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Evidence is mounting that the Amazon river dolphin (Inia geoffrensis), one of only a few remaining dolphin species that live purely in fresh water, is in fact multiple species.

New research reveals that the skulls of these dolphins vary substantially between populations in different river basins in South America. If their physical differences represent evolutionary adaptations rather than just random variation within a population, there could be profound conservation implications for these animals, which have declined sharply in recent years.

Amazon river dolphins, also called botos, have long been sorted into three subspecies geographically isolated by waterfalls and rapids: Inia geoffrensis geoffrensis in most of the Amazon basin, I. g. boliviensis in Bolivia and I. g. humboldtiana in Venezuela鈥檚 Orinoco river basin.

, researchers proposed that a population in the Araguaia river basin in central Brazil should be considered a distinct species (I. araguaiaensis) based on its genetic uniqueness.

But to accept new boto species, biologists require more information on the animals鈥� physical features, which has been lacking, says at the Butantan Institute in S茫o Paulo, Brazil.

Hingst-Zaher and her colleagues have now examined, measured and categorised 46 boto skulls from an international set of museum collections.

Both Bolivian and Venezuelan botos have comparatively delicate skulls. The Bolivian botos have long snouts that are lined with extra teeth, and the Venezuelan botos are particularly small.

But the proposed Araguaia species鈥� skull was difficult to differentiate from that of the Amazonian population, to Hingst-Zaher鈥檚 surprise.

The boto probably consists of a complicated, evolutionary patchwork, with this study unveiling 鈥渢he tip of the iceberg of the variation that is there鈥�, she says.

Honing these distinctions has major implications for boto conservation. 鈥淚f [the boto] is one thing throughout the Amazon basin, it would be of less concern,鈥� says Hingst-Zaher, but 鈥渋f we have three species, the three of them will be endangered鈥� given ongoing threats like the construction of hydroelectric dams.

at the University of Dundee in the UK is concerned that differences between the sexes in skull size and shape could be inadvertently skewing the results. 鈥�Inia is recognised as one of the most sexually dimorphic species of cetaceans for size; the males are enormous compared to females,鈥� he says.

Overall, however, Martin does suspect that various boto groups have embarked on their own evolutionary trajectories and if we assume that each group is unique it can help safeguard against extinction of a potentially unrecognised species or subspecies.

鈥淕iven how long these different groups of animals have been physically separated, it would be astonishing if they are not different and my guess is they should be recognised as full species,鈥� says Martin.

In particular, the toothy jaws of the Bolivian botos look like a big enough difference to consider them a separate species, says Martin.

at the South Australian Museum in Adelaide agrees, and she is interested in how the numbers of vertebrae in the dolphins鈥� spines differ, since this can vary between other dolphin species. Investigating more of the skeleton is one next step for Hingst-Zaher and her team.

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