The zoo said changes to her hormone levels as well as differences in her behaviour meant she could be ready to give birth within weeks.
But it won’t be known for definite whether she is pregnant until shortly before she is due to give birth. It is quite common for pandas to have phantom or pseudo pregnancies, and Tian Tian had one last year.
However, experts say the indications this time are more encouraging.
Key changes in her protein and hormone levels suggest the artificial insemination she underwent on 21 April was successful. Senior keepers have also noted restlessness, a lack of appetite and nesting behaviour such as making a bed of straw.
The zoo said a spike in her level of the hormone progesterone, which is measured by sending urine samples to the Leibniz Institute for Zoo and Wildlife Research in Berlin, was among the key indicators. The spike was first detected on 15 July and confirmed on Wednesday.
At five months, the gestation period is relatively short and if Tian Tian is pregnant, the cub will by born in late August or early September. If so, it would first be displayed to the public on 1 January, 2014.
Iain Valentine, director of the zoo’s giant panda programme, said: “Tian Tian had a pseudo-pregnancy last year but the behaviour wasn’t as pronounced, so when you compare last year to this year, you start to think something is a bit different.
“It’s hugely exciting. There’s never been a baby panda born in the UK.”
He said keepers were more hopeful than last year due to the greater change in hormone levels. “Now that progesterone levels are rising, she is going off her food, spending less time interacting with the keepers and becoming more sleepy,” he said.
Determining if a panda is expecting a baby is notoriously difficult as cubs can weigh as little as 100 grams (3.5oz) and there are few physical changes in the mother-to-be.
Further hormone results will be available by mid-August and if the Tian Tian is not pregnant, her hormone levels will have reduced to zero.
She and male panda Yang Guang were not able to mate naturally in April, so she was artificially inseminated with sperm from him and from Bao Bao, who died at Berlin Zoo last year at the age of 34.
Dr Martin Dehnhard, from the Leibniz Institute, said: “The hormone concentrations we measured in the urine sample from the panda increased.
“That tells me that she is pregnant or pseudo-pregnant.”
Chris West, chief executive of the Royal Zoological Society of Scotland – the charity that runs Edinburgh Zoo – said: “Results so far are very encouraging. But we don’t want to count our pandas until they are born.”
Pandas often have twins which Mr Valentine said would present a unique problem.
Tian Tian can raise only one cub and zoo staff would have to raise the other. The first would remain in Edinburgh for two years – and be displayed to the public for about 18 months – before going to China as part of the agreement with the authorities there. The second, raised by staff, is likely to be sent to China sooner to live in a “panda crèche”.
Some 24 hours before giving birth, Tian Tian would be expected to become restless and start to bleat. Two incubators will be on standby in the zoo’s panda nursery in case of twins. Mr Valentine said: “You could say there’s a 50 per cent chance there could be twins. When we did the artificial insemination, we could only see one follicle, but there were other follicles in development.
“Now whether they fully developed and ovulated since that scan we don’t know, so we have to plan for twins. Basically we don’t know until the birth.
“We are planning for one of our Chinese colleagues to come over to help, and our staff have been to China to work with pregnant pandas, so we’ll be as prepared as we can be.”
News of the possible panda pregnancy sparked a surge in visitor numbers at the zoo, with families travelling from across Scotland and south of the border to catch a glimpse of the animals yesterday.
Staff estimated that between 4,000 and 5,000 flocked there, and the panda enclosure’s daily capacity of 2,100 was reached.
They were all eager to catch a glimpse of Tian Tian, who after sleeping for the morning, awoke early in the afternoon and hung off the edge of her bench, to the delight of visitors.
Zoo guide Cheryl-Ann Beattie, 26, said there was huge excitement among visitors and staff. “We arrived at work and the press were already here,” she said. “People had heard the news at 7am and visitors were lined up outside before we opened. We told the visitors they had come on a very exciting day. It’s going to be mayhem for the next few weeks but very exciting.”
Even bookies are getting in on the act, offering 10/1 odds on a cub being called Wang Lei if male or Zhang Lei if female. They are among the most popular names in China and translate as “rock pile” in English. Other local-themed favourites include Choi (mountain) at 14/1, Bao Bao (castle) at 25/1 and Adingbao Xiongmao (a literal translation of Edinburgh Panda) at 33/1.
Giant pandas are renowned for having a very low birth rate, and artificial insemination is the most common method of breeding them in captivity, as they often lose interest in mating once taken from the wild.
Tian Tian seemed ready to mate in spring, but the decision was taken to intervene after attempts at a natural coupling with Yang Guang failed.
Pandas in the wild usually mate with more than one partner, so Tian Tian was inseminated three times with samples from both Yang Guang and Berlin Zoo’s Bao Bao, whose sperm was frozen before he died last year. This means a twin pregnancy could produce cubs with different fathers.
Identifying whether or not conception has occurred is difficult in pandas, which often experience a pseudo-pregnancy, where they display all the behavioural and hormonal changes seen in a real pregnancy. Even an ultrasound scan may not determine whether a female is pregnant, since a foetus is so small and late in developing that it can be impossible to detect.
The gestation period for a panda ranges from 95 to 160 days, and successfully predicting the due date of a successful conception is further complicated by a phenomenon known as delayed implantation. This means a fertilised egg can float about in the uterus for months before attaching to the womb lining and beginning to develop into a foetus.
Hormone tests have shown encouraging results for Tian Tian. Rises in progesterone occur in females at the time of breeding and again when the embryo implants in the uterus, with experts putting the birth at 40 to 55 days after the second rise. This suggests the pitter patter of tiny panda feet may be expected in Edinburgh between 24 August and 10 September.
If it turns out Tian Tian is experiencing a phantom pregnancy, progesterone levels will drop in the next few weeks.
Results from Tian Tian’s urine tests, which were carried out in Berlin, indicate that she is pregnant and will carry to term, but it won’t be known for certain until a few days before the birth. Signs that she may be about to go into labour include restlessness and bleating.
If Tian Tian gives birth, her cub, or cubs, will be the first born in the UK. But twins will bring added complications for Edinburgh Zoo.
When twin cubs are born in the wild, usually only one will survive, as panda mothers seem unable to produce enough milk for two. She will select the stronger of the two, leaving the other to die.
In the event of twins for Tian Tian, it is expected one will be hand-reared, though innovative techniques have been developed in China to “trick” mothers into rearing two cubs. This involves switching the twins between their mother and an incubator to ensure they are properly fed, kept warm and receive equal maternal nurturing.
Newborns are blind, toothless and tiny, weighing only 90 to 150 grams. They are completely helpless and dependent on their mothers for survival.
They are born pink, but a week or two later, the skin will turn grey in the areas where the hair will eventually be black.
Cubs should start to crawl after 75 to 80 days. They usually stay with their mothers until they are one and a half to two years old.
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