The number of giant pandas born in captivity in China has now passed the critical threshold that will allow a controversial reintroduction of the iconic species into the wild within three years.
Scientists in China have announced that they now have a stable breeding population of 300 pandas, which has long been considered the minimum number necessary for captive-bred animals to be released back into their natural bamboo-forest habitat, where just 2,000 wild pandas are believed to remain.
One of the reasons for the success is that China's main panda-breeding centre in Chengdu, Sichuan, has perfected a technique for boosting the number of twin cubs that survive by tricking mothers into believing they are raising one cub when in fact they are raising two.
About 50 per cent of panda births are twins but in nearly all cases the mother abandons one of the cubs at birth. The Chinese researchers have overcome the problem by rearing the abandoned twin in an incubator and swapping it up to 10 times a day with the other twin so that both cubs are suckled by the same mother.
The remarkable approach has meant that nearly all twin panda cubs born in captivity are now surviving to adulthood at which point they are enrolled into the breeding pool at the Chengdu centre. By the end of this year, this centre alone will have produced a total of about 140 cubs.
Twin swapping is just one of several innovative techniques that the Chinese scientists have pioneered. Artificial insemination using frozen panda sperm and careful analysis of the female panda's short ovulation cycle have both proved pivotal in the success of the Chengdu Research Base of Giant Panda Breeding, which has funded much of its work by lending pandas to foreign zoos for a fee of $1m a year.
China has identified three areas of natural bamboo forest in the mountains of Sichuan in southwest China, where it plans to begin a careful reintroduction programme of its captive-bred pandas. The only previous attempt four years ago ended in failure when a solitary male panda that had been freed was attacked, probably by a rival male.
This has led Chinese authorities to take the forthcoming reintroduction far more seriously, with a phased approach over 15 years centred on three reserves at varying altitudes in the Sichuan mountains. However, critics have argued that a costly reintroduction programme is doomed to fail unless the natural habitat is better protected against human encroachment.
Ten years ago, a study by the Chinese Academy of Sciences, using American spy satellite data from the 1960s, revealed that the main panda conservation reserve in the misty forested slopes at Wolong, a four- hour drive from Chengdu, the provincial capital, had suffered substantial damage from human development even after the panda reserve had been established.
Despite the concerns about habitat conservation, Chinese scientists are convinced that the captive-breeding programme is essential to the long-term future of the giant panda, a species that that literally has become the iconic symbol of the conservation movement, and central to China's environmental credentials abroad.
The remarkable twin-swapping technique at the Chengdu centre has been captured on film by a British television documentary to be broadcast next week on BBC2 as a Natural World special narrated by Sir David Attenborough. A pregnant female called Li-Li is filmed giving birth to twins and tenderly cradling her first-born, but ignoring the second cub which has to be put in an incubator.
Keepers are shown distracting Li-Li with a bowl of water and honey and gently swapping the cubs so that each has a chance of being suckled and reared by its mother. Li-Li is unaware that she is raising both of her twins, the keepers say.
The film makers had unprecedented access to the panda-breeding sanctuary and followed the scientists as they tried to prepare males for mating with females, which come into heat for only 72 hours each year. Panda pregnancies are also notoriously difficult to predict, with births occurring at any time between 11 weeks and 11 months after insemination.
Once the baby is born, the rearing challenges become apparent in the highly under-developed nature of the panda cub, which is one nine-hundredth the size of its mother and is born helpless and blind. Sorrel Downer, the documentary's producer, said everything seemed to weigh against the survival of the giant panda, which has lost some 50 per cent of its natural habitat over the past 20 years.
"Some people question if what is happening at Chengdu is a worthwhile use of conservation money but, as Sir David says in the film, the project is a triumph of science and skill," Ms Downer said.
"It is a fact that by inspiring the logo adopted in 1961 by WWF, the giant panda has become a global symbol of what we've damaged, and what we want to save, and perhaps a reminder that we're honour-bound to return what we shouldn't have taken away," she said.
'Panda Makers' is on BBC2 on Tuesday at 8pm