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The longest river in Asia, the Yangtze River, was home to two different species of dolphin -- the Yangtze finless porpoise and the Baiji dolphin. In 2006, the Baiji dolphin was declared functionally extinct. Its close cousin, the Yangtze finless porpoise, known for its mischievous smile, and a level of intelligence comparable to that of a gorilla, is now critically endangered.

Unlike most porpoises, these don't have a fin on their back or a beak. They can't breathe under water, so have to come to the surface regularly to breathe air. They don't leap out of the water like a dolphin, but they 'spy hop' -- stand up in the water and use their fins to spin round so they can see what's happening.

Porpoises look very similar to dolphins. Generally they are smaller, their body shape is rounder and they don't have the 'beak' of the more well-known dolphins.