South Asian river dolphin

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South Asian river dolphin
Temporal range: 0.012–0 Ma
↓
 Quaternary – recent[1]
File:Platanista gangetica noaa.jpg
Ganges river dolphin breaking the surface
South Asian river dolphin size comparison.svg
Size compared to an average human
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Infraorder: Cetacea
Family: Platanistidae
Genus: Platanista
Wagler, 1830
Type species
Delphinus gangetica [2]
Roxburgh, 1801
Species

Platanista gangetica
Platanista minor

SouthAsianRiverDolphin distribution2019.png
Ranges of the Indus river dolphin and Ganges river dolphin

South Asian river dolphins are toothed whales in the genus Platanista, both of which live in fresh water habitats in the northern 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). More recent genetic and morphological evidence have evaluated them into separate species. 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 rostrums, broad flippers and triangular dorsal fins. They have several unusual features. Their eyes are tiny and lensless, and instead rely on echolocation to navigate through murky river waters. 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 pollution, both chemical and noise.

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 classified the Ganges river dolphin as Delphinus gangeticus in 1801, while Johann Georg Wagler coined the genus name Platanista in 1830,[3] a Latin word derived from the Greek "platanistēs" which may be related to the Greek words platē ("oar") or platē ("flat, broad").[4] In 1853, Richard Owen described a specimen from the Indus and considered it to the same species as the Ganges river dolphin; but a smaller form.[3]

Based on differences in the structure of the skull, vertebrae, blood proteins and lipids, scientists declared them as separate species in the 1970s.[5] The results of these studies were criticized for their small sample sizes and absence of statistical analyses and the classification reverted to the pre-1970s consensus of a single species containing two subspecies by the late 1990s. A 2014 mitochondrial DNA study of the two populations did not display enough differences to support their classification as separate species.[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 led to the conclusion that both were indeed distinct species.[3]

South Asian river dolphins are the only surviving members of the family Platanistidae and the superfamily Platanistoidea.[7] They are not closely related to the other river dolphins of the families Lipotidae, Pontoporiidae and Iniidae, which all independently evolved for freshwater habitats.[8] The following cladogram is based on Gatesey and colleagues (2012) and shows the relationship of South Asian river dolphins among living toothed whale families:[9]

 Toothed whales 
 
 
 
 Delphinoidea 
 
 

 Belugas, narwhals (Monodontidae) Delphinapterus leucas NOAA.jpg

 
 

 Porpoises (Phocoenidae) Vaquita swimming through the void. (Phocoena Sinus).png

 
 
 

 Oceanic dolphins (Delphinidae) Orcinus orca NOAA 2.jpg

 
 
 
 

 Iniidae

 
 

 Pontoporiidae Pontoporia blainvillei.jpg

 
 
 
 

 Beaked whales (Ziphiidae) Ziphius cavirostris NOAA.jpg

 
 
 

 South Asian river dolphins (Platanistidae)

 
 
 
 

 Dwarf sperm whales (Kogiidae) Kogia sima (transparent background).png

 
 

 Sperm whales (Physeteridae) Physeter macrocephalus NOAA.jpg

 
 
 

Evolution

Several fossil species have been classified under Platanistoidea, the earliest of which date back to the late Oligocene; their numbers peaked around the early Miocene and declined afterward. Examples of ancient platanistids include the genera Otekaikea, Waipatia and 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. Fossil species classified under Platanistidae been found in Miocene deposits in Europe and North America. Fossil Platanistoidea showed a diversity of cochlea shapes, though Platanista was still unusual in that it developed flatter spirals with larger gaps between them.[7]

During the middle Miocene, the ancestor of Plantanita entered the Indo-Gangetic Plain, then covered by inland seas, and remained there when sea levels dropped in the late Neogene and their environment converted to freshwater.[8] River dolphins likely traveled from the Ganges River basin to the Indus via stream capture within the last 5 million years. The split between the two species is estimated to have occurred around 550,000 years ago based on mitochondrial DNA.[10]

