The Ganges River Dolphin (Platanista gangetica) is one of only four species of freshwater dolphin. More commonly known as "susu" in India, these dolphins are endemic to the Ganges, Brahmaputra, and Meghna river systems. Their range extends from the Himalayan mountains of Nepal to the costal regions of India and Bangladesh.
Whales and dolphins as a group are thought to have originally evolved in the prehistoric basin sea that closed up as the Indian tectonic plate collided with the Asian plate. Thus, it's expectable that India would have freshwater dolphins.
The dolphin's generic name (Platanista) was given to it by Pliny the Elder in ancient times. Until very recently, it was thought to be the same species as the Indus River Dolphin (Platanista minor).
These dolphins are extremely endangered due to poaching, pollution, and accidents; there are only about 4,000-6,000 of these dolphins alive in the wild. They live in one of the most densely populated (and polluted) areas of the world: the Ganges basin, where 10% of the world's human population resides.
These dolphins are all blind; their degenerated, lensless eyes sense light and darkness, but little else. In their natural habitat, this blindness wouldn't be a problem, since the river water is extremely muddy and hard to see in. However, in this modern era of motorboats and fishing nets, it is very easy for a blind dolphin to get killed in high-traffic waters. Sometimes, they wander into irrigation channels where they get trapped and die. The dolphins are also hunted by Brahmaputra tribespeople for their meat and oil.
Like other dolphin species, they navigate with echolocation; they emit sound pulses through the "melon" in their heads. The sounds bounce off nearby objects and give the dolphin an "image" of their surroundings. Scientists estimate that 81% of the sounds they make are for navigation and 5% are to communicate with other dolphins. The noise created by motorboats can temporarily wreck a dolphin's ability to navigate.
These dolphins differ from other species in some important ways. Their toothed beaks are very long, almost giving their snout a caiman-like appearance, and the dorsal fin on their stocky bodies has been reduced to a triangular hump.
Most strangely, they swim on one side underwater, with one flipper dragging in the mud. This odd side-swimming allows it to move in water as shallow as 30 centimeters. When a dolphin needs air, it arcs up to take a breath, rolls over, and dives again. No other dolphin species swims like this.
Adult females are larger than the males, and they grow up to 2.5 meters in length and 90 kilograms in weight. They are gray in color and often have a pinkish belly. They reach sexual maturity in ten years and are thought to live up to 20 years. Dolphin pregnancies last 10 months, and the babies are weaned and on their own in just a year. Adults generally travel alone or in small groups of two or three individuals.
Some groups are suffering from inbreeding because dams have blocked their routes to breeding grounds. Such inbred colonies typically produce young that are sickly, malformed, and prone to early death.
These dolphins normally breed during the rainy season when the rivers overflow and the tributaries are deepest. They retreat to the main rivers during the dry season; babies are mainly born in the spring dry season (most are born in April and May). A female generally gives birth once every two years.
Despite their poor vision, these dolphins are predators. They eat squid, fish, turtles, shrimp, and crabs. They've even been known to go after ducks and geese.
Many international groups such as the World Wildlife Federation and the International Union for the Conservation of Nature and Natural Resources are working with the Indian government to figure out ways of preserving these river dolphins.
Full taxonomic classification:
Thanks go out to Gorgonzola for the details about cetacean evolution.