The dolphin of the Araguaian river of Brazil is a mystery. It was thought that it was quite solitary, with little social structure that would require communication. But Laura May Collado, a biologist at the University of Vermont, and her colleagues discovered that dolphins can make hundreds of different sounds to communicate, a finding that could help discover how communication evolved in marine mammals.
"We discovered that they interact socially and are making more sounds than previously thought," she says. "His vocal repertoire is very diverse."
The findings of Collado May are his colleagues, were published in the magazine. PeerJ on April 18.
The Araguaian dolphins, also called botos, are a difficult animal to study. First of all, they are difficult to find, and although the waters of the Araguaia and Tocatins rivers are clear, it is difficult to identify the individuals because the dolphins are scary and difficult to approach.
Fortunately, Gabriel Melo-Santos, a biologist at the University of St Andrews in Scotland and project leader, found a fish market in the Brazilian city of Mocajuba, where botos visit regularly to be fed by the people who buy there. Clear water and regular dolphin visits provided a unique opportunity to observe closely how animals behave and interact, and to identify and track several individuals.
The team used underwater cameras and microphones to record sounds and interactions between dolphins on the market, and took some genetic samples. They identified 237 different types of sounds that dolphins make, but even with 20 hours of recordings, the researchers do not believe that they have captured the entire acoustic repertoire of the animals. The most common sounds were the two-part short calls that baby dolphins made when they approached their mothers.
"It's exciting, marine dolphins like bottlenose dolphins use signature whistles for contact, and here we have a different sound used by river dolphins for the same purpose," says May Collado. The river dolphins also made longer calls and whistles, but these were much rarer, and the reasons for this are still unclear. But there are some indications that whistles serve the opposite purpose than in bottlenose dolphins, with botos using them to maintain distance rather than group cohesion.
The acoustic characteristics of the calls are also interesting; They fall somewhere between the low frequency calls used by whale whales to communicate over long distances, and the high frequency calls used by marine dolphins for short distances. Mayo Collado speculates that the river's environment may have shaped those characteristics.
"There are many obstacles such as flooded forests and vegetation in their habitat, so this signal could have evolved to avoid echoes of vegetation and improve the communication range of mothers and their calves," he says.
May Collado and his colleagues later wish to study if the same diversity of communication is observed in other populations of Araguaian river dolphins that are less accustomed to humans, and compare them with their relatives in other parts of South America. The dolphins of Aragua are closely related to two other species, the Bolivian river dolphin and the Amazon river dolphin; the araguaian dolphins were only described as a separate species in 2014, and that classification is still under debate. But there seems to be a lot of variation in the repertoire of sounds that each species makes.
The Amazon dolphins in Ecuador, studied by May Collado in 2005, are generally very calm. "We need more information about these other species and more populations," she says. "Why is a population more numerous than others and how do these differences shape its social structure?"
May Collado says the work could help researchers better understand how communication evolved 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 evolved first and why.
River dolphins are evolutionary relics, represented by a few species throughout the world, and separated from other cetaceans long before other dolphins. So these calls may have first arisen in river dolphins, and then evolved into marine dolphins in whistles and calls, but in a different social context. Or was there a change in the function of calls, with this type of sound used for group identity in killer whales, and individual identity in river dolphins? Calls can also have other functions besides identity, perhaps indicating the identity of the group or providing information about the emotional state.
"We can not say what the evolutionary history is until we know what sounds other river dolphins produce in the Amazon area and how that relates to what we find," she says. "Now we have all these new questions to explore."
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