This September 15, 2015 image, courtesy of the Smithsonian National Zoo and Conservation Biology Institute, shows giant panda Mei Xiang with her male cub, in her enclosure at the the zoo in Washington, DC

An American benefactor has donated $4.5 million for the giant panda conservation program at the US National Zoo, where twin cubs were born last month.

David Rubenstein, a co-chief executive of the Carlyle Group asset management firm, pledged to fund the Washington-based program through 2020.

The is home to female giant panda Mei Xiang, adult male Tian Tian, female two-year-old Bao Bao and an unnamed male cub born on August 22. His twin died of pneumonia within a week of birth.

"The National Zoo's panda program has been a remarkable success—two healthy pandas in just two years—and I am pleased to support it for another five years," Rubenstein said in statement issued by the zoo.

The zoo's panda habitat—its star attraction—is named after the Rubenstein family in recognition of a previous donation to the made in 2011.

Eleven conservation biologists have also been awarded fellowships funded by Rubenstein to study pandas in the United States and China.

A twitter account set up in honor of the newborn panda cub reacted gleefully to the news.

"So wait... $4.5m divided by us 4 here... Holy moly!! Am I a millionaire now????? #NeedanAccountant," he tweeted under the handle @houseofcubs.

A member of the zoo-owning Smithsonian Institution's Board of Regents since 2009, Rubenstein has given a total of $9 million for panda research and another $2 million in 2013 for Asian elephant research.