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The dolphin of the Araguaian River of Brazil is a mystery. It was thought that it was quite lonely, with little social structure requiring communication. But Laura May Collado, a biologist at the University of Vermont, and her colleagues have discovered that dolphins can actually produce hundreds of different sounds, which could help discover how communication has evolved in marine mammals.
"We found that they interacted socially and produced more sounds than previously thought," she says. "Their vocal repertoire is very diverse."
May Collado's conclusions are his colleagues were published in the journal peerj April 18th.
Aragua's dolphins, also called botos, are hard-to-study animals. They are hard to find in the first place, and although the waters of the Araguaia and Tocatins rivers are clear, it is difficult to identify individuals because dolphins are capricious and difficult to approach.
Fortunately, the biologist Gabriel Melo-Santos of the University of St Andrews in Scotland and project leader discovered a fish market in the Brazilian city of Mocajuba, where botos regularly go for food. Clean water and regular visits to dolphins were a unique opportunity to closely examine the behavior and interaction of animals, as well as to identify and closely monitor individual individuals .
The team used underwater cameras and microphones to record sounds and interactions between dolphins on the market, and took genetic samples. They identified 237 different types of sounds produced by dolphins, but even after 20 hours of recording, the researchers do not think they captured the entire acoustic repertoire of these animals. The most common sounds were the brief two-part calls that baby dolphins made when approaching their mother.
"It's exciting; marine dolphins, like big noses, use characteristic whistles, and here we have a different sound used by river dolphins for the same purpose, "says May Collado. River dolphins also made longer calls and whistles, but they were much rarer and their reasons are not yet clear. However, there is some evidence that whistles have the opposite meaning to bottlenose dolphins, with botos using them to maintain distance rather than group cohesion.
The acoustic characteristics of the calls are also interesting. they are somewhere between the low frequency calls used by baleen whales to communicate over long distances and the high frequency calls used by short-distance marine dolphins. May Collado speculates that the river environment may have shaped these features.
"There are many obstacles, such as flooded forests and vegetation in their habitat, so this signal could have evolved to avoid the echoes of vegetation and improve the communication distance of mothers and their young", she said.
May Collado and his colleagues then want to study if the same diversity of communication is observed in other populations of Aragua River dolphins who are less accustomed to humans, and compare them to their parents elsewhere in South America. The Araguaian dolphins are closely related to two other species, the Bolivian river dolphin and the Amazon river dolphin; Araguaian dolphins have only been described as a separate species in 2014, and this classification is still under discussion. But there seems to be a lot of variation in the repertoire of sounds that each species produces.
The Amazonian dolphins in Ecuador, studied by May Collado in 2005, are generally very calm. "We need more information about these other species and more and more people," she said. "Why is a population more talkative than others and how do these differences shape their social structure?"
May Collado believes that this work could help researchers better understand the evolution of communication in marine mammals. Similar calls have been reported in pilot whales and killer whales, for example, and the similarities and differences between different species could help determine which signals were created first and why.
River dolphins are relics of evolution, represented by only a few species in the world, and they have diverged from other cetaceans long before other dolphins. Thus, these calls may have first started in river dolphins, and then evolved in marine dolphins in whistles and calls, but in a different social context. Or has the call function changed, what type of sound is used for group identity in killer whales and individual identity in river dolphins? Calls can also have other functions in addition to the identity, perhaps indicating the identity of the group or providing information on the emotional state.
"We can not say what is the history of evolution until we know what sounds are produced by other river dolphins in the Amazonian region and how that is related to what we discovered, "she says. "We now have all these new questions to explore."
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