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 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.