Fifteen Przewalski's horses, the world's only surviving subspecies of wild horse, were released into a nature reserve in the Xinjiang Uygur autonomous region on Sept 27, as part of efforts to protect the rare subspecies from extinction.

The group includes six stallions and nine mares, and for the first time researchers released five horses over 20 years old, equivalent to a person aged 70 to 80, according to Zhang Hefan, chief engineer of the Wild Horse Breeding Center based in Xinjiang.

"In the past, we selected only among the young and strong horses, usually between 0 and 13 years old, because old animals were more likely to die in the wild," she said.

But the center's researchers believed that it is time to try again, since a stable natural population has formed in the wild thanks to years of preservation efforts.

"We have 221 horses in the wild now, accounting for half the population of the center. We believe the environment has become more supportive to help the old ones survive," she said.

Protection of Przewalski's horses dates back to the 1980s. The wild horse is believed to be even rarer than the giant panda, since its current population worldwide is estimated at about 2,000.

Native to China, the animal was on the Red List of Threatened Species of the International Union for Conservation of Nature from the 1960s to 1996. Then it was reassessed as critically endangered due to successful reintroductions.

China imported 18 wild horses in 1985 from a number of countries and built a wild horse breeding center for their preservation. The center now has 411 wild horses.

Since 2001, the center has started to release wild horses into the Kalamaili Nature Reserve in Xinjiang, which spans more than 1.7 million hectares. The horses are first put into a 200-hectare enclosure so they can adapt before beginning to live in the wilderness.

"Living in natural habitats will help enlarge the gene pool and improve its ability to survive," Zhang said.