Female Tibetan antelopes walking to calving grounds near Zhuonai Lake after crossing the Qinghai-Tibet Railway to reach Hol Xil Nature Reserve in north-west China's Qinghai province on Tuesday.

Each year, tens of thousands of the endangered animals traverse hundreds of kilometres of barren land to reach the lake in order to give birth. It remains a mystery why they make this arduous journey.

Tibetan antelopes mate in December. The females then migrate to Zhuonai Lake in late May or early June the following year, while the males stay behind.

The mothers return to the original habitat with their calves in July to be reunited with the males.