Guess Who's Coming to Dinner

With sophisticated ultrasonic measures and countermeasures, bats and their prey conduct elaborate war games

02-01-1993 // Bruce Fellman

Under A pitch-dark sky, a sleek fighter cruises. Its sonar sweeps through the night and unerringly locates a highflying target. But hold on: The sonar screen is suddenly a blur! And now the target is gone! Persian Gulf images to the contrary, this is not a scene from the high-tech annals of Operation Desert Storm. Rather, the fighter is a mere bat, a flying mammal that for the last 50 million years has used sophisticated sonar (high-frequency sound pulses) to navigate and find its prey in the dark with what has seemed to be exquisite precision.

Now scientists are discovering that bats are not alone: A number of the insects that comprise bat dinner have remarkable abilities of their own to drive their predators, well, batty. "It's a war up there," says James A. Simmons, a biologist at Brown University in Providence, Rhode Island. "The bats and their prey are locked in a battle of 'electronic' measures and countermeasures." This choreography offers scientists insights into the basic biology of hearing and perception. And, yes, it even intrigues the U.S. Office of Naval Research and at least one defense contractor, Raytheon, enough to help fund some bat research.

Researchers have known for 200 years that bats don't need vision to find their way in the dark. The nature of this sixth sense, however, remained a mystery until the 1940s, when Harvard undergraduate Donald Griffin showed an incredulous scientific world that the creatures produce beams of ultrasonic sound waves and then use the returning echoes to "see" at night-an ability he dubbed "echolocation."

The human ear hears few of these predatory deliberations, for most of the bats that use echolocation (well over 80 percent of the 900 or so bat species), emit sonar cries too high-pitched for us to detect. Some bats, including two Southwest species in the United States, do emit sonar just within the range of human hearing.

Humans can't hear sounds above 20 kilohertz (kHz). A number of insect species, in contrast, can detect bat sonar, which is commonly in the 20 to 60 kHz range. The bugs do so with ears similar in basic structure to our own, but with the critical difference that they are tuned to detect higher frequencies. University of Toronto researcher James H. Fullard explains that in comparison to our own complicated system, with its 15,000 to 18,000 cells and delicate anatomy, the insect ear is the "absolute, pared-down, super-economy model"-generally little more than a tiny drum with a handful of nerve cells.

Some insects, including crickets, can hear a range of sounds, from their own communications to bats' high pitches. But the ears of lacewings, praying mantises and the vast majority of moths hear only high pitches. "They're simply bat detectors," says Fullard.

A bat on a reconnaissance mission sends out ultrasonic "clicks" at the rate of 10 to 20 per second. As the hunter detects a possible meal, it moves in rapidly and increases its call rate-presumably for accuracy-which peaks at many hundreds of clicks each second just before the bat snatches its prey.

Fortunately for insects, bats are relatively nearsighted. "Echolocation is a short-range operation," says M. Brock Fenton, a bat biologist at York University in Toronto, Canada. "Most hunting bats are collecting information about the air space 5 to 10 meters in front of themselves." Beyond that range, the echos quickly grow too weak for the bat to pick up. But a moth can hear the bat's initial cries perfectly, even when they come from a distance of 40 meters or more. "Well before the moth even comes up on the bat's sonar screen, the prey knows the predator is there," says Fenton.

When the listener hears the scattered clicks of a cruising bat, the time-honored tactic is to fly away from the sound. But when the insect hears the click rate increase, it knows an attack is imminent-and that it's time for more intricate evasive maneuvers.

The delicate, slow-flying lacewing, for example, changes its flight path by folding its wings into a V and nosediving from the sky to the safety of the underbrush. If this ploy doesn't work, and the click rate tells the insect the bat is in hot pursuit, the plummeting lacewing unfurls its wings to put on air brakes-and sometimes succeeds in throwing off the bat's timing.

The praying mantis practices a variation on the evasion theme-though in about a third of all mantis species, only the males have ears. This makes good evolutionary sense, says David D. Yager, the University of Maryland scientist who has studied these insects. In these sexually dimorphic species, the males are the fliers that nightly risk their lives in the air. The earless females lead a relatively sedentary existence, presumably out of harm's way.

