Washington D.C. [USA], Apr 19 : No matter how friendly a dolphin might appear, the Araguaian river dolphin of Brazil has remained a mystery. A new study has found that there are hundreds of sound that the dolphins use to communicate.
The study was published in the Journal 'PeerJ’.
"We found that they do interact socially and are making more sounds than previously thought. Their vocal repertoire is very diverse,” Laura May Collado said.
The Araguaian dolphins, also called botos, are hard to find and difficult to study.
The team used underwater cameras and microphones to record sounds and interactions between the dolphins and took some genetic samples. They identified 237 different types of sounds that the dolphins make, but even with 20 hours of recordings, the researchers don't think they captured the animals' entire acoustic repertoire.
The most common sounds were short, two-part calls that baby dolphins made when they were approaching their mothers."It's exciting; marine dolphins like the bottlenose use signature whistles for contact, and here we have a different sound used by river dolphins for the same purpose," said May Collado. 
The dolphins also made longer calls and whistles, however, they were rare. "There are a lot of obstacles like flooded forests and vegetation in their habitat, so this signal could have evolved to avoid echoes from vegetation and improve the communication range of mothers and their calves," she said.
The Araguaian dolphins 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 large amount of variation in the repertoire of sounds each species makes.
Collado asserted the work could help researchers gain a clearer understanding of 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 tease out which signals evolved first, and why.
Collado added that the calls may also have other functions in addition to identity, perhaps indicating group identity, or providing information on emotional state.
"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. We now have all these new questions to explore,” concluded Collado.