South Asian river dolphins are toothed whales in the genus Platanista, which inhabit the waterways of the Indian subcontinent. They were historically considered to be one species (P. gangetica) with the Ganges river dolphin and the Indus river dolphin being subspecies (P. g. gangetica and P. g. minor respectively). Genetic and morphological evidence led to their being described as separate species in 2021. The Ganges and Indus river dolphins are estimated to have diverged 550,000 years ago. They are the only living members of the family Platanistidae and the superfamily Platanistoidea. Fossils of ancient relatives date to the late Oligocene.
South Asian river dolphins are small but stocky cetaceans with long snouts or rostra, broad flippers, and small dorsal fins. They have several unusual features. Living in murky river waters, they have eyes that are tiny and lensless; the dolphins rely instead on echolocation for navigation. The skull has large crests over the melon, which help direct their echolocation signals. These dolphins prey mainly on fish and shrimp and hunt them throughout the water column. They are active through the day and are sighted in small groups. Both species are listed as endangered by the IUCN Red List of mammals. Major threats include dams, barrages, fishing nets, and both chemical and acoustic pollution.
Taxonomy
South Asian river dolphins were traditionally considered to be one species, Platanista gangetica, with the Ganges and Indus River populations being subspecies (P. g. gangetica and P. g. minor, respectively). Heinrich Julius Lebeck named the Ganges river dolphin Delphinus gangeticus in 1801, while Johann Georg Wagler coined the genus name Platanista in 1830, a Latin word derived from the Greek "platanistēs", which may be related to the Greek words platē ("oar") or platē ("flat, broad"). This name was first given to the Ganges dolphin by Pliny the Elder in Naturalis Historia back in 77 CE. In 1853, Richard Owen described a specimen from the Indus and considered it to be the same species as the Ganges river dolphin, but a smaller form. However, a 2021 study reanalyzed the two populations and found significant genetic divergence and major differences in skull structure; this led to the conclusion that the two were indeed distinct species.
Several fossil species have been classified under Platanistoidea, the earliest of which date back to the late Oligocene ( million years ago). The number of species peaked around the early Miocene ( million years ago) and declined afterward. Examples of ancient platanistids include the genera Otekaikea and Waipatia and the species Awamokoa tokarahi of late Oligocene New Zealand, the family Allodelphinidae of early Miocene North Pacific, and Notocetus vanbenedeni and Aondelphis talen of early Miocene Patagonia. Platanistidae fossils have been found in Miocene deposits in Europe and North America. Fossil Platanistoidea showed a diversity of cochlea shapes, though Platanista was unusual in that it developed flatter spirals with larger gaps between them.
During the middle Miocene, the ancestor of Platanista entered the Indo-Gangetic Plain, then covered by inland seas, and remained there when sea levels dropped in the late Neogene and its environment converted to freshwater. River dolphins likely traveled from the Ganges River basin to the Indus via stream capture within the last five million years. The split between the two species is estimated to have occurred around 550,000 years ago based on mitochondrial DNA.
Description
South Asian river dolphins are stocky with broad, squared-off pectoral fins; elongated, slender rostrums (snouts); and tiny triangular dorsal fins. Their neck joints give them great flexibility. The Ganges species also has a protrusion near the frontal suture, which distinguishes it from the Indus species. Indus dolphins have more teeth than Ganges dolphins, averaging 33.2 teeth in the upper jaw and the 32.9 in the lower jaw, as compared to 28.4 in the upper jaw and 29.4 in the lower. The ears are adapted to hearing low frequencies, having a short, flattened cochlea with widely spaced spirals. In the 19th century, this species was reported to have occurred throughout the Indus River system, from the Indus River Delta north to Kalabagh just south of the Himalayas, including all the main tributaries. They can be found in pools over deep, but usually dwell in shallower water. They swim on their sides when in shallow water.
thumb|right|Indus river dolphins leaping|alt=Two dolphins leaping out of water
River dolphins are typically seen alone or in groups of up to 10 individuals, though enough natural resources may attract up to 30 dolphins. Individuals do not appear to have strong social bonds, outside of mothers and calves. Their clicks are about one octave below those of oceanic toothed whales of comparable size, meaning that they provide less information about the location of an object, but the dolphins' maxillary crests likely compensate by providing greater directional sensitivity.
River dolphins feed mainly on fish and shrimp. In one meta study, around 46% of prey items were found to be bottom-dwelling species, while 31% were near the surface, and 23% occupied the middle of the column. The most frequently taken prey are bagrid catfish, barbs, glass perches, spiny eels, gobies, and prawns. When hunting at the surface, dolphins listen for the movements of schooling fish which are then herded with spins, side-swimming, and lobtailing. Echolocation signals are not frequently used at the surface, since many fish at this level can hear ultrasound. At the mid-surface level, the dolphins use more echolocation clicks to find prey hidden in clutter and vegetation as far as away. They flush out bottom-dwelling prey by digging around.
Little is known about reproduction in these river dolphins. Calves are born around a year later. Births in the Ganges river dolphin appear to be most frequent between December and January and between March and May. For Indus river dolphins, newborns are most commonly seen between April and May. Growth layers in the teeth suggest South Asian river dolphins can live up to 30 years.
Conservation
thumb|right|Man holding Ganges river dolphin in [[Bhagalpur|alt=Man standing in water while holding dolphin]]
As of 2022, the IUCN Red List of mammals lists both South Asian river dolphins as endangered. Two assessments in 2014 and 2015 estimated populations of 3,500 for the Ganges river dolphin and 1,500 for the Indus river dolphin. A survey of Indian rivers between 2021 and 2023 found over 6,000 Ganges river dolphins and only three Indus river dolphins. The habitat of these river dolphins intersects with some of the most densely populated areas, leading to intense competition for water and resources. Dolphins captured in fishing nets are usually accidental, but dolphin oil is sought after as a fish lure, and thus fishermen may be motivated to kill caught dolphins.
South Asian river dolphins are protected by law in all the states they inhabit. They can be found in numerous protected areas, The Ganges and Indus river dolphins are considered to be the national aquatic animals of India and Pakistan respectively.
See also
- Irrawaddy dolphin
- List of cetaceans
- Makara – water creature from Hindu mythology that is sometimes depicted dolphin-like
- Project Dolphin (India)