You’d think it would be easier to spy on a Risso’s dolphin. The species frequents nearly every coast in the world. Their bulging heads and streaky gray and white patterning make them some of the most recognizable creatures in the ocean. And as with other cetaceans, they travel in groups and constantly chitchat: Clicks, buzzes, and whistles help them make sense of their underwater existence. Their social world is a sonic one.
“They’re a very vocal species,” says Charlotte Curé, a bioacoustics expert. “Sound is very important for them.”
Curé works for France’s Joint Research Unit in Environmental Acoustics, where she uncovers how cetaceans use the sounds of their environment to make intelligent decisions. Dolphins are known to communicate directly with one another, and to echo-locate their prey before striking. But many years ago, she wondered whether they could also pick up messages from other dolphins that were not intended for them.
But the problem is, even though dolphins are chatty, neither Curé nor Fleur Visser, her collaborator and an expert in Risso’s behavior, speak the language. So instead of snooping on what the dolphins appeared to be saying, they focused their attention on how they move. In their experiment, Curé’s team tested how dolphins responded when the researchers parked their boats overhead and played them social noises recorded from other groups.
After four years of field studies, Curé’s team reported their results: the first evidence of cetaceans eavesdropping on each other and using that information to decide where to swim next. For example, social recordings of males, which are known to harass females, calves, and antagonize other males, drove most dolphins away. Their study appeared last month in Animal Cognition.
The work is a masterclass in animal espionage, according to Caroline Casey, a marine mammal acoustic communication expert from UC Santa Cruz who was not involved with the study. “It’s just like in humans,” she says of the dolphins’ eavesdropping. “And I love when experiments can show what seems obvious to us, but hasn’t been previously demonstrated in an animal that is pretty elusive.”
After all, while Risso’s dolphins are easy to spot, it’s harder to listen in on their secrets. But since cetaceans are so intelligent and dependent on language, studies of their communication could help us understand the origins of our own language. More practically, knowing how to entice and repel these dolphins suggests a new tool for their conservation.
Dolphins aren’t the only noisy, nosy animals. Scientists have proven that male red-winged blackbirds, which clash over territory, eavesdrop on each others’ fights to gauge a potential rival’s aggression. Female great tit songbirds check out male singing contests, then cheat on their mates with more dominant tweeters. Birds and bats also eavesdrop when searching for mates and food. In each case, researchers suspect that vocal sounds trigger some known behavior. So to test how the animals respond, researchers play a recording of those sounds over a speaker and watch what happens.
But Curé’s team was curious about animal communication happening below sea level, and that’s been more mysterious. Until about a decade ago, researchers didn’t have the right tools to prove that such large ocean mammals can overhear distant chatter and react. “Now we have some tools,” Curé says. Along with a boat toting an underwater speaker, the researchers used drones to track movement from overhead as well as tags—suction-cupped acoustic sensors—to mark their test subjects.
They followed about 14 individual dolphins and groups of dolphins they had tagged off the coast of Terceira Island, in the Azores. Dolphins will normally swim in a straight line. But Curé hypothesized that sounds revealing social information could make them deviate. Sitting aboard the “playback vessel,” she would cue up three types of sounds. One was the clicks and buzzes of dolphins foraging—a “dinner bell” assumed to be an attractive signal that others would swim toward. Another recording featured the social whistles and “burst pulse” sounds of males, assumed to be a threatening signal that would repel females and competing males. They also played the chatter from females and calves, thought to be neutral.