The Chinese can give no greater gift
They're proud, partly, because no animal in the world provides cachet for a zoo like a panda does. "The Chinese can give no greater gift than a pair of pandas," says Valentine. He calls it "panda diplomacy": China bestows the favour of the bears - their sacred national animal - to establish links with another country. The tradition goes back to the Tang dynasty of the seventh century, and was revived in the Fifties, although the first pandas to set foot on British soil were brought here by an American hunter in 1938. The modern form began in 1984, when the former premier Deng -Xiaoping began his policy of economic reform of China. Instead of giving the pandas as outright gifts, China began giving them on 10-year leases, at a cost to the receiving zoo - currently -$1 million per year, per pair.
"In the early years," says Valentine, "they used it in a way that said, 'Please let us be your friend'. Now it's very much about establishing mutual relationships; the countries they give them to are carefully chosen." The UK, the United States, Germany and Japan.
The two pandas I see - Tian Tian lounging outside, on a wooden platform, periodically scratching herself and yawning; Yang Guang in his enclosure, which is strewn with bamboo ("He's the messy one," I'm told. "Tian Tian always keeps hers spotless") - therefore represent a significant investment of goodwill from China. And they also represent a non-negligible fraction of the total world population of the bears. "There are about 45 pandas in zoos outside China," says Valentine, and perhaps 1,500 in total, wild and in captivity, in the world. All of which makes Tian Tian and Yang Guang rather high-value residents of the zoo.
"Japan had one, Xing Xing, which died," Valentine tells me, shuddering at the thought. "In 2010. While it was under general anaesthetic for an artificial insemination." Valentine says that in China, "perhaps 25 or 30 pandas" are born in captivity every year. Outside China, it's usually two or three. "All in all," he marvels, "a panda cub is a pretty special thing."
The bears aren't all that difficult to keep alive once they're grown, although they're somewhat prone to a variety of diseases, but the newborn cubs are delicate; bringing one to adulthood is quite an achievement.
International joke
The seemingly eternal struggle to get pandas to breed in captivity has become an international joke as hackneyed as the one about Tory MPs. But according to Forbes Howie, a biomedical scientist at the University of Edinburgh who is helping the zoo with some groundbreaking research, it's very different in the wild.
"I normally say it's like Newcastle on a Friday night," he says, when I meet him for coffee at the Queen's Medical Research Institute. "The female goes to the top of the hill and says come and get me boys, and then mates with four or five of them." This approach means that even though females only come into oestrus for two or three days a year, they will get pregnant around 90 per cent of the time when they do. They're quite successful creatures, in the wild; the reason they're rare is because their habitat has been devastated, not because they aren't very good at having sex.
The fact that female pandas, outside China, do not have the availability of several males is one reason it is so hard to achieve successful pregnancies in captivity. But there are several others, the main one of which is that surprisingly little is known about panda reproduction. Even a question as basic as "how long is a panda pregnancy" can only be guessed at.
"I'd say probably 24 to 30 days," says Valentine; very short, compared with humans' 280 or so, because newborn panda cubs are so tiny. "But we just don't know." The reason we don't know is because pandas have a very strange reproductive system. One major difference from other mammals, such as humans, is that, unlike us, pandas don't get pregnant immediately after sex. Pregnancy begins when the fertilised egg becomes implanted in the uterus, which in humans happens seven to 10 days after ovulation. But pandas delay this implantation - sometimes by months. The time between mating and birth is between 95 and 160 days, but how much of this is gestation is unknown.
And what triggers the implantation, after the unspecified period in which the egg is simply floating around in the panda's reproductive tract, is also unknown. It could be something to do with light levels changing with the seasons, says Howie; or it could be to do with the bamboo forests, says Valentine; or both. "But there must be some chemical signal, some hormone which guides these things through. There's something we're missing," Howie says, sighing.
'Nutritional knife-edge'
Giant pandas can't be separated from their habitat; at different points of the year, the nutritional quality of the bamboo changes. Pandas' ancestors, a mere four million years ago, were mainly carnivorous like their fellow bears. Now they almost exclusively eat bamboo (although Tian Tian, says Valentine, occasionally catches and eats birds in her enclosure, "in full view of the watching children"). Bamboo isn't particularly nutritious at the best of times, and pandas aren't very good at digesting it, so they live on what Valentine calls a "nutritional knife-edge"; they even sniff pieces of bamboo before eating them, rejecting the lower-quality bits, because they simply can't afford to waste time digesting it. Having a baby is expensive, from an energy point of view, and pandas don't have much energy to spare. So they have evolved the ability to put pregnancy off until an auspicious moment.
But that makes things difficult for scientists. The usual pregnancy tests that you'd use on humans are hard to do, because they measure hormones that appear several days after implantation. A woman then has nine months before she gives birth. With pandas, nobody knows exactly when implantation happens and there is then only a relatively short period of gestation. This is why "is the panda pregnant" stories drag on for so long, and is why Howie, who normally works with humans, has been brought in to help.
