Frequency fliers 

Bats are blind, and other myths

Sitting quietly on the front stoop as a lingering summer evening fades to night I suddenly see a few dark shapes dart out of nowhere and flit crazily, noiselessly overhead. The hour of the bat has arrived.

I know that these creatures of fairy tale and horror story will not get tangled in my hair or suck my blood, but I still admit to a few involuntary flinches—until my curiosity, that age-old courage pill, takes over.

These night-time hunters could be the little brown myotis (myotis meaning "mouse-eared") that sometimes hide out in attics or under eaves during the day, then emerge at dusk to feed on swarms of aquatic insects like midges and mosquitoes. With three-inch bodies and 10-inch wings, these little guys are super-maneuverable, zigging and zagging in pursuit of erratic prey in bursts of up to 20 miles per hour.

click to enlarge BRUCE D. TAUBERT
  • Bruce D. Taubert

Whatever kind they are, the bats are master hunters, thanks to their wonderfully adapted bodies and wings. The wings of bats—the only mammals capable of true flight—are marvels of invention. Although not as efficient as bird wings for sustained flight, they are far more elastic. Imagine tiny forearms sprouting from your shoulders and ending in fingers as long as your body. These bony digits are like the ribs of an umbrella that allow the wing to open and collapse instantly for tricky twists and turns. Instead of feathers, a skin-like membrane stretches between a bat's fingers and tail, creating a mean catcher's mitt that can scoop up prey on the wing, contain it and flip it into an open mouth. I get dizzy watching them above me, executing dives, rolls, loops and about-faces without pause.

Montana has a bunch of different bat species. The largest is the hoary bat, so named for its grizzled coat; the smallest is the palm-sized western small-footed myotis. Some, like the unimaginatively named big brown bat, are relatively abundant and widespread; others, like the spotted bat, are among the rarest species in the state.

Spotted bats are not only rare, but also flashy-looking. A spotted bat's ears are huge, about half the length of its body, and its fur is dark with large white spots on rump and shoulders, a triad like the pawnbrokers' symbol. The animals mostly live in the southern portion of Montana, where they roost in caves and crevasses of cliffs and forage over rough, dry ground. You have a much better chance of seeing a spotted bat this summer if you're camping, say, in the Bighorn Canyon National Recreation Area, where they have been, er, spotted. In fact, you might even hear one.

Spotted bats are unusual in that the metallic clicking sounds they make to target prey are in the range that humans can hear.

Most bats have perfectly good eyesight, but it's sound and echo they use to navigate in unbelievably precise ways.

According to University of Montana ecology professor Erick Greene, when some bats are hunting they produce constant frequency clicks and frequency modulated, or FM, clicks. When constant frequency clicks bounce off an object, a bat can tell roughly where the object is in the air and whether it's coming closer or moving away. Once a bat is "locked on" to a target, Greene says, it speeds up the number of clicks and switches over to FM "sweeps" that go up or down in pitch depending on the species. From these return echoes, bats can determine the size, shape, even texture of an object.

Just so you don't feel too sorry for the pursued moth or beetle, Greene says many insects can hear bat sonar. Sometimes they detect the bat before it homes in, and fly out of its way. But if a bat is approaching fast, bugs have their own repertoire of evasive maneuvers. Some can even produce their own ultrasounds that either are extremely loud to a bat and deafen its sensitive ears, or that sound to bats like many different objects moving in different directions. Greene calls it "sonic shrapnel."

My evening entertainers' flight might look chaotic, but it's actually highly directed, and not at all noiseless—I just can't hear it. As professor Greene says, I'm tuned to the wrong channel.

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