Characteristics

Full body (above) and skull of Indus river dolphin

South Asian river dolphins are stocky in build with broad, square-tipped pectoral fins, elongated, slender rostrums and tiny, triangular dorsal fins. Their neck joints give them great flexibility.[3][11][12] Unusual among cetaceans, the blowhole is cleft-shaped,[12] and the skeletal digits can be seen through the flippers.[11] South Asian river dolphins possess some features that are "primitive" for a cetacean, such as a cecum and air sacs near the blowhole. The testes of the males are located closer to the underside than marine dolphins and descend more.[13] Their skin ranges from gray to grayish brown in colour, though the rostrum and surrounding areas may have some pinkish colouration. The Indus species tends to be more brownish.[3]

The skull of these river dolphins have an unusual morphology. The maxilla has pneumatic extensions or "crests" on each side which curve around the melon and protrude forward over the rostrum. These likely help them focus their echolocation signals in their riverine environment.[14] The Ganges species additionally has a protrusion near the frontal suture, which distinguishes it from the Indus species.[3] The curved teeth of these river dolphins are longer at the front, where they remain exposed when the jaws are closed.[12] Indus dolphins have more teeth, the upper jaw averaging 33.2 teeth and the lower jaw 32.9, while the Ganges dolphin has an average of 28.4 and 29.4 for the upper and lower jaw respectively.[3]

Living in murky waters, South Asian river dolphins are nearly blind, their tiny eyes having a flattened cornea and no lens. The retina, with its thick receptor layer, thin bipolar and ganglion cell layers and reduced optic nerve, does not form images but instead merely discerns light. It also lacks a pigmented epithelium, and the animal instead relies on a sphincter-like muscle around the eye to control access to the retina and prevent light scattering, similar to a pinhole.[15] The ears are adapted to hearing low-frequencies; having a short, flattened cochlea with widely-spaced spirals.[7]

In one sample study, the maximum recorded length for a Ganges river dolphin was found to be 267 cm (8.76 ft), while the maximum recorded weight was 108 kg (238 lb). For the Indus species, the maximum length and weight was 241 cm (7.91 ft) and 120 kg (260 lb). Female Ganges dolphins are generally longer than Indus dolphins of both sexes, while Ganges males, by contrast, are shorter than both. Indus dolphins tend to be proportionally heavier than Ganges dolphins, independent of sex.[3]

Distribution and habitat

Ganges river dolphin in the Sundarbans, Bangladesh

South Asian river dolphins inhabit the northern waterways of the Indian subcontinent. The Ganges river dolphin lives in the Ganges, Brahmaputra, Meghna, Karnaphuli and Sangu rivers and tributaries. They range from the Himalayan foothills to the Sundarbans; in southern Nepal, India and Bangladesh.[3][12] They have also been seen in the Budhabalanga River, presumably entering via an outflow of freshwater form the Ganges to the Bay of Bengal.[16] 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 the mainstream Indus River of Pakistan; with three subpopulations between the Chashma, Taunsa, Guddu and Sukkur barrages. Two other populations exist south of Sukkur and in the Beas River of India.[3][17] In the 19th century; this species was reported to have occurred throughout the Indus River system, from the delta north to Kalabagh just under the Himalayas, and included all the main tributaries.[3] The Indus river dolphin is reported to have disappeared between the Jinnah and Chashma barrages since 2001.[17]

South Asian river dolphins inhabit major river channels during the dry season and travel to smaller tributaries for the monsoon. They are most commonly found in stream pools, meanders, confluences, and around islands and shoals which produce relatively stable waters.[16] They can be found in waters over 30 m (98 ft) deep.[12]

Behaviour and life history

South Asian river dolphins appear to be active throughout the day. Living in ever-flowing waters, they swim almost constantly with only brief periods of sleep, which add up to seven hours per day.[18] They swim on their sides when in shallow water.[19] River dolphins generally surface with the rostrum, head and dorsal fin breaking the water and rarely rise the tail fluke or breach, though this can vary based on age class, distance from shore and time of day. Diving may last from ten seconds to almost eight minutes. Adult and subadult dives are nearly equally as long and more so than juveniles and newborns, while juveniles can dive for longer than newborns.[20]

