For a moment, the fat black-and-white bear sitting at the base of the tree paused as she shredded bamboo stalks with her teeth and glanced over at the hubbub.

Just outside her enclosure at the National Zoo on Monday, photographers, diplomats and assorted VIPs were gathered to announce big goings-on at the giant panda compound, thanks to the generosity of a local benefactor.

Alas, Mei Xiang, the zoo’s 13-year-old female giant panda, might be left out of the action.

As the zoo celebrated the donation by philanthropist David M. Rubenstein of $4.5 million to its giant panda research program, its scientists said it seems increasingly likely that Mei will be replaced.

She has not produced a cub in five years, and experts said research shows that such bears never become pregnant again.

“Mei, God bless her, but she’s only had one baby in her entire life,” said Donald E. Moore, the zoo’s associate director of animal care.

As a result, when giant panda experts gather at a zoo facility in Virginia next month, they will probably scour the digest of captive female pandas for a suitable replacement — perhaps a younger panda whose family has a good breeding record.

But Mei still has some time left, and one more breeding season to try for a cub.

And as the speakers took to the podium Monday, she went back to her bamboo.

While she munched, officials from the zoo, the Chinese Embassy and the Smithsonian Institution, which operates the zoo, lavished thanks on Rubenstein.

“Around the world, David is well known as an astute businessman, a great community leader and a great, generous donor,” Dennis W. Kelly, the zoo’s director, said. “Today, David, we’re going to add a new title to your résumé: panda enthusiast.”

Wayne Clough, secretary of the Smithsonian, said that in honor of the gift, the zoo’s giant panda habitat was being named the David M. Rubenstein Family Giant Panda Habitat.

“Panda enthusiasts, zoogoers . . . and scientists here . . . and around the globe have every reason to celebrate with us today,” Clough said. “David, your gift is a difference maker.”

Rubenstein is co-founder and managing director of the Carlyle Group, a global asset-management firm. He is also a member of the board of regents at the Smithsonian and chairman of the John F. Kennedy Center for the Performing Arts.

“There are probably 10 million species on the face of the earth,” Rubenstein said Monday. “And I doubt that any one . . . is more popular and beloved than the giant panda. . . . So we’re very pleased to make this holiday gift to the people of Washington and to the National Zoo and the people of our country.”

The donation will cover the cost for five years of giant panda reproduction efforts at the zoo and fund extensive panda research here and in China.

This year, the zoo reached a new lease agreement with China that extends the stay of its two giant pandas — Mei, 13, and Tian Tian, 14, a male — for five years. The agreement replaced a 10-year lease that expired last year. The new agreement expires Dec. 6, 2015.

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

The new agreement lowered the annual cost of leasing the pandas from $1 million to about $500,000 — still a substantial sum that the zoo was struggling to raise in difficult economic times.

Although the zoo wanted to explore acquiring new pandas right away, it agreed with the Chinese to give Mei and Tian until 2012 to produce a cub. Kelly said this is the start of the annual “love season,” during which the zoo attempts to breed the giant pandas.

As Mei goes into heat, probably in the next few weeks, the zoo will let her try to mate naturally with Tian and, if that fails, resort to artificial insemination.

The zoo’s only giant panda pregnancy resulted in the 2005 birth, via artificial insemination, of Tai Shan, who was sent to a breeding program in China last year.

“There’s still a cohort of pandas that fail to reproduce,” David Wildt, head of the Center for Species Survival at the Smithsonian Conservation Biology Institute, told the gathering. “When we evaluated all the panda breeding records . . . we found that no female has ever successfully reproduced after five consecutive failures.

“Although we are working for success, we are discussing the possibility of a panda exchange with our Chinese colleagues, which may include transporting Mei Xiang, and perhaps Tian Tian, back” to China, he said.

The population of captive giant pandas has grown from about 120 in 1998 to more than 330 today. Wildt said the goal is 500, after which scientists want to try introducing captive giant pandas into the wild.

About 1,600 giant pandas are thought to survive in the wild, but Wildt said the Chinese are engaging in a new count.