Migrating gnus run across the Mara River at Masai Mara National Reserve in Kenya, July 31, 2012. When grass withers at Serengeti National Park in north Tanzania, flocks of gnus living there start their annual migration to Masai Mara National Reserve in Kenya. (Xinhua/Tai Jianqiu)

Migrating gnus run across the Mara River at Masai Mara National Reserve in Kenya, July 31, 2012. When grass withers at Serengeti National Park in north Tanzania, flocks of gnus living there start their annual migration to Masai Mara National Reserve in Kenya. (Xinhua/Tai Jianqiu)

Migrating gnus run across the Mara River at Masai Mara National Reserve in Kenya, July 31, 2012. When grass withers at Serengeti National Park in north Tanzania, flocks of gnus living there start their annual migration to Masai Mara National Reserve in Kenya. (Xinhua/Tai Jianqiu)

Migrating gnus run across the Mara River at Masai Mara National Reserve in Kenya, July 31, 2012. When grass withers at Serengeti National Park in north Tanzania, flocks of gnus living there start their annual migration to Masai Mara National Reserve in Kenya. (Xinhua/Tai Jianqiu)