Chinese scientists have experimented with giving the male anti-impotence drug, Viagra, to giant pandas in an effort to get them to breed.

The trials began last year at Wolong Nature Reserve in the south-western province of Sichuan, but have had no success.

In the last few years, we've given Zhuang Zhuang many chances but he simply can't do it

Guo Feng, Researcher at Wolong Nature Reserve

Pandas, which have relatively low fertility rates even in the wild, lose virtually all interest in sex once they are in captivity.

The scientists have now abandoned the experiment, but some experts say the researchers should simply find new candidates.

Chinese panda breeding programmes have struggled for years to help the endangered species, using what has been dubbed "panda porn" - explicit video of pandas mating - traditional herbs, and artificial insemination.

Not interested

The first panda to be given Viagra, 16-year-old male Zhuang Zhuang, which translates as Strong Strong, did not live up to his name.

"No result on him at all," said Wang Pengyan, deputy director of the reserve, adding that his treatment had been an experiment and there were no plans to give the drug to other pandas.

There are few baby pandas born in captivity

But other experts said it could be worth trying on candidates other than middle-aged Zhuang Zhuang.

"You can't say Viagra has no results on pandas. We used the wrong panda. That panda basically has no capability," said Guo Feng, a scientific researcher at Wolong.

"In the last few years, we've given Zhuang Zhuang many chances but he simply can't do it."

Mr Wang said that by contrast, the "panda porn" did seem to have some effect.

Release planned

Next year, Wolong hopes to begin releasing pandas born in captivity into the wild, where they are more likely to mate as they have the opportunity to indulge in the species' elaborate mating rituals.

These see males compete aggressively with each other for females on heat.

More than 60% of male pandas in zoos or sanctuaries exhibit no sexual desire at all, while just a tenth of them will mate naturally, according to Xinhua.

Thus, the vast majority of panda births in captivity are the result of artificial insemination.

Captive births are vital to the survival of the species. There are only about 1,000 giant pandas left in the wild, mostly in Sichuan.