"It's a nice change," he says. "Usually, at dinner parties, when someone asks me what I do and I say, 'biomedical research', they fall asleep. Now I can say, 'I'm working on making baby pandas', and everyone listens. Everyone loves a panda."
In particular, Howie is following up an idea that researchers at the University of Miami had in 2011. Given that the fertilised egg of a panda could not be detected via a hormone test, they set out to find another biological signal, and started investigating the female's immune system. Since half of the fertilised egg is made up of the male's DNA, they asked themselves, might there not be an inflammatory response to a foreign body following conception? Such a response could be detected if there was a rise in the levels of a protein called ceruloplasmin in the female panda's urine. Tests bore this out: a rise in ceruloplasmin correlated, later, with pregnancy. Howie is repeating their work, to see if it stands up.
It's only an uncertain indicator - all sorts of other things, including stress and other foreign bodies, could provide false positives, and Howie is careful to say that even if the results look positive at first, they need more time to be able to tell if it's the real thing. A fertilised egg doesn't mean pregnancy - a lot are rejected - and even a pregnancy doesn't mean a birth, as often they're reabsorbed or aborted. But a positive result would be a step along a long road.
Even getting to this stage is complicated. The brief panda mating period means that researchers can't simply inseminate the female at any time; they have to keep a careful eye on whether or not she's ovulating. Luckily there is a test for this as well.
Glamorous work it's not
Howie shows me around his lab. The equipment they use, he says, is pretty ancient stuff; the sort of things they used to identify the presence of progesterone and oestradiol (the hormones that indicate ovulation) in humans in the Seventies. The hormones play the same role in pandas. He pats a solid, chunky machine on a shelf with a glowing green text-only computer screen and a whirring centrifuge. "Don't make them like that anymore," he says, grinning.
Unfortunately, to do the ovulation test they need a urine sample and Tian Tian won't pee on a stick, so the designers of her enclosure have put in a clever workaround - a rippled floor, with divots in which her urine will gather. Then, when she walks away, someone rushes in with a syringe and sucks it up for testing.
Glamorous work it's not, but it meant that this year they were able to determine that, on April 13, Tian Tian ovulated. Ideally, Yang Guang then would have done his male duties in the natural way, but, as so often with captive pandas, the pair would not mate. Instead the zoo team, with various experts from China and Berlin in attendance, put Tian Tian under general anaesthetic and performed an artificial insemination with Yang Guang's sperm. Now, Valentine and Howie can only wait.
If there is, eventually, an offspring, the baby panda will - like its parents - be the property of China, and the hope is that eventually it will be reintroduced into the wild. A million dollars a year is a lot of money to spend on borrowing a couple of pandas, when you don't even get to keep the progeny; but, says Valentine, it's money well spent, for everyone concerned.
The money goes toward the conservation of giant pandas in the wild, and breeding and reintroduction programmes in Chinese zoos and reserves. And for Edinburgh Zoo, the pair represent an enormous draw: the number of people coming leapt by 200 per cent in the first months they were there, and while numbers have been less spectacular since, they are still pulling in the crowds - 40 per cent above average last year. Valentine says they have paid for themselves comfortably. A panda cub would bring in even more.
The problem for pandas is that they have become, in the past three decades or so, symbols of a particular kind of conservation: focusing on big, impressive creatures, so-called "charismatic megafauna", such as pandas or elephant, rather than on the entire ecosystem of which they are a part. This has attracted criticism, but Valentine counters it in two ways.
"First, you have to raise money somehow," he says. "Dung beetles are vital to various ecosystems in Africa, but it's harder to raise money for the conservation of dung beetles, so people focus on the elephant and use the money to protect the whole system.
"Second, you can't separate the panda from its environment. The panda is as vital for the bamboo forests as the forests are for the panda; top predators have a huge impact on their ecosystems, and the way to think of the panda is as an apex predator whose prey happens to be bamboo."
It occurs to me that I haven't asked Valentine an obvious question: why pandas? He's dedicated his life to these creatures: what is it that he loves so much about them? He blinks a bit, as though he's never been asked that before.
"They're a fascinating species, biologically," he begins, "and there's lots we don't understand. But they're intelligent, and they learn." They're not just docile bamboo-chompers, but intelligent creatures, who communicate using complex chemical signals.
The scientist also has something in common with Tian Tian and Yang Guang - he and his wife have been trying for a baby themselves for several years. Happily, the Valentines are now expecting. Does he think Tian Tian will follow suit? Both Valentine and Howie are supremely keen to say that they won't know, for weeks and perhaps months, whether she's pregnant, and they won't know whether it will go to full term until they see the breathing cub in front of them. Howie's tests can hint that there might be a fertilised egg, but that's all.
But with all those caveats, he eventually shows me. The ceruloplasmin in Tian Tian's urine has been going steadily up. There might, possibly, be a baby on the way.
