An Acoustic Arms Race
Bats and other animals use sound as a hunting tool—but their prey has also evolved ways to thwart detection
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.