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You would think it will be easier to spy on Riso’s dolphin. The species visits almost every coast in the world. Their protruding heads and grooved gray-white patterns make them one of the most recognizable creatures in the ocean. And as with other cetaceans, they travel in groups and chat all the time: Clicks, buzzes and whistles help them make sense of their underwater existence. Their social world is sound.
“They’re a lot of vocal types,” says Charlotte Kure, a bioacoustic expert. “Sound is very important to them.”
Cure works for France’s Joint Research Unit on Environmental Acoustics, where he reveals how cetaceans use the sounds of their surroundings to make intelligent decisions. Dolphins are known to communicate directly with each other and echolocate their prey before striking. But many years ago, she wondered if they, too, could take messages from other dolphins that weren’t meant for them.
But the problem is that although the dolphins are talkative, neither Cure nor Fleur Visser, her collaborator and expert on Riso’s behavior, speaks the language. So instead of eavesdropping on what the dolphins seem to be saying, they turned their attention to how they move. In their experiment, the Curé team tested the dolphins’ reaction when the researchers parked their boats on top and made social noises recorded by other groups.
After four years of field research, the Cure team reported its results: the first evidence that cetaceans eavesdrop on each other and use this information to decide where to swim next. For example, social records of men known to harass women, calves, and antagonize other men have driven away most dolphins. Their study appeared last month in Knowledge of animals.
The work is an animal espionage master class, according to Caroline Casey, a marine mammal acoustic communication expert at the University of California, Santa Cruz, who was not involved in the study. “Just like humans,” she says of eavesdropping on dolphins. “And I love it when experiments can show what seems obvious to us, but hasn’t been demonstrated before in an animal that’s quite elusive.”
After all, while Riso’s dolphins are easily spotted, it’s harder to listen to their secrets. But because cetaceans are so intelligent and language-dependent, studying their communication can help us understand the origins of our own language. More practically, knowing how to lure and repel these dolphins offers a new tool for their conservation.
Dolphins are not only noisy, curious animals. Scientists have proven that male red-winged hairs that collide across the territory eavesdrop on each other’s battles to assess the aggression of a potential rival. Female magnificent songbirds check out male singing competitions, then cheat on their friends with more dominant whistles. Birds and bats also eavesdrop when looking for friends and food. In any case, researchers suspect that vocal sounds provoke some behavior. To check how the animals react, the researchers record these sounds through a loudspeaker and observe what happens.
But Kure’s team was curious about the communication with animals that takes place below sea level, and it was more mysterious. Until about a decade ago, researchers did not have the appropriate tools to prove that such large oceanic mammals could hear distant chatter and react. “Now we have some tools,” says Cure. Along with a boat carrying an underwater speaker, the researchers used drones to track movement from above, as well as tags — suction acoustic sensors — to mark their subjects.
They were followed by about 14 individual dolphins and groups of dolphins that had marked off the coast of Terceira Island in the Azores. Dolphins usually swim in a straight line. But Kure suggested that sounds revealing social information could lead them astray. Sitting aboard the reproduction ship, she would make three kinds of sounds. One was the clicking and buzzing of food-seeking dolphins, a “dinner bell” that was seen as an attractive signal for others to swim to. Another recording included social whistles and a “pulsating pulse” of men, which were supposed to be a threatening signal that would repel women and competing men. They also played the chatter of women and calves considered neutral.
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