| South Asian river dolphin | |
|---|---|
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| Ganges river dolphin breaking the surface | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Suborder: | Whippomorpha |
| Infraorder: | Cetacea |
| Family: | Platanistidae |
| Genus: | Platanista Wagler, 1830 |
| Type species | |
| Delphinus gangeticus[2] Lebeck, 1801 | |
| Species | |
 | |
| Current ranges of theIndus river dolphin andGanges river dolphin | |
South Asian river dolphins aretoothed whales in the genusPlatanista, which inhabit the waterways of theIndian subcontinent. They were historically considered to be one species (P. gangetica) with theGanges river dolphin and theIndus river dolphin being subspecies (P. g. gangetica andP. 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 havediverged 550,000 years ago. They are the only living members of the familyPlatanistidae and the superfamily Platanistoidea. Fossils of ancient relatives date to thelate Oligocene.
South Asian river dolphins are small but stockycetaceans with long snouts orrostra, 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 onecholocation for navigation. The skull has large crests over themelon, which help direct their echolocation signals. These dolphins prey mainly on fish and shrimp and hunt them throughout thewater column. They are active through the day and are sighted in small groups. Both species are listed asendangered by theIUCN Red List of mammals. Major threats include dams,barrages, fishing nets, and both chemical andacoustic pollution.
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 andP. g. minor, respectively).Heinrich Julius Lebeck named the Ganges river dolphinDelphinus gangeticus in 1801, whileJohann Georg Wagler coined the genus namePlatanista in 1830,[3] aLatin word derived from theGreek "platanistēs", which may be related to the Greek words platē ("oar") or platē ("flat, broad").[4] This name was first given to the Ganges dolphin byPliny the Elder inNaturalis 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.[3]
Based on differences in skull and vertebrae structure, blood proteins, andlipids, scientists declared them to be separate species in the 1970s.[5][6] The results of these studies were criticized for their small sample sizes and the absence of statistical analyses; by the late 1990s, the two populations were again considered to be two subspecies of a single species.[7][6] A 2014mitochondrial DNA study found insufficient differences to support their classification as separate species.[6] 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.[3]
South Asian river dolphins are the only surviving members of the familyPlatanistidae and the superfamily Platanistoidea.[8] They are notclosely related to otherriver dolphins of the familiesLipotidae,Pontoporiidae, andIniidae, which allindependently adapted to freshwater habitats.[9] The followingcladogram is based on Gatesy and colleagues (2012) and McGowen and colleagues (2020); and shows the relationship of South Asian river dolphins to other living toothed whale families:[10][11]
| Toothed whales |
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Several fossil species have been classified under Platanistoidea, the earliest of which date back to thelate Oligocene (c. 25 million years ago). The number of species peaked around theearly Miocene (c. 19 million years ago) and declined afterward. Examples of ancient platanistids include the generaOtekaikea andWaipatia and the speciesAwamokoa tokarahi of late Oligocene New Zealand, the familyAllodelphinidae of early Miocene North Pacific, andNotocetus vanbenedeni andAondelphis talen of early Miocene Patagonia. Platanistidae fossils have been found in Miocene deposits in Europe and North America. Fossil Platanistoidea showed a diversity ofcochlea shapes, thoughPlatanista was unusual in that it developed flatter spirals with larger gaps between them.[8]
During the middle Miocene, the ancestor ofPlatanista entered theIndo-Gangetic Plain, then covered byinland seas, and remained there when sea levels dropped in the lateNeogene and its environment converted to freshwater.[9] River dolphins likely traveled from theGanges River basin to theIndus viastream capture within the last five million years. Thesplit between the two species is estimated to have occurred around 550,000 years ago based on mitochondrial DNA.[12]
South Asian river dolphins are stocky with broad, squared-offpectoral fins; elongated, slenderrostrums (snouts); and tiny triangulardorsal fins. Their neck joints give them great flexibility.[3][13][14] Unusual among cetaceans, theblowhole is slit-shaped.[14] The finger bones can also be seen through the flippers.[13] South Asian river dolphins possess some features that are "primitive" for a cetacean, such as acecum connected to the gut and air sacs near the blowhole. The testes of the males are located closer to the underside than in marine dolphins and descend more.[15] Their skin ranges from grey to greyish-brown in colour, though the rostrum and surrounding areas may have some pinkish colouration. The Indus species tends to be more brownish.[3]
In one study sampling 46 Ganges river dolphins, the maximum length and weight recorded were 267 cm (8.76 ft) and 108 kg (238 lb). For the Indus species, the maximum length and weight were 241 cm (7.91 ft) and 120 kg (260 lb) (80 individuals sampled). Female Ganges dolphins are generally longer than Indus dolphins of both sexes, while male Ganges dolphins are shorter than Indus dolphins of both sexes. Indus dolphins tend to be proportionally heavier than Ganges dolphins, independent of sex.[3]
South Asian river dolphin skulls have unusual features. Themaxilla (fixed upper jawbone) haspneumatic extensions or "crests" on each side which curve around themelon and protrude forward over the rostrum. These likely help them focus their echolocation signals in their riverine environment.[16] The Ganges species also has a protrusion near thefrontal suture, which distinguishes it from the Indus species.[3] The teeth of South Asian river dolphins are curved and longer in the front, where they remain exposed when the jaws are closed.[14] 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.[3]
Living in murky waters, South Asian river dolphins are nearly blind, their tiny eyes having flattenedcorneas and nolens. Theretina—which connects to a reducedoptic nerve—does not form images but instead merely discerns light. The animal relies on asphincter-like muscle around the eye to control access to the retina and preventlight scattering, similar to apinhole.[17] The ears are adapted to hearing low frequencies, having a short, flattened cochlea with widely spaced spirals.[8]
