The National Zoo has reached a new agreement with China that extends the stay of its two giant pandas for five more years, continuing the romance with the iconic, rotund animals that have become symbols of Washington and objects of its intense affection.

That's five more winters of black and white bears rolling in the snow. Five more summers of them munching on fruit on warm afternoons. And five more years of waiting and hoping that the pandas might produce just one more giant panda cub.

Government and zoo officials said Wednesday that they were delighted, although the agreement did not fulfill all the zoo's hopes. They were trying to get one or even two new pandas. In 10 years, the zoo's giant pandas - Tian Tian, the 13-year-old male, and 12-year-old Mei Xiang - have produced just one cub.

Dennis W. Kelly, the zoo's director, has made it clear that he would like to replace one or both of the giant pandas to enhance the likelihood of having more cubs at the zoo.

"I think this is a wonderful new agreement for both pandas and the National Zoo," he said. "We have agreed in writing to put a very heavy emphasis on why this particular pair has not produced a cub in five years."

A high-level signing ceremony to formally announce the agreement was scheduled for 10 a.m. Thursday at the zoo, with Kelly, Interior Secretary Ken Salazar and Zang Chunlin, secretary general of the China Wildlife Conservation Association, in attendance.

Cost reduced

The agreement - a bit of panda diplomacy that was months in the making - replaces a 10-year lease that expired Dec. 6.

Among other things, it lowers the annual cost of leasing the pandas from $1 million to about $500,000. The money is to go toward panda conservation programs in China.

It calls for an intensive, China-U.S. study of the pandas this year and next year to try to determine why they have produced only one cub.

And it leaves the door open to the possibility of replacement pandas if the study concludes that one or both animals are unsuitable for breeding, officials said.

Zoos in Atlanta and San Diego have had more success breeding pandas, producing more than six cubs in recent years.

The new deal coincides with the U.S. state visit of China's president, Hu Jintao.

"This new agreement really signals that we're moving our relationship to a whole new level that's really based in trust," at least on the issue of biodiversity, Kelly said. "We're not Americans. We're not Chinese. We're people concerned about biodiversity for future generations.

"Having a panda cub here is of primary importance to both us and my Chinese colleagues," he added.

Salazar, whose U.S. Fish and Wildlife Service was involved in the negotiations, said he was elated.

"It is a win-win agreement," he said. "It's a win for countless Americans who love these amazing animals. ... It's a win for the Chinese, as the loan of the giant pandas ... plays an important role in supporting the protection and recovery of species and their habitats in China."

The agreement also allows any panda cub born at the zoo to stay for four years.

The old agreement required a cub to be taken to China after two years, although China allowed the zoo's only giant panda cub, Tai Shan, a two-year extension of that requirement.

For the past six weeks, Tian Tian and Mei Xiang, had been staying on a temporary lease extension. The new agreement expires Dec. 6, 2015.

Kelly traveled to China last June for talks and again Dec. 12 to present Chinese experts with a new scientific plan to study giant panda cub behavior and to request the new deal.

He returned from China on Dec. 16, and American and Chinese officials had been hammering out the new arrangement since then.

Giant pandas are native to China, and the Chinese own and lease all giant pandas in U.S. zoos.

The zoo's pandas - both born at the China Research and Conservation Center for the Giant Panda in Wolong, Sichuan Province - arrived in Washington on Dec. 6, 2000, amid international fanfare, a police escort and the hope that they would produce many cubs.

But reproduction of the animals is notoriously complex and mysterious, and in recent years zoo experts have tried in vain - with natural and artificial methods - to impregnate Mei Xiang.

Her only offspring, Tai Shan, was born via artificial insemination on July 9, 2005. He was sent to China on Feb. 4 to join a breeding program.

Male 'performing fine'

Recent science suggests that if a female giant panda has not become pregnant for several years, it is increasingly unlikely that she will be able to do so.

"We've not had a cub here in the last five years," Kelly said in an interview in July. "What we are talking about is, what can we do to ensure that there is a cub in our future?"

"There's some data that suggests that after four [breeding] tries, there's something [negative] going on," he said. Pandas generally mate once a year.

"Our male seems to be performing fine," Kelly said. "Seems to be that we're focused on Mei Xiang."

"As much as I love these two animals, it's important that we're always thinking about the genetic health of the 300 pandas in the captive panda colony . . . all over the world."

Meanwhile, zoo scientists have started monitoring Mei Xiang to determine when she goes into heat so they can begin the breeding process once again.

Last year, she went into heat in early January. After she and Tian Tian failed to mate successfully, she was artificially inseminated Jan. 9 and 10. Following a four-month vigil, the zoo announced April 28 that she was not pregnant.

A total of nine cubs have been produced at the four U.S. zoos that have giant pandas. In addition to three born at Zoo Atlanta - the latest on Nov. 3 - and one in Washington, five have been born at the San Diego Zoo.

The Memphis Zoo's pair - residents since 2003 - have not produced any offspring. The female appeared to be pregnant with twins last year but then apparently miscarried, a spokeswoman said.

Scientists do not know exactly why pandas at some zoos reproduce better than others.

Kelly said experts believe that in the wild, panda reproduction takes place after the female goes into heat and alerts local males by scent-marking. The female then watches from afar as the gathering males fight for dominance, and the winner gets the girl.