There is a unique peninsula in Pu'er Sun River National Park, which is formed naturally because streams in the forest erode year in and year out. During the long rainy season, steam water keeps flooding and ebbing, so after a long time, the silt in the valley is washed up onto the peninsula to form a semi-circular island. On the Island, commonly seen animals are mainly "gibbons", which are the first class national protected animal. Gibbons are the only living anthropoids in China, which as well as orangutans, gorillas and chimpanzees are called the four great apes, and are advanced primates second only to humans. In Pu'er Sun River National Park, tourists can see white-cheeked gibbons, which are classified as the first class national protected animals in China.