The Araguaian river dolphin is different from most other dolphin species. Besides inhabiting fresh, not salty, water it lives a relatively solitary life, instead of forming large pods. It's the latter feature that led biologists to expect Araguaians would have much less complex communication tools than their marine counterparts, but this has turned out to be wrong. Instead, there are hundreds of sounds in the Araguaian's vocabulary, a discovery that may shed light on how marine mammals in general developed their communication tools.
Dr Laura May Collado of the University of Vermont studied the inhabitants of the Tocantins River and its tributary the Araguaia, which the dolphins are named after. "We found that they do interact socially and are making more sounds than previously thought," May Collado said in a statement. "Their vocal repertoire is very diverse."
Normally the dolphins are more wary of humans than their marine counterparts – a wise move considering what we've done to other freshwater dolphin species. However, in the town of Mocajuba fish market customers have taken to feeding dockside visitors, and the lure of easy food has overcome the dolphins' natural caution.
In PeerJ May Collado reports underwater microphones identified 237 distinct Araguaian sounds in the space of 20 hours of recording, considerably more than some grumpy teenagers seem to manage.
Some of the sounds were relatively simple ones between calves and their mothers. Others, however, were longer and more complex. In news that will warm the hearts of introverts everywhere, May Collado has a theory these are designed to help Araguaia dolphins keep their distance from each other, rather than as part of group bonding.
Rare footage of Araguaian river dolphins. Paulo Castro
May Collado also reports that the Araguaia's calls have a structure like an orca's but a pitch much lower than that of marine dolphins – indeed half way to the depth of baleen whales, which use low frequencies to communicate over great distances. May Collado believes this is to prevent sounds echoing off underwater vegetation in a habitat that expands to include flooded forests in the rainy season.
Although it is unlikely these sounds could have evolved from scratch in the short time the dolphins have been feeding off human fish scraps, May Collado is keen to see if river dolphins with less human interaction are just as chatty. Studies of the closely related Amazon dolphin have indicated they are much quieter.
Elsewhere in Brazil a small group of bottlenose dolphins that partner with humans to ensure both catch more fish than either would alone have been found to have formed their own society within the pod, which has persisted over generations.
Meanwhile, the highly endangered Irrawaddy dolphins of the Mekong River were reported to be making a comeback, although their numbers are still a tiny fraction of pre-1970s levels.