South Asian river dolphin facts for kids

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South Asian river dolphin
Temporal range: 0.012–0Ma
Quaternary – Recent
Ganges river dolphin leaping out of the water
Size compared to an average human
Conservation status
Scientific classification
Genus: Platanista
Species: gangetica
Subspecies

Platanista gangetica gangetica
Platanista gangetica minor

Ranges of the Ganges river dolphin and of the Indus river dolphin

The South Asian river dolphin (Platanista gangetica) is an endangered freshwater or river dolphin found in the region of Indian Subcontinent which is split into two subspecies, the Ganges river dolphin (P. g. gangetica, ~3,500 individuals) and the Indus river dolphin (P. g. minor, ~1,500 individuals). The Ganges river dolphin is primarily found in the Ganges and Brahmaputra Rivers and their tributaries in India, Bangladesh and Nepal, while the Indus river dolphin is now found only in the main channel of the Indus River in Pakistan and active channels connected to it between the Jinnah and Kotri barrages. From the 1970s until 1998, they were regarded as separate species; however, in 1998, their classification was changed from two separate species to subspecies of a single species (see taxonomy below). The Ganges river dolphin has been recognized by the government of India as its National Aquatic Animal and is the official animal of the Indian city of Guwahati. The Indus river dolphin has been named as the National Mammal of Pakistan.

Description

The South Asian river dolphin has 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; however, 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. They are unique among cetaceans in that they swim on their sides. 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. The oldest recorded animal was a 28-year-old male, 199 cm in length. 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.

Platanista gangetica skull cast
Skull cast

Biology

Schnabeldelphin-drawing
Indus river dolphin, 1927 illustration

Life cycle

Ganges river dolphin skeleton
Ganges river dolphin skeleton specimen exhibited in Museo di storia naturale e del territorio dell'Università di Pisa

Births may take place year round, but appear to be concentrated between December to January and March to May. Gestation is thought to be approximately 9–10 months. After around one year, juveniles are weaned and they reach sexual maturity at about 10 years of age. During the monsoon, South Asian river dolphins tend to migrate to tributaries of the main river systems. Occasionally, individuals swim along with their beaks emerging from the water, and they may "breach"; jumping partly or completely clear of the water and landing on their sides.

Diet

The South Asian river dolphin relies on echolocation to find prey due to their poor eyesight. Their extended rostrum is advantageous in detecting hidden or hard to find prey items. The prey is held in their jaw and swallowed. Their teeth are used as a clamp rather than a chewing mechanism.

The species feeds on a variety of shrimp and fish, including carp and catfish. The Ganges subspecies may take birds and turtles. They are usually encountered on their own or in loose aggregations; the dolphins do not form tight interacting groups.

Vision

The species lacks a crystalline eye lens and has evolved a flat cornea. The combination of these traits makes the eye incapable of forming clear images on the retina and renders the dolphin effectively blind, but the eye may still serve as a light receptor. The retina contains a densely packed receptor layer, a very thin bipolar and ganglion cell layer, and a tiny optic nerve (with only a few hundred optic fibers) that are sufficient for the retina to act as a light-gathering component. Although its eye lacks a lens (this species is also referred to as the "blind dolphin"), the dolphin still uses its eye to locate itself. The species has a slit similar to a blowhole on the top of the head, which acts as a nostril.

The dense pigmentation in the skin overlying the eye prevents light from reaching the retina from any entrance except for a pinhole sphincter-like structure. This structure is controlled by a cone-shaped muscle layer that extends from the posterior eye orbit to the overlying eye skin layer. The sphincter-like structure is capable of sensing light and may be able to sense the direction from where the light was emitted. However, the muddy waters, or low light conditions, that the Platanista gangetica inhabit negate the use of the little vision that remains.

Vocalization

The Ganges subspecies shows object-avoidance behavior in both the consistently heavily murky waters of its habitat and in clear water in captivity, suggesting that it is capable of using echolocation effectively to navigate and forage for prey. However, there is limited information on how extensively vocalization is utilized between individuals. This subspecies of river dolphin is capable of performing whistles but rarely does so, suggesting that the whistle is a spontaneous sound and not a form of communication. The Ganges river dolphin most typically makes echolocation sounds such as clicks, bursts, and twitters. Produced pulse trains are similar in wave form and frequency to the echolocation patterns of the Amazon river dolphin. Both species regularly produce frequencies lower than 15 kHz and the maximum frequency is thought to fall somewhere between 15 and 60 kHz.

Echolocation is also utilized for population counts by using acoustic surveying. This method is still being developed and is not heavily utilized due to cost and technical skill requirement.

Distribution and habitat

The South Asian river dolphins are native to the freshwater river systems located in Nepal, India, Bangladesh, and Pakistan. They can be most commonly found in water with high abundance of prey and reduced flow. They migrate seasonally—downstream in colder conditions with lower water levels and upstream in warmer conditions with higher water levels.

The Ganges subspecies (P. g. gangetica) can be found along the Ganges-Brahmaputra-Meghna and Karnaphuli-Sangu river systems of Bangladesh and India, and the Sapta Koshi and Karnali Rivers in Nepal. A small subpopulation may still be found on the Ghaghara River, but requires verification.

Being a mammal, the Ganges River dolphin cannot breathe in the water and must surface every 30–120 seconds. Because of the sound it produces when breathing, the animal is popularly referred to as the 'Susu'. The Ganges River dolphin favours deep pools, eddy counter-currents located downstream of the convergence of rivers and of sharp meanders, and upstream and downstream of mid-channel islands.

The Indus subspecies (P. g. minor) today only occurs in a 1,000 km stretch of the Indus River itself and several connecting channels between the Jinnah and Kotri barrages. In the past it was to be found along 3,400 km of the Indus, its tributaries and neighboring river systems. Its range has contracted by about 80% since 1870. Since the two originally inhabited river systems - between the Sukkur and Guddu barrage in Pakistan's Sind Province, and in the Punjab and Khyber Pakhtunkhwa Provinces - are not connected in any way, it remains unknown how they were colonized. It is improbable that the river dolphins made it from one river to another through the sea route since the two estuaries are very far apart. A possible explanation is that several north Indian rivers like the Sutlej and Yamuna changed their channels in ancient times while retaining their dolphin populations.

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