One Mammal that is shrouded in mystery are the Araguaian river dolphins of Brazil. The dolphins was considered to be quite solitary, with little social structure that would require communication. But a biologist at the University of Vermont, Laura May Collado, and her colleagues have discovered that the dolphin can indeed make hundreds of different sounds to communicate, finding that could help uncover how communication revolved in marine mammals. The biologists published their findings in the journal Peerj.
Collado said that they discovered that the dolphin interacts socially and are making more sounds than previously thought. They have quite a diverse vocal repertoire.
The biologists used underwater cameras and microphones to record sounds and interactions between the dolphins at the market and took some genetic samples. They were able to identify 237 different types of sounds the dolphins make, but even with 20 hours of recordings, the researchers don't believe they captured the entire acoustic repertoire of the animals. The most common sounds were short, two-part calls that baby dolphins made when they were approaching their mothers.
May Collado explained that it is exciting that the 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. Also, these animals made longer calls and whistles, but these were much rarer, and the reasons for them are not yet clear to the biologists. There is some indication, however, that whistles serve the opposite purpose than in bottlenose dolphins, with the botos using them to maintain distance rather than for group cohesion.
The characteristics of the acoustic of the calls are also exciting. These calls fall somewhere between the low-frequency calls used by baleen whales to communicate over long distances, and the high-frequency ones used by marine dolphins for short distances. May Collado speculation is that the river environment may have shaped those characteristics.
The next step for May Collado and her colleagues is to study whether the same diversity of communication is seen in other populations of Araguaian river dolphins that are less accustomed to humans. They will also compare them to their relatives elsewhere in South America.
The Araguaian dolphins are closely related to two other species, the Bolivian river dolphin and the Amazon river dolphin. The sole description of the Araguaian dolphins is that of a separate species in 2014, and that classification is still under debate. But this appears to be a large number of variations in the repertoire of sounds each species makes.
May Collado concluded by saying that they cannot say what the evolutionary story is yet until they get to know what sounds the other river dolphins produce in the Amazon area, and how that relates to what they found. These are a new set of questions to explore.
