Who's who? Only their keeper can tell
Even to the trained eye, Tuan Tuan and Yuan Yuan look identical.
The man who has seen more giant pandas than anyone else, Wei Fuwen , secretary general of the China Zoological Society, cannot tell them apart.
Professor Wei prowled the treacherous mountains of Sichuan and Shaanxi provinces for nearly three decades, tracking hundreds of wild panda families, often just by droppings or broken bamboo.
Sometimes a panda just stumbled across his path.
'Unfortunately, they all look the same; so even if a giant panda is keen enough to meet a human twice, I could not recognise it as an old friend,' he said. 'You would have to be superhuman to tell the difference between them at a glance.'
But Tuan Tuan and Yuan Yuan's keeper, Qu Chunmao , can spot the difference a mile off. The two giant pandas looked as different as they could be, Ms Qu told Xinhua yesterday.
One of the easiest ways to tell them apart, she said, was to compare heads: Tuan Tuan's is round and big, and Yuan Yuan's is small with a somewhat pointed chin.
Curiously, Yuan Yuan means 'round and round' in Putonghua.
Other differences include, but are not limited to, the patches around Yuan Yuan's eyes which are bigger and darker than Tuan Tuan's; Yuan Yuan has an appetite for carrots whereas Tuan Tuan favours buns; and Tuan Tuan has a mild temperament while Yuan Yuan is sometimes a bully. Tuan Tuan is also skinnier - well, as far as pandas go.
The least obvious but most fundamental difference is that Tuan Tuan is male and Yuan Yuan female.
Huang Zhi , chief keeper at the Wolong Giant Panda Breeding and Research Centre in Sichuan, predicted the two might mate this spring.
'Their overall mental and physical state is excellent,' Xinhua quoted Mr Huang as saying.
Mr Huang said the centre had the world's most advanced technology to stimulate sexual desire in the endangered species, and experts from Wolong were willing to share their experience and knowledge with their Taiwanese counterparts so the pair, isolated from other giant pandas and living together since 2006, could begin their sex life as quickly as possible.
'When they become aroused, the Taiwanese keepers will be in touch with us through various channels, including e-mail,' Mr Huang said.
Both giant pandas will be five years old next year.