In the murky rivers of South Asia, there lives two species of dolphins that are almost entirely blind. These rivers are so muddy and silted that eyesight of any kind would be useless, so over the years the Indus and Ganges River dolphins have lost their eye lenses and developed a sophisticated sonar system, also called echolocation to help them navigate in the dark rivers. Where the eye of a non-blind dolphin would be, the Indus and Ganges dolphins have a small skin fold containing a tiny eye with no crystalline lens. The dolphin can only detect direction and intensity of light. It can't see shapes or colors. They use high frequency sound to navigate, socialize and locate their prey in much the same way as a modern submarine uses SONAR. The dolphins also swim on their side when underwater, trailing one flipper along the bottom of the river. The physical touch gives the dolphins important information about their surroundings and helps them find food.
There are two species of blind river dolphins in south Asia - the Indus (Platanista minor), and Ganges (Platanista gangetica) River dolphins. The Indus river dolphin is found in the Indus and Chenab rivers of Pakistan, while the Ganges river dolphin is found in the Ganges, Meghna, Brahmaputra, and Karnaphuli rivers that flow through India, Nepal, Bhutan and Bangladesh. Both varieties are endangered.
These dolphins are more primitive than other species of dolphins and are considered "living fossils". They are two of only five species that have, over a period of millions of years, adapted to living in fresh water. River dolphins are considered to be much more primitive than their saltwater cousins. Their brains are not compressed and their skulls have not gone through the streamlined transformation that ocean-going dolphins have. As a result, their foreheads are bulbous and their rostrums are very long. They are small, usually no longer than nine or ten feet. River dolphins, despite being more primitive than other dolphins, are still very "smart" and have a brain/body ratio that is very close to that of bottlenose dolphins. All river dolphins have many teeth, more than 200 in some species. Their neck vertebrae are not attached, to allow for free movement of the head, which may assist them in capturing prey, navigating in narrow waterways, or in scanning their surroundings with echolocation. Their flippers are quite short and broad. Their dorsal fins are low, and The tail flukes are broad in relation to the body size.
The river dolphins do leave the river beds one time each year. When the spring floods come, and the surrounding forest is covered in water, they leave the river and travel out amongst the flooded forest floors heading for specific waterways and usually unobtainable deep pools to mate. There they will congregate for a short time for mating, then return to their murky rivers until the next year. Recently, however, the proliferation of dams on the rivers have prevented the dolphins from migrating to their ancestral mating spots, and the resulting inbreeding is seriously affecting the health of the dolphin population
The Indus River Dolphin is considered the most endangered cetacean (toothed whale) in the world today. A recent survey (May, 2001) found only 1,000 dolphins in the entire lower reaches of the Indus River. This was actually good news, as previous estimates had been as low as 400 remaining Indus susu, as the dolphins are called. The Ganges dolphin, also known as Ganges susu is also endangered, although their numbers are not as devastatingly low as that of the Indus dolphin. There are only 4000 – 6000 Ganges dolphins today.