Only a few rivers in the world have dolphins living in them. The Yangtse River in China, the Ganges River in India and Pakistan, and the Indus River which flows through Nepal, Bhutan, and India all currently have populations of river dolphins. Those populations are declining at a rapid rate however, and in the Yangtse and Indus, the dolphins are close to extinction. Only a few hundred animals remain. The boto and tucuxi are cetaceans and belong to the same scientific Suborder, the Odontoceti because they have teeth.
Scientists think that their recent ancestors are different though.
The boto has a long narrow beak and flexible neck and is more closely linked to prehistoric cetaceans. The tucuxi has a more compact shape like a modern marine dolphin. It is thought that the boto has lived in the Amazon basin longer than the tucuxi and is better adapted to the shallow waters. Because dolphins are mammals, they are warm blooded and breathe air. Millions of years ago, a land mammal began to adapt to life in the water.