are a limited number of bat detectors, volunteers can share equipment to cover different survey routes.

“Librarians in Gustavus (Kate Boesser) and Haines (Jessie Morgan) are the reasons why this Southeast program can work. They are outfitted with a driving survey kit and they recruit community members to participate in the program.” Kohan said.

Next year, Kohan hopes to expand the program to Sitka and perhaps Yakutat and Petersburg. She is preparing to do presentations for this year’s volunteers.

“It’s exciting to see it come full circle and give a presentation of results,” she said.

The volunteers help find areas to focus future monitoring or that might prove fruitful for catching bats. The ADFG research team has outfitted some bats with miniature radio transmitters to track where the tiny creatures forage and where they spend the winter. So far, little brown bats in Southeast have been found overwintering in rock slides or tangles of tree roots.

As you dress up in Halloween finery, think about the tiny mysterious beasts out in the woods, more amazing than any fairy tale.

For more information on participating as a bat observer, or for forms to report bat sightings, visit the Citizen Science page of the Alaska Department of Fish and Game website (http://www.adfg.alaska.gov/index.cfm?adfg=citizenscience.bats)or contact: Michael Kohan, 907-465-8155, P.O. Box 110024, Juneau, AK 99811, or by email at michael.kohan@alaska.gov.

• Beth Peluso is a freelance writer, illustrator, and avid birder. She enjoys spying on wildlife around Alaska. She now really wants to try a bat detector.

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