Rare River Dolphin Species Neither Solitary Or Quiet Says New Study 

A rare species of river dolphin that was believed to be solitary and largely silent has been revealed to make hundreds of sounds. The discovery of Araguaian river dolphin sounds raises some interesting questions about the evolution of the way marine mammals communicate. The species is closely related to the Amazon river dolphin.

Hard to identify individuals

One major issue when it comes to studying the behaviour of river dolphins is they do not perform conspicuous displays at the surface. This means it is very difficult to identify individuals in the field. This species of dolphin is also extremely shy and tends to move quickly away from anyone seeking to observe it.  Researchers are taking advantage of a unique opportunity to study a small group of dolphins that pay regular visits to a fish market in the Araguaia-Tocantins river system in Brazil.

Acoustic technology and observation

The researchers made use of acoustic technology and underwater observations as the dolphins visited the market to be fed by both tourists and locals. River dolphins have received very little academic attention and as a result the researchers thought they would find more acoustic signals than had been previously recorded. Even with that expectation, they were still very surprised by the actual results.

At par with marine dolphins

The published findings suggest that instead of being mainly silent as was previously believed, Aragaian dolphins communicate with another at par with their marine counterparts. The team was able to distinguish between 237 different sound types and they still think their sample size wasn’t large enough to capture then entire repertoire. The researchers think that the river habitat is probably a key driver in the evolution of the communication frequency.

Similar behaviour to toothed whales

Rivers have plenty of obstacles such as vegetation in their habitat so have probably developed a signal that avoids vegetation echoes and improves the range of communication between mothers and their calves. The non-linear nature of their calls is quite similar to those made by toothed whales. The information carried by these calls is believes to be regarding group identity as well as information that maintains social cohesion and even individual identity and emotional state.

Evolutionary relics

River dolphins are sometimes referred to as being “evolutionary relics” and the history of their evolution is still being debated. One theory suggests their ancestors were coastal marine dolphins that ended up being stranded in fresh water systems as seal levels dropped about 20 million years ago.

More work to be done

There is still far more work required and studies must be undertaken in parts of the river where dolphins unused to humans roam in order to determine whether there are any possible differences between populations. The researchers says they cannot tell us what the evolutionary story is just yet until it is understood what sounds are produced by other river dolphins in the Amazon and how those sounds relate to what has been discovered.