Dolphins May Eavesdrop on Each Other, Study Suggests

Anna Petherick
for National Geographic News
January 18, 2006
Synchronized swimming in tight formations might help wild dolphins
listen in on one another's mapping and foraging, according to a
recent study.

Thomas Götz, a marine biologist at the University of Tübingen in Germany, reached that conclusion after analyzing the sounds made by dolphins swimming near the island of Gomera in the Canary Islands (see map).

By dragging a recording device behind a boat, Götz captured the echolocation signals of several small groups of rough-toothed dolphins (Steno bredanensis).

Like all dolphins, these marine mammals gather information about their environment by emitting high-pitched sounds, or clicks, and analyzing the returning echoes.

Götz's recodrings showed that when the dolphins swam in loose, scattered formations, several individuals produced echolocation signals.

But 80 percent of the samples from groups moving in close, synchronous clusters contained the clicks of just one animal.

"You can tell if there are two animals echolocating or if there's just one by comparing the sound spectrum and the interval pattern of different clicks in a recording sequence," he said.

The findings led Götz to suggest that wild dolphins can tune in on each other's signals and use the information to navigate as a group.

Language Barrier?

To make sense of others' echolocation, a dolphin would have to be able to link the returning echoes to the outgoing clicks.

Such a task would be much easier to perform if the eavesdropper stays close to the animal making the sound, Götz says.

"[The dolphins'] behavior almost looks like a military formation, because all the fins are in one line, and they go down and come up in exactly the same formation," he said.

Steve Dawson is a marine mammal researcher at the University of Otago in New Zealand who was not involved in the study. He also thinks eavesdropping is the most likely explanation for the findings.

Dawson says the theory would explain why dolphins have not evolved to use language beyond a simple range of sounds to communicate with others in a pod, despite their relatively large brains.

For dolphins, echolocation "can carry all sorts of complex information about whether a dolphin is pregnant, what mood it's in, and what's around it," he said.

Listening in to each other's mapping and foraging efforts would make group communication even easier.

"In essence, if you were to sense a fish, you wouldn't need to tell me there's fish over there, because I would already know," Dawson said.

Dawson adds that the animals' ability to eavesdrop may have contributed to the evolution of cooperative behavior in dolphins.

Acting selfishly would be harder, he said, because others in the group would "be in on whatever you might try to get away with."

Enough Evidence?

Götz's findings are not the first evidence that dolphins can understand each other's echolocation signals.

Previous research on captive Atlantic bottlenose dolphins (Tursiops truncatus) demonstrated that they are capable of deciphering the echoes from another individual's clicks.

The 1996 study showed that a dolphin could correctly identify objects without seeing them or using echolocation. The animal simply listened to another dolphin's echolocation signals as it was presented with an item.

Like dolphins, bats also use echolocation to assess their surroundings.

For example, when a bat approaches an insect, it speeds up the repetition rate of its signals to get a better reading on its prey. The change indirectly alerts other bats to the snatch, says Hans-Ulrich Schnitzler, Götz's supervisor and study co-author.

"But we don't have the situation where one bat emits a signal and all others are waiting for the returning echo," Schnitzler said. "[Bats] can tell if something's going on, but dolphins truly eavesdrop."

Götz, however, admits that his data, detailed online last month in the British science journal Biology Letters, does not definitively prove the theory.

Whitlow Au, chief scientist for the Marine Mammal Research Program at the University of Hawaii, is also not convinced the wild rough-toothed dolphins were necessarily eavesdropping.

"I think it's questionable whether the evidence is strong enough," he said.

"But understanding how marine animals do things in the wild is extremely difficult."

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