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.