When a mantis hears a distant bat, it too simply turns away. But as the insect detects a rising click rate, "it makes a very steep, sudden, banking roll, then it spirals straight down," says Yager. "Unlike the lacewing maneuvers, these are power dives. The mantis keeps flapping, and it accelerates to twice its original flight speed." With only one ear, mantises don't appear to be able to tell which direction the ultrasonic clicks are coming from. But these animals compensate with an unpredictable roll and downward spin. "Randomness is just what you want," says Yager.

Moths are also unpredictable, executing a wild array of barrel rolls, loop-theloops and power dives when clicks signal an attacking bat. And at least one group has carried defense to an entirely new level. Scientists have learned that tiger moths, an unpleasant tasting (to bats) group of insects more properly known as arctiids, generate their own ultrasonic clicks. "These clicks may startle the bat, or they may warn the hunter that the moth is distasteful," says Lee A. Miller of Odense University in Denmark. They also sometimes seem to interfere with the bat's ability to pinpoint prey-essentially jamming the sonar.

If a bat has never heard the arctiids' sounds, or if it doesn't encounter the clicking moths very often, the hunter's response is no different than yours would be if someone unexpectedly yelled "Boo!" at you in the dark. The startled predator pulls up short and beats a hasty retreat.

"We actually had a bat jump up and fly out of its cage when it heard the clicks," says Miller. "But if the moths keep clicking, the bats soon habituate to the noise. It's just like crows on the side of a road. If it's deserted and a single car drives by, that gets their attention. When the road is busy, though, they learn to ignore the cars."

If a bat captures an arctiid, it quickly learns it has made a big mistake. Tiger moths taste vile, due to the plants the insects consume as caterpillars, and the bats immediately learn to associate clicks with wretched meals. And because the hunters have long memories, a predator that has had an arctiid experience only has to hear the charactistic clicks of moth ultrasound, and it will look elsewhere for a meal.

In 1979, Fenton, Fullard and Simmons found that the clicks could do more than startle the predators and advertise arctiid distastefulness. The scientists suspected that because the noises resembled the hunter's own sonar signals, perhaps a clicking tiger moth was flooding a bat's brain with "ghosts." In the past few years, however, Lee Miller has shown that though the hunter is indeed confused, the confusion is anything but ghostly. "It looks like the bat still knows there's one object out there," he says, "but it can't determine where it is."

The defense is not foolproof. The Danish scientist discovered that the sonar-jamming clicks worked only when they arrived at a hunter's ears precisely at the beginning of the bat's returning echo. That window of opportunity is open for only a thousandth of a second. "If the arctiid gets its clicks in through that window, the bat makes mistakes," says Miller. "But if the sounds arrive anywhere else on the echo, they're not there for the bat."

And so bats still catch insects despite he sophisticated evasion tactics. On any given night, bats will succeed in capturing more than half the prey they attack. Their main advantage, of course, is their sonar. But there are also "whispering" bats that insects never hear, as well as hunters that use sonar frequencies either below or above the detection range of their prey. Fullard has studied the so-called "gleaning" bats, tropical bats that rely on ultrahigh frequency sonar (as high as 212 kHz) to capture insects hiding in dense vegetation. "The moths never hear the bats coming," says Fullard.

One insect counter-counterstrategy is to be active at times when bats aren't likely to be hunting. Another is to stick to the relative safety of shrubs. And then there's a moth Fullard examined in Panama, a country loaded with gleaning bats-the tropical equivalent of stealth fighters. The bats hover for an instant as they get close to the moths, which in this case can't hear high-frequency sound but can detect whiffs of air from beating , wings. "This hovering," says Fullard, "gives the moth a last-ditch chance to escape." The insect, which then drops to the ground, is covered with sensors that monitor wind movement.

No doubt, at this very moment, a bat is developing a counter-counter-counterstrategy. As Fullard puts it, "Evolution never stops."

Bruce Fellman is a Connecticut free-lance writer.

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