River Dolphins Provide New Clues to How Marine Mammals Communicate

Because they’re so elusive, we’ve known very little about the critically endangered Araguaian river dolphins that are native to Brazil. Scientists didn’t even discover them as a separate species until just five years ago. Also called botos, these dolphins, with long beaks they use to hunt for fish, were believed to be loners—until now. Researchers recently discovered they actually communicate with each other using hundreds of different sounds.

“We found that they do interact socially and are making more sounds than previously thought,” Laura May Collado, a biologist at the University of Vermont who participated in the study, said in a news release. “Their vocal repertoire is very diverse.”

The researchers believe this is the first time contact calls have been described for river dolphins. To be able to observe and record these mysterious creatures, scientists from the University of Vermont and University of St. Andrews went to the northeastern Brazil town of Mocajuba, where shoppers at a local fish market feed the river dolphins. The clear water made it easy to identify individual dolphins and observe their behavior.

Using underwater cameras and microphones, the researchers recorded 237 different types of sounds within 20 hours. They believe these dolphins probably make even more sounds than the ones that they recorded.

It’s interesting that the river dolphins used signature whistles, much like bottlenose dolphins do in the ocean. However, the river dolphins seemed to make this sound to maintain distance, while marine dolphins use it to gather groups together. Most of the sounds they recorded were the short calls calves made as they swam to their mothers.

The frequencies of the river dolphins’ sounds were higher than the ones used by baleen whales to communicate over long distances but lower than those used by marine dolphins for shorter distances. Collado thinks this particular frequency may have evolved to avoid echoes from the dense vegetation in the river dolphins’ habitat as well as to enable mothers to communicate more easily with their calves.

River dolphins are “evolutionary relics,” according to the University of Vermont. Only a few species of them exist in the world. They diverged from other cetaceans long before other dolphin species. This means that the whistles and calls that marine dolphins make may actually have originated in river dolphins.

Araguaian river dolphin

An Araguaian river dolphin in Cantăo State Park, Brazil.
Photo credit: Rio Cicica/Wikimedia Commons

The researchers next plan to study the sounds of Araguaian river dolphins that haven’t had as much contact with humans and compare those sounds to those of other river dolphins in South America.

Araguaian river dolphins are closely related to Bolivian and Amazon river dolphins. Collado studied Amazon river dolphins in 2005 and said they are relatively very quiet. “Why is one population chattier than others and how do these differences shape their social structure?” she asked.

The findings from the Araguaian river dolphin study, which was published April 18 in the journal PeerJ, could lead to a better understanding of the evolution of communication in marine animals. Pilot whales and killer whales use similar calls to communicate with each other, so determining the similarities between different species could lead to a discovery of what sounds evolved first and why.

“We can’t say what the evolutionary story is yet until we get to know what sounds are produced by other river dolphins in the Amazon area, and how that relates to what we found,” Collado said. “We now have all these new questions to explore.”

Photo credit: University of Vermont/YouTube

76 comments

Jennifer H
Jennifer H9 days ago

Interesting and fascinating but worrisome at the same time, Another species they have found and can mess with.

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Cindy M. D
Cindy M. D13 days ago

Listening to the sounds of a dolphin is one of the most peaceful sounds on the planet.

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Richard B
Richard B20 days ago

Thank you

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Frances Bell
Frances B27 days ago

More importantly than speculating why they make sounds, we could put this knowledge to better use to understand what impact human generated noise has on these species and put safeguards in place - before we do to them what we're already doing to marine life.

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HEIKKI R
HEIKKI R28 days ago

thank you

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Brandy S
Brandy S28 days ago

Thank you.

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Terri S
Terri S28 days ago

They didn't know that dolphins communicate with each other??? I'm sorry, but I don't get the purpose of this study. Just leave them alone to live and freely communicate as they have for hundreds of years. Are we so nosy that we need to understand every noise that other species make??

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Callie R
Callie R28 days ago

Thanks.

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Laura R
Laura R28 days ago

Thank you.

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