An Acoustic Arms Race
Bats and other animals use sound as a hunting tool—but their prey has also evolved ways to thwart detection
Animals hunt using sound in two distinct ways. Some listen passively for the noises produced by their prey. If you have ever watched an owl strike a vole moving under a blanket of dry leaves or snow, you have observed the effectiveness of passive listening. In contrast, most bats and some toothed whales (including dolphins) are active listeners. They project sounds into their surroundings and detect telltale echoes, a process called echolocation or biological sonar. The echoes allow them to orient within their environment and also to detect and track prey; in essence they “see” using sound. Echolocation evolved more than 65 million years ago in bats, and more recently in toothed whales. Biological sonar is a marvel of sophistication that continues to inspire engineers who develop its technological counterparts, the radar and sonar systems used by ground-based stations as well as airborne and underwater vehicles. The echolocation of animals and the radar and sonar systems of humans show extraordinary parallels in how signals are produced, transmitted, received and processed. Perhaps most interesting of all, both paths culminate in countermeasures that include stealth technology and signal jamming.
Radar, which originally was an acronym for “radio detection and ranging,” uses pulses of radio waves as the signal that is sent out; an antenna detects the reflections of the signal off solid objects. If the object is moving, the reflected signals will be shifted in frequency, allowing detection of the target’s velocity. Radio waves are used because they can travel long distances in air, even in the presence of fog or precipitation. Sound waves propagate better underwater, hence the development of sonar (“sound navigation and ranging”) for aquatic use. Other than the difference in the signal used, it operates on similar principles to radar.
In his 2007 book Blip, Ping, and Buzz: Making Sense of Radar and Sonar, physicist Mark Denny also was interested in comparing the remote sensing technologies of humans and nonhuman animals. As Denny describes, the history of the development of radar is peppered with such familiar names as Nikola Tesla, the great Serbian-American inventor, and Guglielmo Marconi, the Italian-British engineer who first transmitted a radio signal across the Atlantic. The tale also includes less well-known contributors from around the globe—the many fathers of radar. The development of functional radar systems was driven and accelerated by the approach of World War II. The earliest of these was a series of radar stations called the Chain Home system along the southern and eastern coasts of England. The stations were an early warning system that alerted the British that German bombers were massing in the airspace over France, which allowed the Royal Air Force to meet the incoming waves with Spitfire and Hurricane fighters already at high altitude.
The development of human-designed sonar predates the development of radar by some 30 years, but it too was a technological response to weapons of warfare, specifically the submarines of World War I. The first devices were passive-listening arrays of underwater microphones (or hydrophones) developed for German ships such as the heavy cruiser Prinz Eugen, which could both detect the sounds of approaching torpedoes and target distant ships. The sinking of the Titanic likewise spurred the development of sonar because the technology could also detect icebergs in darkness and fog. The years between the two world wars saw the development of active listening, or true sonar. By the beginning of World War II most U.S. and British warships carried anti-submarine sonar.
Researchers studying biological sonar and human-produced devices frequently crossed paths. Sir Hiram Maxim, a prolific American-British inventor of the early 20th century, proposed developing a batlike system to protect oceangoing ships from collisions. Unfortunately, knowledge of bat echolocation was rudimentary at the time, and he failed to produce a functional device. Maxim thought that bats were using low- frequency signals produced by their flapping wings to orient themselves. George Washington Pierce—who took a leave of absence from the physics department at Harvard University to work in the Anti-Submarine Laboratory of the U.S. Navy at New London, Connecticut— later assisted Harvard zoologist Donald Griffin in determining the true nature of bat echolocation. Pierce developed a microphone based on piezoelectric materials (which produce electricity in response to mechanical stress) that allowed Griffin to be the first to record the ultrasonic cries of bats, which form the basis of their echolocation system.
The processes of radar and sonar, either biological or mechanical, begin with the production of a signal punctuated in pulses. The basic rule of signal production is that the wavelength of the signal is proportional to the size of the structure producing it. Early radar devices had a wavelength of 12 meters (the long wavelength permitted detection of objects at farther distances), which meant that the device that produced it had to be very large. The Chain Home stations consisted of four towers, each 110 meters high and 55 meters apart, with a net of magnetized steel cables strung between them. The cables produced the outgoing signal. The vocal cords of a bat, in contrast, are tiny and produce sound waves that have very short wavelengths (see box in Figure 1 for the relationship between frequency, wavelength and the speed of propagation).
