South Asian river dolphin
The South Asian river dolphins are two species of toothed whales in the genus Platanista, both of which live in fresh water habitats in northern South Asia.[2] Both species in the genus are endangered.
Until 1998, they were regarded as two separate species; in 1998, the classification was changed from two separate species to subspecies. More recent studies have recovered them as distinct species, and they have thus been reclassified as such.[3]
Until the 1970s, the South Asian river dolphin was regarded as a single species. The two subspecies are geographically separate and have not interbred for many hundreds if not thousands of years. Based on differences in skull structure, vertebrae, and lipid composition scientists declared the two populations as separate species in the early 1970s.[4]
In 1998, the results of these studies were questioned and the classification reverted to the pre-1970 consensus of a single species containing two subspecies until the taxonomy could be resolved using modern techniques such as molecular sequencing. The latest analyses of mitochondrial DNA of the two populations did not display the variances needed to support their classification as separate species.[5][6] However, a 2021 study re-analyzed the two populations and found significant genetic divergence and major differences in the structure of their skulls; this, when combined with the fact that the Ganges and Indus basins had not been contiguous for over 40 million years (indicating that the event that split the two species was likely a chance event with no gene flow between the split populations afterwards) led to the conclusion that both were indeed distinct species.[3]
An assessment of divergence rates in mitochondrial DNA of the two species indicates that they diverged from a common ancestor around 550,000 years ago, likely in a case of stream capture that briefly connected the Indus and Ganges basins, allowing dolphins to travel through both, up until the rivers reverted to their previous state and isolated the dolphins in new habitats.[3] The ancestor of the genus Platanista is thought to have been a marine platanistid inhabiting the epi-continental seas in South Asia during the sea level rises in the middle Miocene.[5] The earliest fossil identified as belonging to the genus only 12,000 years old.[1]
The South Asian river dolphins have the long, pointed nose characteristic of all river dolphins. Their teeth are visible in both the upper and lower jaws even when the mouth is closed. The teeth of young animals are almost an inch long, thin and curved; as animals age, the teeth undergo considerable changes and in mature adults become square, bony, flat disks. The snout thickens towards its end. Navigation and hunting are carried out using echolocation.[7] They are unique among cetaceans in that they swim on their sides.[8] The body is a brownish color and stocky at the middle. The species has only a small, triangular lump in the place of a dorsal fin. The flippers and tail are thin and large in relation to the body size, which is about 2-2.2 meters in males and 2.4–2.6 m in females. Mature females are larger than males. Sexual dimorphism is expressed after females reach about 150 cm (59 in); the female rostrum continues to grow after the male rostrum stops growing, eventually reaching approximately 20 cm (7.9 in) longer.
