Fantastic Bats!

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As they fly through the night, microbats emit a pulsating stream of high-frequency squeaks and clicks through either their nose or mouth, depending on the species. When the sound waves strike an obstacle or object—say, a tree limb or a tasty blood-engorged mosquito — they bounce back to the bat as echoes. The bat instantly processes the data and responds accordingly: swerving in the case of the branch, attacking in the case of the juicy mosquito, and often doing both at the same time. Bats use this sonarlike system, called echolocation, for both navigation and hunting. It is amazingly sophisticated and precise: An echolocating bat can detect objects as small as a human hair; it can use riverbanks, vegetation and other terrain features as acoustic landmarks; and it can determine not only a target’s speed and direction, but also its size and surface texture. Researchers have found that bats can perceive not just an object’s position, but also its 3-D form. The echo from a deciduous tree is different from the echo of an evergreen. The echo from an unappetizing hard-shelled beetle is different from a tender gnat.

Scientists are struggling to understand fully how echolocation works. They do know that different bat species emit sounds in different ways, using varying combinations of frequencies, harmonics and intensities. As a bat searches for prey, it sends out a relatively low number of sound pulses; typically about 10 per second. When it detects prey and flies progressively closer, it speeds up its clicks and shortens their duration. Just before the bat strikes, the pulses may be only a fraction of a millisecond long and may number as many as 200 per second; scientists call this a “feeding buzz.”

Most bat sounds are well above the range of human hearing, so as we watch them diving and swooping at night, we’re not aware of the volume of their screams. Bats that hunt in the open, such as the big brown bat, generate amazingly high-volume sound: 110 decibels measured at 4 inches from the bat’s mouth — the loudness of a smoke detector 4 inches from your ears. Bats that forage in closer quarters, such as a forest, emit low-intensity sound: about 60 decibels, or the level of normal human conversation.

We can’t hear bats because we’re only human. But some insects can, moths in particular. They’ve evolved their own bat radar and will dive for cover when they detect a bat’s clicks. Some moth species, in fact, can make their own ultrasonic clicks that effectively “jam” the bat’s sounds and confuse it.

Beauty in the Beasts

Take another look at those bizarre, alienlike folds and wrinkles on insect-eating bats’ faces, and at the odd, cone-shaped growth (called a tragus) projecting from each ear on most species — they’re all parts of the animals’ echolocating bug detectors. And those tiny, sharp, daggerlike teeth? They’re for piercing the exoskeletons of insects, not for biting humans or other animals. Microbats are the world’s single most effective controllers of flying-insect populations. Just one big brown bat can consume up to 1,200 mosquitoes an hour. Those 20 million Mexican free-tailed bats in Bracken Cave gobble up 200 tons of insects every night. Fruit- and pollen-eating megabats, meanwhile, are critical pollinators of countless tropical plants. Fish- and frog-hunting tropical bats play critical roles in their ecosystems, too.

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