A second relationship is also important: The shorter the wavelength, the higher the resolution. Long wavelengths can efficiently detect large objects such as ships at sea, but they are less effective at resolving smaller targets. Indeed, the early radars had difficulty distinguishing the number of incoming planes; they could only warn that some were coming. Bats, with their short-wavelength sonar, can detect items as tiny as the mosquitoes, beetles and moths that make up their diets. Jim Simmons at Brown University has gathered evidence that bats are capable of sensing at a resolution down to the micrometer scale, which would allow them to detect even the texture of the surface of their targets.
A radar device is composed of an array of elements, or transmitters. Together they emit signals in the form of a beam much like the beam of a flashlight. Narrow beams are desirable because they allow the sending device to concentrate its power in one direction and thus detect more distant targets. Narrowly focused beams also allow target direction to be determined with greater accuracy. Radar engineers can control the shape of the beam by manipulating the distance between the transmitting elements of the antenna and the total length of the antenna. Animals can also control beam shape. A recent collaboration between Rolf Müller at Virginia Tech, Zhiwei Zhang of Shandong University in China and Son Nguyen Truong of the Vietnamese Academy of Sciences determined that the exotic nose “leaves” of Bourret’s horseshoe bat allow them to produce a highly focused sonar beam, optimizing their ability to detect insect prey.
One could also imagine situations in which it would be useful to control beam shape dynamically. Some flashlights, for example, allow the user to vary the beam shape. Although narrow beam shapes are great for finding distant targets and detecting target direction, wider beams allow the user to scan a larger, but closer, area. Some toothed whales—most notably beluga whales, but also recently discovered in false killer whales—have the ability to focus their echolocation beam shape by adjusting the shape of an oil-filled acoustic lens on their forehead called the melon. Recent research by Lasse Jakobsen, John Ratcliffe and Annemarie Surlykke of the Sound Communication Group at the University of Southern Denmark has shown that bats can also vary their sonar view in adaptive ways. By opening their mouths wide and increasing call frequency, they can produce a narrower beam for probing at a distance; by doing the converse they can produce a broader beam for sampling a wider area.
Other tricks of the trade have been discovered by both engineers and their nonhuman counterparts. For example, there are notable advantages to producing a signal that sweeps across frequencies. This so-called broadband chirp can increase the range resolution of a system by two orders of magnitude. The distinctive sweep from high to low frequency of a frequency-modulating bat (sometimes called an FM bat) accomplishes this range resolution beautifully. FM bats hunt in complex environments, catching their prey between branches of trees and on top of vegetation or even off the surface of water. Other bats produce a longer, constant frequency signal (and are called CF bats). The advantage for these bats is that the constant frequency allows the use of Doppler shifts to measure the relative velocity of prey. Like the increase in the frequency of a train whistle as it approaches you, the bat can detect the increase in the frequency of the echo from a moth moving in its direction.
Although we once classified bats into groups based on their FM and CF calls, it has become obvious that bats are better than that. Some species have the best of both worlds. They use narrow-band signals for the detection of prey at a distance and then switch to frequency-modulated (broadband) signals as they move in for the kill, when range resolution becomes more critical for the catch.
The transmission characteristics of the medium determine whether electromagnetic radiation or sound makes for a better signal. Electromagnetic waves (in radar) travel through air with little attenuation, or signal loss. Sound travels far less well in air and is more subject to the vagaries of the environment such as wind and rain; in addition it’s also wavelength (and frequency) dependent. Infrasound (with frequencies of less than 25 cycles per second) travels relatively far in air, and elephants and other large land animals use it for long-distance communication (over ranges of about 2 kilometers). In contrast, the ultrasound used by bats (with frequencies of over 20,000 cycles per second) is quickly absorbed by air molecules, which limits its effective use to just meters.
