South Asian river dolphin
South Asian river dolphin Temporal range: Quaternary – recent[1]
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Ganges river dolphin breaking the surface | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Mammalia |
Order: | Artiodactyla |
Infraorder: | Cetacea |
Family: | Platanistidae |
Genus: | Platanista Wagler, 1830 |
Type species | |
Delphinus gangeticus[2] Lebeck, 1801
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Species | |
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Current ranges of the Indus river dolphin and Ganges river dolphin |
South Asian river dolphins are
South Asian river dolphins are small but stocky
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).
Based on differences in skull and vertebrae structure, blood proteins, and lipids, 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 2014 mitochondrial 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]
Evolution
South Asian river dolphins are the only surviving members of the family Platanistidae and the superfamily Platanistoidea.[8] They are not closely related to other river dolphins of the families Lipotidae, Pontoporiidae, and Iniidae, which all independently adapted to freshwater habitats.[9] The following cladogram 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 the
During the middle Miocene, the ancestor of Platanista entered the
Description
South Asian river dolphins are stocky with broad, squared-off
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. The maxilla (fixed upper jawbone) 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.[16] The Ganges species also has a protrusion near the frontal 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 flattened
Distribution and habitat
South Asian river dolphins inhabit the northern waterways of the
The Indus river dolphin mainly lives in the 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][20] 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.[3] The Indus river dolphin is reported to have disappeared between the Jinnah and Chashma barrages after 2001.[20]
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, and confluences, and around river islands and shoals, 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]
Behaviour and life history
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 rarely breach 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 100 milliseconds apart.[24][25] 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.[16] Vocalisations used for communication include bursts and twitterings.[26]
River dolphins feed mainly on fish and shrimp. In one
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]
Conservation
As of 2022, the IUCN Red List of mammals lists both South Asian river dolphins as endangered.[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] The Ganges species appears to be decreasing, while the Indus species may be increasing.[18][30] 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][32]
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.[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.[32] The heavy extraction of water in these dense populations also puts the dolphins at risk.[19]
River dolphins
South Asian river dolphins are protected by law in all the states they inhabit. They can be found in numerous protected areas,
See also
- Irrawaddy dolphin
- List of cetaceans
- Makara – water creature from Hindu mythology that is sometimes depicted dolphin-like
- Project Dolphin (India)
References
- ^ "Platanista Wagler 1830 (toothed whale)". Paleobiology Database. Archived from the original on 12 December 2021. Retrieved 26 March 2018.
- OCLC 62265494.
- ^ hdl:10023/21691.
- ^ "Platanista". Merriam-Webster.com Dictionary. Archived from the original on 22 September 2022. Retrieved 22 September 2022.
- ^ Pilleri, G.; Marcuzzi, G.; Pilleri, O. (1982). "Speciation in the Platanistoidea, systematic, zoogeographical and ecological observations on recent species". Investigations on Cetacea. 14: 15–46.
- ^ S2CID 14113756.
- ISBN 9781891276033.
- ^ from the original on 5 April 2023. Retrieved 31 October 2022.
- ^ PMID 11296868.
- (PDF) from the original on 9 June 2015. Retrieved 4 September 2015.
- PMID 31633766.
- S2CID 14113756.
- ^ PMID 11027333.
- ^ ISBN 978-0-12-409542-7.
- ^ PMID 24924188.
- ^ PMID 23573197.
- from the original on 2 August 2023. Retrieved 1 December 2019.
- ^ . Retrieved 17 October 2022.
- ^ PMID 24924188.
- ^ .
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- ^ S2CID 89753459.
- JSTOR 24111514.
- PMID 23573197.
- S2CID 22758375.
- ^ Mizue, K.; Masaharu, N.; Akira, T. (1971). "The underwater sounds of Ganges River dolphin (Platanista gangetica)". The Scientific Reports of the Whales Research Institute. 23: 123–128.
- ISBN 978-0-12-804327-1.
- ^ Kasuya, T. (1972). "Some information on the growth of the Ganges dolphin with a comment on the Indus dolphin". The Scientific Reports of the Whales Research Institute. 24: 87–108.
- hdl:10023/20641.
- ^ . Retrieved 14 October 2022.
- ^ from the original on 22 September 2022. Retrieved 27 August 2021.
- ^ PMID 32211183.
- S2CID 204918073.
- ^ "Appendix I". Convention on International Trade in Endangered Species of Wild Fauna and Flora. 22 June 2022. Archived from the original on 5 October 2011. Retrieved 6 January 2023.
- ^ "National Aquatic Animal – National Symbols – Know India: National Portal of India". National Portal of India. Archived from the original on 22 January 2013. Retrieved 23 March 2012.
- ^ "National Symbols of Pakistan". Government of Pakistan. Archived from the original on 28 November 2016. Retrieved 23 October 2016.
External links
- Media related to Platanista at Wikimedia Commons