Gnus prepare to run across the Mara River at Masai Mara National Reserve in Kenya, Aug 29, 2011. When grass withers at Serengeti National Park in north Tanzania, flocks of wildebeests, zebras and antelopes living there start their annual migration to Masai Mara National Reserve in Kenya. [Photo/Xinhua]

Gnus prepare to run across the Mara River at Masai Mara National Reserve in Kenya, Aug 29, 2011. [Photo/Xinhua]