Underwater, however, sound is king. It can travel great distances and is absorbed at a rate of only about 1 percent per kilometer. Because of this characteristic, whales can likely communicate across the vastness of oceans. Long-distance communication underwater is facilitated by the presence of naturally occurring sound transmission channels at different depths: As one goes deeper, the speed of sound is affected by changes in temperature, salinity and pressure. Gradients in the speed of sound bend the sound waves, focusing them at specific depths, effectively creating channels. In the ocean there is a shallow channel where the sound waves are bent toward the surface and bounce along it, allowing communication over longer distances. At greater depths there is a second channel (called the SOFAR, for “sound fixing and ranging”) that traps sounds at a depth and projects them laterally rather than spherically. Both channels facilitate sound transmission because the normal three- dimensional spreading of sound is limited to two dimensions. Both submariners and marine mammals take advantage of the sound transmission channels for more efficient use of sound power.
Electromagnetic radiation works fine in air, but most wavelengths are absorbed so strongly by water (99.99 percent by one meter of water) that it is not of much use in aquatic media. The exception to this rule is the narrow wavelength band from 400 to 700 nanometers that corresponds to visible light. Aquatic organisms take full advantage of this transmission window for their visual communication.
The sensors used in radar and sonar, respectively, are dipole antennae (for example, like the “rabbit ears” on an old-style television) and piezoelectric hydrophones. Bat ears are typical mammalian sound detectors with an eardrum that vibrates in sympathy with airborne sound and a fluid-filled cochlea that converts the mechanical vibrations into the electrical language of the nervous system. The middle ear bones, called the ossicular chain, match the incoming signal’s impedance—a measure of the resistance a signal experiences when it tries to enter a system. This matching allows for more efficient transmission of the sound vibrations from the air to the fluids within the cochlea. The large, mobile pinnae (the outer, visible part of the ears) are the most conspicuous features of the sound detection system. They function as parabolic reflectors, funneling sound into the ear canal. Here again we see the size rule: In many bat species the size of the ear is matched to the frequency range of the sonar signals they produce. High frequency means smaller ears; low frequency leads to larger ears. The largest ears are found in gleaning bats, which tend to hunt prey from ground and water surfaces, and use passive listening to find their targets (as opposed to aerial hawking bats, which seek moths in flight). They can hear the beat of a moth’s wings or the footsteps of a centipede walking nearby. The pinnae also play an important role in sound localization. Each acts as a directional acoustic antenna with the strength of incoming signal each receives dependent on the azimuth (or its horizontal angle in relation to the direction the bat is facing) and elevation of the sound. The fact that there are two ears allows for comparisons of time of arrival, phase and intensity of the signal to localize sound.
The bat inner ear follows the basic mammalian plan with a basilar membrane—a stiff structure that separates the two fluid-filled coils of the cochlea—that contains inner hair cells, which respond to vibration. The basilar membrane is laid out like a reversed piano keyboard, with its stiffer and narrower base vibrating in response to the high frequencies and its less stiff and broader apex vibrating in sympathy with low frequencies; a gradual transition between the two conditions occurs along the length of the membrane. The basilar membrane thus carries out a frequency analysis of any incoming signal. The hair cells along the length of the basilar membrane then transmit this information through the auditory nerve to the brain for further processing. The most interesting specialization of the basilar membrane is found in constant frequency bats. In these animals a disproportional amount of the length of the basilar membrane is devoted to a narrow band of frequencies around the constant frequency of the bat’s call. This part of the basilar membrane has been referred to as an “auditory fovea” in parallel to the part of the eye’s retina that contains a concentration of light receptors. Its presence indicates that these bats have an enhanced resolution of frequencies for the detection of Doppler-shifted echoes.
Knowing the precise time of arrival of an echo is critical because it allows the bat to determine the target range. Picking an echo out of background noise is a formidable task. The intensity of the echo may be 1,000 times less powerful than the background noise. The task is rendered possible by a mathematical function called cross-correlation. The transmitter keeps a copy of the outgoing signal and then continuously compares what it hears to the copy until it gets a good match. The match is achieved by multiplying the copy times the input; when everything matches up nicely the product of the two curves reaches a peak telling the receiver that the echo has arrived. Here’s a visual way to understand the process: Hold your two hands in front of your face, one facing toward you and one away. One hand represents the copy of the output signal and one respresents the echo. Slide your two hands past each other from right to left. It is obvious when the two hands match; you have a good cross-correlation and the echo has arrived. This method becomes even more effective when the outgoing pulse is frequency modulated, in the form of a chirp.