Indus river dolphins leaping

River dolphins are typically seen alone or in groups of up to 10 individuals, though a mass of resources may attract up to 30 dolphins. Individuals do not appear to have strong social bonds, outside of mothers and calves.[12] Living in a shallow, river environments with more acoustic obstacles, dolphins echolocate using repetitive clicks which have a proportional low centroid frequency of 61.4±4.9 kHz and are octave below those of oceanic toothed whales of comparable size. The maxilla crests likely help them maintain a directionality similar to that of their marine relatives.[14] The gaps between each click lasts 10 to 100 ms for adults and young over six months old.[21] Vocalizations used for communication include bursts and twitterings.[22]

River dolphins feed mainly on fish and shrimp. Around 46% of prey items are bottom-dwelling species while 31% are near the surface and 23% occupy the middle of the column. The most frequently taken prey are catfish, barbs, glass perches, spiny eels, gobies and prawns. When hunting at the surface, dolphin use their ears to detect the splashing of schooling fish which are then herded with spins, side-swimming and lobtailing. Echolocation signals are not frequently used as the fish at this level can heard 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. Bottom-dwelling prey is flushed out by moving around substrate.[19]

Little is known about reproduction in these river dolphins.[12] 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 ending with one of the males earning the right to mate.[13] Calves are born around a year later.[23] Births in the Ganges river dolphin appear to be greatest between December and January and between March and May. For Indus River dolphins, newborns are most commonly seen between April and May.[12] One sample study of Indus river dolphins found that calves were around 70 cm (28 in) long at birth, and may nurse for up to a year. They eat their first soild food within a couple months. Sexual maturity is reached at around ten years, though males may not reach their adult size until they are 20 years.[24]

Conservation

Man holding Ganges river dolphin in Bhagalpur

As of 2022, the IUCN Red List of mammals lists both South Asian river dolphins as endangered.[25][26] Two assessments in 2014 and 2015 estimated populations of 3,500 for the Ganges river dolphin and 1,500 for the Indus river dolphin.[16][27] The Ganges species appears to be decreasing while the Indus species may be increasing.[25][26] The habitat of these river dolphins intersects with some of the most densely populated areas in the world and they compete with humans. for freshwater.[16][27] The creation of dams and barrages in the Indus River system have heavily fragmented the range of the Indus river dolphin, leading to a population decline of 80% since the 19th century.[27] Around 50 such structures have been built in the historical range of the Ganges species.[16]

River dolphins accumulate high amounts of persistent organic pollutants, pesticides and heavy metals in their system due to being at the top of their riverine food chain.[16][27] Hence, they are seen as bioindicators for the health of river systems. Dolphins captured in fishing nets are usually accidental, but dolphin oil is sought after as a fish lure and thus fisherman may be motivated to kill caught dolphins.[16] Being nearly blind and relying on echolocation for navigation, river dolphins are also negatively affected by noise pollution from boats.[28]

South Asian river dolphins are protected by law in all the states they inhabit, and they can be found in numerous protected areas,[25][26] including ones established specifically for them, such as the Indus Dolphin Reserve in Pakistan and the Vikramshila Gangetic Dolphin Sanctuary in India.[16][27] International trade is prohibited by the listing of the South Asian river dolphins on Appendix I of the Convention on International Trade in Endangered Species.[29] The Ganges and Indus river dolphins are considered to be the national aqautic animals of India and Pakistan respectively.[30][31] On 20 May 2013, India's Ministry of Environment and Forests declared dolphins ‘nonhuman persons’ and as such has forbidden their captivity for entertainment purposes; keeping dolphins in captivity must satisfy certain legal prerequisites.[32]

See also

References

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  28. ^ Dey, M; Krishnaswamy, J; Morisaka, T; Kelkar, N (2019). "Interacting efects of vessel noise and shallow river depth elevate metabolic stress in Ganges river dolphins". Scientific Reports. 9 (15426): 15426. doi:10.1038/s41598-019-51664-1. PMC 6817857. PMID 31659202. S2CID 204918073.
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