Monitoring data show that Cao Cao, a 16-year-old giant panda raised in captivity, has mated with a male in the wild－a rare occurrence for the endangered species－according to a panda research center in Sichuan province.

Cao Cao was released in the Liziping Nature Reserve in early February in time for mating season from March to May.

In March, researchers reported seeing several males fight for the right to mate with her and began checking data sent by the GPS tag on her neck－which is fitted with a recording device－every five days.

"When we checked the recording on March 27, we heard her making noises similar to those made by females while mating," said Zhang Hemin, head of the China Conservation and Research Center for the Giant Panda, where Cao Cao was raised. "It lasted one minute and 30 seconds."

If the panda is pregnant, she will give birth in July or August, "at which point we will bring her and her cub back to the center", Zhang said.

Cao Cao arrived at the center when she was 2 years old, after being found in poor health inside the Wolong National Nature Reserve. She has already mothered two cubs: male Tao Tao and female Zhang Xiang. Both are 2 years old and have been released into the wild in the Liziping reserve.

The research center, which was set up in the Wolong reserve in 1983, has released seven captive pandas since 2006 in an effort to boost the wild giant panda population.

There are 471 captive pandas worldwide, 234 of which are from the Wolong center－enough to prevent extinction, experts say.

In the early 1990s, the government banned the capture of wild pandas to mate with captive ones. Captive pandas have mated with few of their captive peers, which hurts biodiversity, researchers say.

The center formulated the plan last year to have captive pandas mate with wild ones. Cao Cao is the first to be chosen.

"She has a close bond with humans, and researchers have helped feed her since she was released," Zhang said.

Cao Cao is the equivalent of 48 years old in human terms, and she has only one or two years left before she is too old to participate in a breeding program.

"If she fails to get pregnant this year, she will have one last opportunity to mate with a wild panda next spring," Zhang said.

More captive pandas will be selected to mate wild ones, he said.