Doppler shift processing is immensely powerful. It allows the receiver to have an additional piece of information beyond azimuth, elevation and range—it gives relative speed as well. Imagine a bat flying over the landscape. All background items such as ground, trees and bushes will have a relative speed equal to the flight speed of the bat. Now imagine a lone moth flying toward the bat. It will have a different Doppler-shifted frequency and will stand out as a potential target. Constant frequency bats dedicate an inordinately large portion of their brain and processing power to measuring tiny Doppler shifts. Researchers in the laboratory of Han-Ulrich Schnitzler in Germany have shown that the mustached bat can use the Doppler shift to detect the movement of a moth’s wings as it flutters—information potentially useful in distinguishing prey types.
The bane of sonar and radar reception is clutter—echoes from nontarget items. Clutter includes the echoes returning from rain (so-called volume clutter) or background surfaces—anything that decreases the signal-to-noise ratio. Imagine attempting to detect a low-flying plane from above when it is flying against a background of city buildings giving off myriad complex echoes. For a bat this situation would be the equivalent of detecting a moth as it flies near a bush, with each leaf presenting a confusing echo. Again Doppler shifts come to the rescue, but a second mechanism is also useful. Some bats send out sounds in pairs (or strobe groups). By altering the frequency of the first and second emissions, the bat can keep track of and sort out the incoming echoes more efficiently, rendering clutter less effective.
Nonetheless, some insect prey take advantage of clutter by hiding in it. Earless ghost swift moths become “invisible” to echolocating bats by forming mating clusters close (less than half a meter) above vegetation and effectively blending into the clutter of echoes that the bat receives from the leaves and stems around them. Many insects probably use this strategy, which is a close analogy to crypsis in the visible world—camouflage and other methods for blending into one’s visual background.
Radar and sonar engineers are in an ever-escalating competition. Each improvement in target detection generates efforts to design a countermeasure to make detection more difficult. Most recently engineers have made efforts to reduce the radar cross-section (RCS), a measure of the detectability of a target, to minuscule proportions. We know the result as stealth aircraft. One seldom attends an air show or large sporting event without being treated to a flyover by an impressively strange-shaped B-2 stealth bomber. Its RCS has been decreased by hiding bulky engines and control surfaces inside the wing, emphasizing angles that deflect reflections away from radar receivers, and using composites and paints that absorb or otherwise impede radar reflections. It is rumored that stealth bombers have an RCS as small as a postage stamp.
Not to be outdone, insects can play the stealth game, too. Some prey insects may use stealthlike mechanisms to dampen their echo signature to bats. Jinyao Zeng, Shuyi Zhang and their colleagues at East China Normal University in Shanghai have suggested that the scales on moth wings may decrease the amplitude of the echoes they return to bats by absorbing the bats’ echolocation cries. This feature would give the moth a small but significant advantage in avoiding detection. The scales of the nocturnal moths more than double the absorption factor of the wings for sounds at frequencies between 40 and 60 kilohertz, resulting in a decrease of echo intensity of up to 2 decibels over scaleless wings—and making it more difficult for the bat to detect the moth at a distance. Control butterfly wings, which also have scales, did not show this effect. Although the mechanism of sound absorption remains to be determined, moth scales frequently have spaces between them and are typically covered with micropores and lacunae (oblong spaces) reminiscent of manmade sound-absorbing materials. It seems likely that researchers have just scratched the surface in this regard, and many more stealthlike examples will be found that foil echolocating predators.
As soon as engineers developed effective sonar and radar, others set about thwarting it by interfering with the reception or processing of echoes—a process called jamming. Two organizations have contributed enormously to this electronic warfare: the legendary Lockheed Martin “Skunk Works” and a nonprofit international organization called the Association of Old Crows (the name is a play on the Allied radar equipment and operators in World War II, which were known by the code name Raven). Both groups have developed electronic countermeasures and counter-countermeasures in the military arena. This escalation of ploy and counterploy is reminiscent of the evolutionary arms races that fascinate biologists.
