A herd of Tibetan wild asses pass under a bridge of the Qinghai-Tibet Railway in the Hoh Xil Natural Reserve in northwest China's Qinghai Province, May 11, 2012. The number of the rare Tibetan antelopes and Tibetan wild asses has continued to increase in the Hoh Xil Natural Reserve in Qinghai Province, as social awareness and the measures of environmental protection have been enhanced in recent years. [Photo: Xinhua]