South Asian river dolphins inhabit the northern waterways of theIndian subcontinent. Ganges river dolphins live in the Ganges,Brahmaputra,Meghna,Karnaphuli, andSangu rivers and theirtributaries. Their range extends from theHimalayan foothills to theGanges Delta, across the countries ofNepal,India andBangladesh.[3][14] It is unknown if they are present inBhutan.[18] Outflows of freshwater into theBay of Bengal have allowed them to swim along the coast, and there is at least one record of an individual entering theBudhabalanga River, around 300 km (190 mi) southwest of the Ganges Delta.[19] This species has maintained much of its range since the 19th century but has disappeared from some northern and western rivers and waterways.[3]
The Indus river dolphin mainly lives in theIndus River ofPakistan, with three subpopulations between theChashma,Taunsa,Guddu, andSukkurbarrages. Two other populations exist south of Sukkur and in theBeas River of India.[3][20] In the 19th century, this species was reported to have occurred throughout the Indus River system, from theIndus River Delta north toKalabagh just south of the Himalayas, including all the main tributaries.[3] The Indus river dolphin is reported to have disappeared between theJinnah and Chashma barrages after 2001.[20]
South Asian river dolphins inhabit major river channels during thedry season and travel to smaller tributaries for themonsoon. They are most commonly found instream pools,meanders, andconfluences, and aroundriver islands andshoals, which produce relatively stable waters.[19] They can be found in pools over 30 m (98 ft) deep, but usually dwell in shallower water.[14]
South Asian river dolphins appear to be active throughout the day. Living in flowing waters, they swim almost constantly with only brief periods of sleep, which add up to seven hours per day.[21] They swim on their sides when in shallow water.[22] River dolphins generally surface with the rostrum, head, and dorsal fin breaking the water and rarelybreach or raise the tail fluke, though surface activity can vary based on age, distance from shore and time of day. Diving may last as long as eight minutes among adults and subadults; dives of newborns and juveniles are not as long.[23]
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.[14] Living in shallow river environments with acoustic obstacles, these dolphins echolocate using repetitive clicks spaced 10 to 100milliseconds apart.[24][25] Their clicks are about oneoctave 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.[16] Vocalisations used for communication include bursts and twitterings.[26]
River dolphins feed mainly on fish and shrimp. In onemeta study, around 46% of prey items were found to bebottom-dwelling species, while 31% were near the surface, and 23% occupied the middle of the column. The most frequently taken prey arebagrid catfish,barbs,glass perches,spiny eels,gobies, andprawns. When hunting at the surface, dolphins listen for the movements of schooling fish which are then herded with spins, side-swimming, andlobtailing. 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 20 m (66 ft) away. They flush out bottom-dwelling prey by digging around.[22]
Little is known about reproduction in these river dolphins.[14] Courtship and mating behaviour for the Ganges species has been documented from March to May, when the water level is lower, and involves multiple males chasing one female and ends with one of the males earning the right to mate.[15] Calves are born around a year later.[27] 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.[14] Indus river dolphins calves are around 70 cm (28 in) long at birth and may nurse for up to a year. They eat their first solid food within a couple months. South Asian river dolphins reach sexual maturity at around ten years, though males may not reach their adult size until 20 years.[28] Growth layers in the teeth suggest South Asian river dolphins can live up to 30 years.[29]
As of 2022, theIUCN Red List of mammals lists both South Asian river dolphins asendangered.[18][30] Two assessments in 2014 and 2015 estimated populations of 3,500 for the Ganges river dolphin and 1,500 for the Indus river dolphin.[19][31] A survey of Indian rivers between 2021 and 2023 found over 6,000 Ganges river dolphins and only three Indus river dolphins.[32] The habitat of these river dolphins intersects with some of the most densely populated areas, leading to intense competition for water and resources.[19][31][33]
The creation of dams and barrages in the Indus River system have heavilyfragmented the range of the Indus river dolphin, leading to a population decline of 80% since the 19th century.[31] Around 50 such structures have been built in the historical range of the Ganges species.[19] Fragmentation of populations makes these dolphins more vulnerable to inbreeding.[33] The heavy extraction of water in these dense populations also puts the dolphins at risk.[19]
River dolphinsaccumulate high amounts ofpersistent organic pollutants,pesticides, andheavy metals in their system due to being at the top of their riverinefood web.[19][31] Hence, they are seen asbioindicators for the health of river systems.[19] Fishermen compete with these animals for fish of certain sizes.[33] Dolphins captured in fishing nets are usuallyaccidental, but dolphin oil is sought after as a fish lure, and thus fishermen may be motivated to kill caught dolphins.[19] Being nearly blind and relying on echolocation for navigation, river dolphins are also negatively affected bynoise pollution from boats.[34]
South Asian river dolphins are protected by law in all the states they inhabit. They can be found in numerous protected areas,[18][30] including ones established specifically for them, such as theIndus Dolphin Reserve in Pakistan and theVikramshila Gangetic Dolphin Sanctuary in India.[19][31] International trade is prohibited by the listing of the South Asian river dolphins on Appendix I of theConvention on International Trade in Endangered Species.[35] The Ganges and Indus river dolphins are considered to be thenational aquatic animals of India and Pakistan respectively.[36][37]
Media related toPlatanista at Wikimedia CommonsAuthority control databases: National |
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