South Asian River Dolphin

## Description

The South Asian river dolphin is an endangered freshwater dolphin found in the region of the Indian subcontinent. It has the long, pointed nose characteristic of all river dolphins. Its 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 body is brownish in color and stocky in the middle. In the place of a dorsal fin, South Asian dolphins have only a small, triangular lump.

## Distribution

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 a 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.

## Behavior

South Asian dolphins are not gregarious and prefer to spend time singly. However, they may occasionally gather in small groups of up to 30 individuals where the best fishing spots are. South Asian river dolphins have poor eyesight and rely on echolocation to find prey. Their extended rostrum is advantageous in detecting hidden or hard-to-find prey items. The prey is held in their jaws and swallowed. Their teeth are used as a clamp rather than a chewing mechanism. During the monsoon, South Asian river dolphins tend to migrate to tributaries of the main river systems. Some individuals may 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. South Asian river dolphins are highly vocal and most typically communicate with the help of echolocation sounds such as clicks, bursts, and twitters.

## Diet

South Asian river dolphins are carnivores (piscivores). They feed on a variety of shrimp and freshwater fish, including carp and catfish.

## Reproduction

South Asian river dolphins may breed year-round, but the peaks appear to be concentrated between December to January and March to May. Females give birth to a single calf after the gestation period that lasts approximately 9-10 months. After around one year, calves are weaned and become independent. They reach reproductive maturity at about 10 years of age.

## Population

### Threats

The most serious threat to South Asian river dolphins comes from human activities. These animals have been adversely affected by human use of river systems in South Asia. Entanglement in fishing nets as bycatch can cause significant damage to local populations, and individuals are taken each year by hunters; their oil and meat are used as a liniment, as an aphrodisiac, and as bait for catfish. Poisoning of the water supply from industrial and agricultural chemicals may have also been a contributing factor towards population decline, as these chemicals are biomagnified in the bodies of the dolphins. Perhaps the most significant issue is the building of more than 50 dams along many rivers, causing the segregation of populations and a narrowed gene pool in which dolphins can breed. An immediate danger for the populations in National Chambal Sanctuary is the decrease in river depth and appearance of sand bars dividing the river course into smaller segments, as irrigation has lowered water levels throughout their range.

### Number

According to the IUCN Red List, the total population size of the South Asian river dolphin is fewer than 5,000 individuals. Currently, this species is classified as Endangered (EN) on the IUCN Red List.

### Ecological niche

South Asian river dolphins play an important role as top predators in their river ecosystem. They prey on a range of fish and crustacean species, thus controlling their populations.

## Fun Facts

The South Asian river dolphin is split into two subspecies, the Ganges river dolphin and the Indus river dolphin. 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 the National Mammal of Pakistan.

South Asian river dolphins are unique among cetaceans in that they swim on their sides. This adaptation aids them to find prey in the murky water by using one flipper that helps feel for prey.

Being mammals, Ganges river dolphins cannot breathe in the water and must surface every 30-120 seconds. Because of the sound, they produce when breathing, these dolphins are popularly referred to as susu.

The South Asian river dolphin is also referred to as the "blind dolphin". Its eye lacks a 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. However, the eye may still serve as a light receptor and the dolphin still uses it to locate itself.

South Asian river dolphins have a slit similar to a blowhole on the top of the head, which acts as a nostril.

South Asian river dolphins have very flexible necks and are able to turn their heads while searching for food. They can also swim upside down.

## References

https://animalia.bio/south-asian-river-dolphin