Evolutionary Convergence in Mammalian Bio-sonar

Source: reprinted from materials provided by the Acoustical Society of America

Though they evolved separately over millions of years in different worlds of darkness, bats and toothed whales use surprisingly similar acoustic behavior to locate, track, and capture prey using echolocation, the biological equivalent of sonar. Now a team of Danish researchers has shown that the acoustic behavior of these two types of animals while hunting is eerily similar. The findings were made possible by a new type of whale tag that allows scientists, for the first time, to track whales’ foraging behavior in the wild (Figure 1). The researchers present their results at the Acoustics 2012 meeting in Hong Kong, a joint meeting of the Acoustical Society of America (ASA), Acoustical Society of China, Western Pacific Acoustics Conference, and the Hong Kong Institute of Acoustics.

Figure 1. A dolphin in the Mediterranean (123rf stock photos)

Bats and toothed whales (which include dolphins and porpoises) had many opportunities to evolve echolocation techniques that differ from each other, since their nearest common ancestor was incapable of echolocation. Nevertheless – as scientists have known for years – bats and toothed whales rely on the same range of ultrasonic frequencies, between 15 to 200 kilohertz, to hunt their prey. (For comparison, the human hearing range is between 20 hertz to 20 kilohertz.) This overlap in frequencies is surprising because sound travels about five times faster in water than in air, giving toothed whales an order of magnitude more time than bats to make a choice about whether to intercept a potential meal.

Now, thanks to new technology that records what a whale hears as well as how it moves in the wild, Peter Teglberg Madsen of Aarhus University in Denmark and Annemarie Surlykke of the University of Southern Denmark have uncovered more similarities in the animals’ acoustic tactics.

Bats increase the number of calls per second (what researchers call a “buzz rate”) while in pursuit of prey (Figure 2). Whales were thought to maintain a steady rate of calls or clicks no matter how far they were from a target. But the new research shows that wild whales also increase their rate of calls or clicks during a kill – and that whales’ buzz rates are nearly identical to that of bats, at about 500 calls or clicks per second.

Figure 2. A rhinolophid bat from the Southern Caucasus (123rf Stock Photo)

“On a purely physical basis, you would predict that whales and bats would operate at different [echolocation] rates and frequencies,” Madsen says. “But instead, they operate at the same rates and frequencies.” The similarities support the idea that the acoustic behavior of bats and whales may be defined by the auditory processing limitations of the mammalian brain.

Until now, Madsen continues, “it was not known how [a whale] would coordinate its acoustic behavior” in the wild to intercept its prey.

To track whales’ hunting behavior in the wild, researchers relied on a new device called the DTAG, which was developed by electrical engineer Mark Johnson at the Woods Hole Oceanographic Institution in Woods Hole, Mass. The DTAG attaches to a whale’s skin via suction cup and records ultrasonic frequencies (allowing scientists to analyze what a whale hears) as well as inertia and pressure readings (which allow scientists to reconstruct a whale’s movements in the water in three dimensions).

By making it possible for scientists to track whales’ foraging behavior in more detail, the new tags will also help conservationists to assess environmental impacts on whales’ behavior, Madsen says.