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.
The ploy worked—dolphins noticed the distant commotion. Based on analyses of how tagged individuals changed course upon hearing the playback, the team caught a glimpse into dolphin decisionmaking: Male sounds drove the animals away—similar to the way you’d cross a street to avoid a belligerent group arguing outside of a bar. The “dinner bell” and female-calf noises, on the other hand, enticed the animals toward the boat.
“It's one of those things that’s like, of course—of course dolphins are listening to one another,” says Casey. “It takes experiments like this to really demonstrate that they have very dynamic ways of interacting with one another.”
The discovery of eavesdropping isn’t just a cool quirk that showcases dolphin intelligence—it may prove handy for conservation. Cetaceans swimming near a port or bay sometimes get trapped in these waters, unable to return to the open sea. “There are so far no really good methods to avoid this,” says Curé. But helpful humans playing natural sound could guide the stranded animals back to where they should be. “And it’s worked, actually,” Curé says. In 2017, her team helped rescue a group of Orcas that was blocked in a Norwegian bay. They played attractive social sounds, which effectively lured them to safety. Curé envisions similar uses for Risso’s dolphins, and other species, to prevent stranding and keep them away from fishers’ nets.
Scientists focus on clear links between animal behavior and their communication, in part, because it can shape how we understand our own use of language. Human language may have evolved not from unique adaptations, but rather from cognitive machinery that already existed in other animals. Bird songs are the most common models to study. But marine mammals, as closer relatives, could reveal even more. Dolphins are also thought to be more intelligent than birds, and they’ve recently been shown to cooperate on tasks with vocal signals.
The sound cues that let a dolphin decide whether to approach or avoid another group may be just a snippet of how rich their language is. And it’s still an open question what most known sounds mean to marine mammal listeners. “I hope that more people are inspired to set up the same type of experimental design,” Casey says. “This area of the field is still kind of untapped.”
The catch is, however, still the catch. Spying on animals that live in a whole different medium than us is hard. “Sometimes we could track some animals but we lost them,” Curé recalls. “Sometimes we could get the tag on, but the tag comes off.” Some days they couldn’t spot any Risso’s at all. The project was a grind.
But the intel may be worth the hassle. To Curé, it reinforces her team’s theories about how dolphins use sound and their intelligence to maintain social boundaries—like how individuals can avoid awkward run-ins with a threatening male. It’s an encouraging enough sign that she wants to conduct similar experiments with other cetaceans, such as pilot whales. Researchers will have to be careful, though. They may hear us coming.