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Discovered as a unique species just five years ago, the Brazilian dolphin, Araguaian River, is a fascinating but misunderstood aquatic creature. As new research shows, these dolphins produce a surprising range of sounds – an important clue to understanding how and why dolphins have developed their ability to communicate.
The Araguaian dolphins, also called botos, were first identified in 2014. The Botos live exclusively in the basins of the Amazon, Orinoco and Tocantins rivers in South America, where they use their long beaks to chase the fish.
These dolphins are considered relics of evolution, having diverged from other cetaceans (a family including dolphins and whales) earlier than other dolphins. Because of their unique position in the cetacean family tree, scientists can study these creatures to better understand the ancestors of marine dolphins, such as the bottlenose dolphin. In addition, by studying botos in the wild, scientists can gain new insights into the origins of certain dolphin behaviors, such as their communication skills. Biologists would like to know, for example, whether these iconic clicks and whistles are the consequence of river or ocean life.
Botos are notoriously elusive. Unlike sea dolphins and their spectacular screens, botos do not make noise when they take in the air. They tend to be lonely and shy, living in small social groups. These dolphins are in critical danger of extinction and there can only be 1,000 left. Little is known about their ability to make sounds or communicate with each other, but research done a few years ago has shown that they are able to make noises such as clicks, whistles, jaw clicks and other sounds. Beyond that, we did not know much.
"The majority of studies on Amazon's dolphins, as well as on other dolphins around the world, have reported few sounds used for communication," said Gabriel Melo-Santos, lead author of the new study and marine biologist at the University of St. Andrews, in an email to Gizmodo. "Some studies would even indicate that botos had a simple communication system composed of few types of sounds."
Fortunately, however, there is a group of botos in the Tocantins River, in the city of Mocajuba, Brazil, who have become acclimatized to humans. The inhabitants of this city feed the dolphins in a fish market along the river. Melo-Santos and biologist Laura May-Collado of the University of Vermont visited this market to study this particular population. Their new research, published today in PeerJ, shows that Araguaian dolphins are capable of producing hundreds of different sounds to communicate.
Using microphones and underwater cameras, researchers recorded the sounds and behaviors of dolphins. Genetic samples were collected to determine relationships. Nearly 400 sounds were recorded, which the researchers classified into different types, including 13 types of sounds and 66 types of pulsed calls.
"It was a big surprise when we discovered more than 200 types of sounds and our results indicate that there is still much to discover," said Melo-Santos. "Very interesting, we discovered that the most commonly produced sounds seem to play an important role in mother-small communication."
The most common sounds emitted by the botos were short calls in two parts. Calves accounted for 35% of these short calls, which they produced when they joined their mother. This type of sound is probably a signature whistle, in which calves can identify with others – a behavior also observed in marine dolphins. What is interesting is that it suggests the origin of the whistle in the old dolphins.
River dolphins also made longer calls and whistles, but less frequently. The exact purpose of these sounds is not immediately clear. Interestingly, similar calls by bottlenose dolphins and orca whales "carry information about the group's identity" and are used to "maintain social cohesion," the authors write in this study. The calls made by the river dolphins, however, were used to "keep a distance from each other, rather than to promote social interactions as in marine dolphins," they wrote. Fascinating.
Moreover, the frequency range of the signals emitted by the botos was not as low as the sounds emitted by some whales to communicate over great distances, nor as high as the sounds used by the marine dolphins to communicate over short distances. . It could have something to do with life in the river environment.
"There are many obstacles, such as flooded forests and vegetation in their habitat. This signal could have evolved to avoid echoes of vegetation and improve the communication distance of mothers and their young, "said May-Collado in a press release.
As previously stated, this population of dolphins was accustomed to humans and the study took place close to a bustling market. These factors may have interfered with the results.
"What could have happened is that we were able to capture sounds associated with market behavioral contexts or interactions. As our analysis indicates, there is still a lot to discover if you continue to browse recordings, "said Melo-Santos. "Nevertheless, the animals we have recorded in this study are wild individuals at large who interact with other dolphins of the same population. The sounds we found are therefore representative of this new species. It is therefore a very important first step towards understanding a fundamental aspect of the biology of a dolphin so little known. "
Researchers would like to study other populations of river dolphins, including those of other species (there are three other known species of river dolphins) and dolphins of Araguaian rivers unaccustomed to humans. . Further analysis will provide a better understanding of the communication capabilities of botos and the evolving roots of this capability.
"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, "said May-Collado. "We now have all these new questions to explore."
[PeerJ]
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