Methods of jamming come in two varieties: passive and active. Passive methods include chaff, materials (such as thin aluminum strips or metalized glass fibers) jettisoned by an aircraft to confuse enemy radar about the aircraft’s precise location and movements. Active methods are electronic signals designed to blind or delude the tracking radar. Noise jammers overwhelm the radar receiver with powerful electronic noise that makes it difficult for the receiver to detect the relatively faint target echo. A repeater jammer provides a copy of the real echo but with inappropriate timing, seducing the receiver into detecting a “phantom” object headed in the wrong direction. Active jamming is a tricky business, because the jammer can inadvertently give the radar receiver a new signal to lock onto.
It might seem unlikely that insects could play similar tricks on echolocating bats, but the coevolutionary arms race of bats and insects has been going on for 65 million years—plenty of time to develop sophisticated measures and countermeasures. Members of my laboratory discovered that Ecuadorian tiger moths in the genus Bertholdia (of the subfamily Arctiinae and family Erebidae) produce a cacophony of clicks when they are targeted by an echolocating bat. The moths intercept the sonar signals of an approaching predator using “bat detectors”—ears tuned to high frequencies—and answer them. The anti-bat sounds are produced by blisters of cuticle called tymbal organs located on either side of the thorax. Each tymbal has 30 or so ridges on it, arranged in a striated band. During activation, underlying muscles deform each ridge in succession, producing a train of clicks. The tymbal produces a second train as it returns elastically to its original shape. The anti-bat clicks are produced at a rate of up to 4,500 clicks per second, meaning that over half of the time that the bat is trying to process echoes, it is also receiving spurious moth-created clicks. This behavior is the hallmark of a sonar jammer. The tymbal organs are a characteristic of Bertholdia and its relatives, and their taxonomic distribution suggests that the organ is an ancient weapon against sonar-wielding bats.
My graduate student Aaron Corcoran determined that Bertholdia’s broadband clicks cause hunting bats to miss their targeted prey both in the laboratory and in the field. How do these jamming sounds work? The logic is the same as that described for radar systems. Some have suggested that if moth clicks are sufficiently similar to returning prey echoes in spectral and temporal characteristics, bats might misperceive them as echoes from objects that do not exist, or “phantom targets.” Second, if clicks are sufficiently numerous and intense, they might mask the presence of echoes, rendering the target invisible. A third mechanism is also possible. Clicks that overlap with or closely precede echoes may diminish a bat’s precision in determining target range. The three jamming hypotheses can be differentiated by what the bat perceives: multiple objects surrounding the moth for the phantom target hypothesis, no target for the masking hypothesis and a blurred target for the ranging interference hypothesis. Corcoran’s best efforts so far indicate that the last hypothesis appears to be the answer. Bats miss jamming moths by a degree predicted by the ranging interference hypothesis.
Corcoran also showed that jamming moths produce their signals only when the bat has “locked on” to them and they are in great danger. The moth determines this threat by sensing a combination of increasing bat cry intensity and a decrease in the interval between bat cries. The threshold for sound production in Bertholdia is closely matched to these parameters and allows the moth to unambiguously determine whether it has been targeted.
The latest known escalation of the bat–moth arms race is the discovery of a stealth bat by Holger Goerlitz, Marc Holderied and their colleages at the University of Bristol. The aerially hawking bat Barbastella barbastellus has lowered the intensity of its echolocation calls by 10 to 100 times. This shift allows them to remain undetected by eared moths until they are very close and the outcome is a fait accompli.
We call the ploy and counterploy of bats and moths a diffuse arms race, with multiple species of bats in an evolutionary battle with multiple species of moths. We are just beginning to understand this arms race, and one can be sure that there will be more surprises to come. For one, the tiger moths comprise approximately 11,000 species worldwide. There are an estimated 200,000 other kinds of moths plying the nighttime sky, and that doesn’t even touch on the beetles, katydids, crickets, mole crickets, flies, lacewings, locusts, nocturnal butterflies and mantids that are also jetting about. Any insect that flies after dark must have a strategy for dealing with nature’s ultimate nocturnal predators— echolocating bats. The race is on.
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