Freshwater dolphins are evolutionary relics, and their calls give clues to the origins of cetacean communication in general. Christopher Intagliata reports.
Freshwater dolphins live in many of the world's biggest rivers, from the Amazon to the Ganges, and they differ in many ways from their better-known oceangoing cousins. "They have a flexible neck. They have different types of teeth. They can also move their flippers independently in different directions, so they can swim backwards."
Gabriel Melo-Santos, a marine biologist at the University of St. Andrews in Scotland. “When I decided I was going to be a biologist I decided I wanted to study dolphins. And being born in the Amazon, it was only natural to go for the river dolphins."
He says another thing that sets his study subjects apart from marine dolphins are their calls <river dolphin calls>.
Over several years, Melo-Santos has recorded the sounds of Araguaian river dolphins that came calling at the Mocajuba fish market. That's on Brazil's Tocantins River. Using sound analysis software, his team fished 237 distinct sound types from the recordings—indicating the dolphins have a wide repertoire.
The call collection, published in the journal PeerJ, has only a few whistles. Instead, three quarters of the collected sounds were short two-parters, like this one <call> produced by a female calf as she rubbed her head on her mother's belly. It’s a call that's more similar in structure to the ones orcas and pilot whales make to identify a family or social group than to the social whistles of marine dolphins. [Gabriel Melo-Santos et al, The newly described Araguaian river dolphins, Inia araguaiaensis (Cetartiodactyla, Iniidae), produce a diverse repertoire of acoustic signals]
"These are older lineages, right? So if you understand how these dolphins communicate, we might have a sense to understand how the communication system evolved in different lineages of cetaceans." Meaning perhaps these calls between mother and calf are, like the river dolphins themselves, an evolutionary relic.
[The above text is a transcript of this podcast.]