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The Ganges river dolphin is one of the only surviving members of a once diverse group of whales and dolphins.
New research has now shown that the hearing of these dolphins is unlike any other cetacean alive or extinct, and may have been a key reason why they alone survived when all other relatives died out.
The toothed whales, which includes all dolphins as well as some whales such as the sperm whale and beluga, are the most successful marine mammals currently swimming the oceans.
It is thought that their success and diversity has been driven by the evolution of echolocation. This would have allowed for the animals to rely less on their eyesight and move into more extreme environments such as the deep sea and murky river systems.
But there is one dolphin that lives in rivers that is unlike any other: the Ganges river dolphin.
While these dolphins look very similar to other species of riverine dolphins (such as those that live in the Amazon), they and the recently described Indus river dolphin are in fact the last surviving species of a once far more diverse group of marine mammals.
New research has shown that the inner ear of the Ganges river dolphin, which relates to how it uses and hears its echolocation, is vastly different to any other dolphin, alive or extinct.
Dr Travis Park, a researcher at the Museum, was involved in the study that compared the inner ear structure of the dolphin to it extinct fossil relatives. The findings are published in the journal Paleobiology.
'Within living dolphin species, the Ganges river dolphin is a bit strange,' says Travis. 'It has a very long, thin snout, and it is basically blind, so it relies really heavily on its echolocation abilities to make sense of its surroundings.
'But our research has found that even within its own group, it is an outlier.'
The Ganges river dolphin is found only within the freshwater river systems of the Ganges-Brahmaputra-Meghna and Karnaphuli-Sangu river systems of Bangladesh and India, while the recently described Indus river dolphin lives in the Indus river system of Pakistan.
They are the last surviving members of a once diverse group of toothed whales known as platanistoids. These marine mammals were found globally and occupied many of the same habitats as today's toothed whales, which belong to the delphinoid group.
'In terms of the Ganges river dolphin's evolutionary history, it is a bit of an odd ball,' explains Travis. 'It is one of just two surviving species of this once massive group that were the dominant toothed whales up until about 12-15 million years ago, when more modern dolphins appeared on the scene and eventually replaced them.'
Living in these murky waters, sight is effectively useless, so the Ganges river dolphins have reduced their eyes to such an extent that they even lack functioning lenses. Instead, they rely solely on echolocation.
The shape of the inner ear of these dolphins is linked to how the species uses echolocation. It is also markedly different from any living species, including those which also live in riverine environments.
'So in terms of the cochlear spiral it is really quite flat, and the actual spiral itself is really spaced out,' says Travis. 'We think it is part of how it is dealing with high frequencies within the cochlear canal and that is related to its different use of echolocation.'
The researchers wanted to find out whether this is because the dolphins have had to adapt their echolocation to deal with the relatively shallow and turbid water in which they live, or they shared their peculiar ear shape with their now-extinct relatives.
Travis and his colleagues scanned the fossil skulls of extinct platanistoids and comparing the shapes of their inner ears to the Ganges river dolphin and other living dolphin species.
Dr Mariana Viglino is a postdoctoral researcher at the Instituto Patagónico de Geología y Paleontología, Argentina, who was also involved with the project.
'We found that there is no typical platanistoid inner ear shape,' explains Mariana. 'Even within this group, the Ganges river dolphin is an outlier.
'So then we thought that maybe the unusual ear shape is why it survived when the rest of its group went extinct. Of course, there could be other reasons too, but this is something that could be contributing to it.'
While this research is still in its early days, Travis suspects that the shape of the inner ear might have a role in filtering out particular sounds and removing noise from the dolphin's echolocation, which is helping them to 'see' in the rough waters of the rivers.
Unfortunately, despite persisting for the last 15 million years, the Ganges river dolphin is now facing yet another battle for survival.
Over the past century, the rivers in which these dolphins live have been changed beyond recognition. Once free to swim for thousands of kilometres from the uplands down to the estuary, the dolphin populations are becoming more fragmented as the rivers are dammed for irrigation and electricity generation.
The waters have become more polluted, and the animals are caught by accident in fishing nets. It is now thought that despite ongoing conservation efforts, there are only 1,000 Ganges river dolphins and 1,000 Indus river dolphins.
But while the loss of any species is a tragic event, the prospect of the Ganges river dolphin fading into extinction would in some sense be an even bigger hit.
'Overall the outlook is not particularly positive for them,' says Travis. 'We are losing the only surviving representative of this once huge group.
'By evolutionary metrics, this would be a much bigger loss.'