Discovered to be a unique species just five years ago, the Araguaian river dolphin of Brazil is a fascinating, yet poorly understood, aquatic creature. As new research shows, these dolphins produce a surprising array of sounds—an important clue in our understanding of how and why dolphins evolved the capacity for communication.
The Araguaian river dolphins, also known as botos, were first identified in 2014. Botos live exclusively in the Amazon, Orinoco, and Tocantins River Basins of South America, where they use their long beaks to hunt for fish.
These dolphins are considered evolutionary relics, having diverged from other cetaceans (a family that includes dolphins and whales) earlier than other dolphins. Because of their unique position within the cetacean family tree, scientists can study these creatures to better understand the ancestors of marine dolphins, such as the bottlenose dolphin. What’s more, by studying botos in the wild, scientists can acquire new insights into the origin of certain dolphin behaviours, such as their communication skills. Biologists would like to know, for example, if those iconic clicks and whistles emerged as a consequence of river life or ocean life.
Botos are notoriously elusive. Unlike marine dolphins and their dramatic breaching displays, botos don’t make a fuss when they come up for air. They tend to be solitary and shy, living in small social groups. These dolphins are critically endangered, and there may be only 1,000 of them left. Not much is known about their ability to make sounds or communicate with one another, but research done a few years ago suggested they’re able to make noises like clicks, whistles, jaw-snaps, and other sounds. Beyond this, not much was known.
“The majority of studies with Amazonian River dolphins, as well as other river dolphins around the world, reported few sounds used for communication,” explained Gabriel Melo-Santos, the lead author of the new study and a marine biologist at the University of St. Andrews, in an email to Gizmodo. “Some studies would even state that botos had a simple communication system composed by few sound types.”
Fortuitously, however, there’s a group of botos in the Tocantins River in the town of Mocajuba, Brazil, that have become acclimated to humans. People in this town feed the dolphins at a fish market along the river. Melo-Santos, along with biologist Laura May-Collado from the University of Vermont, visited this market to study this particular population. Their new research, published today in PeerJ, shows that Araguaian river dolphins are capable of producing hundreds of different sounds to communicate.
Using underwater microphones and cameras, the researchers recorded the sounds and behaviours of the dolphins. Genetic samples were collected to determine relationships. Nearly 400 sounds were recorded, which the researchers classified into various types, including 13 types of tonal sounds and 66 types of pulsed calls.
“It was a great surprise when we discovered more than 200 sound-types, and that our results indicate that there’s more to discover,” said Melo-Santos. “Very interestingly, we discovered that the most commonly produced sounds seem to play important role in mother-calf communication.”
The most common sound emitted by the botos were short, two-part calls. Calves accounted for 35 per cent of these short calls, which they produced when joining their mothers. This type of sound is likely a signature whistle, in which calves can identify themselves to others—a behaviour also observed in marine dolphins. Excitingly, this suggests an early origin of the signature whistle in ancient dolphins.
The river dolphins also made longer calls and whistles, but not as frequently. The exact purpose of these sounds isn’t immediately clear. Interestingly, similar calls made by bottlenose dolphins and orca whales “carry information on group identity” and are used for “maintaining social cohesion,” the authors wrote in the study. The calls made by the river dolphins, however, were used for “keeping distance between each other, rather than promoting social interactions as in marine dolphins,” they wrote. Fascinating.
Also, the frequency range of the signals produced by the botos wasn’t as low as the sounds produced by certain whales to communicate over vast distances, nor were they as high as the sounds used by marine dolphins to communicate over short distances. This might have something to do with life in river environments.
“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,” explained May-Collado in a press release.
As noted, this population of dolphins was habituated to humans, and the study took place next to a busy market. These factors may have interfered with the results.
“What could have happened is that we might have captured sounds associated with the behavioural contexts of the market or interactions, and as our analysis indicate there is more to discover if you keep looking through recordings,” explained Melo-Santos. “Nevertheless, the animals we recorded on this study are wild free-ranging individuals that interact with other dolphins from that same population, so the sounds we found are representative of this new species. Therefore this is a very important first step towards understanding more of a fundamental aspect of the biology of such a poorly known dolphin.”
Looking ahead, the researchers would like to study other populations of river dolphins, including those from other species (there are three other known river dolphin species) and from Araguaian river dolphins that are not accustomed to humans. Further analysis will shed more light on the communication abilities of botos, and the evolutionary roots of this capacity.
“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,” said May-Collado. “We now have all these new questions to explore.”
[PeerJ]