Araguaian river dolphins are highly elusive creatures. Identified as a unique species just five years ago (though the classification is subject to debate), these cetaceans are solitary, shy of humans and endangered. Only 1,000 may be alive today.
As a result, these dolphins are difficult to observe and there is much that remains unknown about them, including the nature of their communication. Previous theories posited that the animals’ vocal repertoire was more limited than that of their chatty marine relatives—like bottlenose dolphins—since Araguaian river dolphins are thought to spend much of their time alone. But as Gizmodo’s George Dvorsky reports, a new study suggests that Araguaian river dolphins are quite talkative indeed.
The mysterious animals belong to a South American group of river dolphins known as “botos,” which are found only in the Amazon, Orinoco, and Tocantins river basins, according to the new paper, published in the journal PeerJ. Though botos are typically skittish, there is a group of Araguaian dolphins that has become accustomed to humans; the animals hang out near a market in the Brazilian town of Mocajuba, where human shoppers feed them tasty fish.
A team led by Gabriel Melo-Santos, a marine biologist at the University of St. Andrews in Scotland, set out to record these unusually bold botos, in the hopes of learning more about Araguaian dolphin communication. The researchers used underwater cameras and microphones to track the animals’ sounds and interactions, and also took some genetic samples to pinpoint the relationships between them.
The botos, as it turns out, were chatting up a storm. Across 20 hours of recordings, the researchers identified 237 different sounds, and they believe the dolphins’ acoustic repertoire is likely greater than that. The most common sounds were short, two-part calls that baby botos made when approaching their mothers.
“It’s exciting,” says study co-author Laura May-Collado, a biologist at the University of Vermont. “[M]arine dolphins like the bottlenose use signature whistles for contact, and here we have a different sound used by river dolphins for the same purpose.”
The botos did infrequently emit some longer calls and whistles, but unlike bottlenose dolphins who use whistles to foster social cohesion, botos seemed to use the sound to maintain distance.
The acoustics of botos’ calls were also unique, falling between the low frequency of the calls that baleen whales rely on to communicate over long distances, and the high frequencies that marine dolphins emit when communicating over short distances. This could, May-Collado suggests, have something to do with the river environment that botos call home.
“There are a lot of obstacles like flooded forests and vegetation in their habitat,” she explains, “so this signal could have evolved to avoid echoes from vegetation and improve the communication range of mothers and their calves.”
Scientists are interested in learning more about river dolphin communication because they are, as the study authors write, “evolutionary relics.” The few river dolphin species that exist across the globe diverged from other cetaceans much earlier than marine dolphins, so by studying river dolphin communication, experts may be able to get a better sense of how other cetacean calls developed. For example, the calls emitted by boto calves are similar to ones used by orcas and pilot whales to transmit information about group identity.
“Given these similarities,” the researchers write, “we propose these two-component signals could have evolved early in the evolutionary history of toothed whales as social contact signals, likely for mother-calf interactions and later in the lineage leading to delphinids it evolved into a group recognition signal.”
But there is still much research to be done. The study authors can’t say, for instance, whether other groups of Araguaian river dolphins are as talkative as the one that has become used to humans. Not only do scientists have a complete understanding of the communication patterns of other river dolphins, like the closely related Amazon river dolphin and Bolivian river dolphin.
“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,” May-Collado says. “We now have all these new questions to explore.”