President Richard Nixon once mused on the sex lives of the National Zoo's giant pandas.

In 2015, first lady Michelle Obama and the first lady of China, Peng Liyuan, attended the naming of one of the cubs.

Now, in light of the U.S.-China trade war, the giant pandas could again take on a high political profile.

The National Zoo's beloved black-and-white bears, which have delighted Washingtonians for generations and have created joyous episodes of pandamania and profit, have often been on the world stage.

But next year, the extended 20-year Chinese lease of the two adults -- Mei Xiang, a female, and Tian Tian, a male -- will be up on Dec. 7.

The zoo said it has not started discussions with the Chinese about the lease and could not speculate on an outcome. And the U.S. political landscape by late 2020 is a mystery.

"Our agreements are based on science surrounding the giant pandas," zoo spokeswoman Pamela Baker-Masson said. "We've accomplished a lot over the last 40-plus years. Now both sides have to take a look at what the future science goals should be and they go from there."

The zoo's only other giant panda, Bei Bei, who turned 4 on Thursday, is slated to be gone within the next few months.

By prior agreement with the Chinese, all giant panda cubs born in U.S. zoos must be sent to a breeding program in China once they turn 4.

Two of the zoo's cubs have been shipped to China in recent years, and plans for the transfer of Bei Bei are underway, the zoo said.

The adults are present under different terms from their offspring.

The National Zoo hopes China will not leave Washington pandaless and has said collaboration with Chinese scientists on giant pandas has been highly beneficial for both counties.

Should giant pandas be caught up in Chinese-American affairs, it would not be the first time.

Milestones in the pandas' tenure in Washington have been celebrated by top officials from both countries.

During World War II, China gave New York's Bronx Zoo two pandas in gratitude for American war relief.

In February 1972, at a dinner in Beijing, first lady Patricia Nixon told Chinese Premier Zhou Enlai how fond she was of giant pandas, the zoo's website says.

Eager for improved relations with the United States, the zoo said, Zhou replied: "I'll give you some."

The visit by President Nixon and his wife had made worldwide headlines. The United States and Communist China had been bitter geopolitical foes for 20 years. And the offer of the pandas was part of a historic thaw in relations.

That April, China gave the United States two young giant pandas, Ling-Ling, a female, and Hsing-Hsing, a male. Both were 18 months old.

They were a gift, not a loan, the zoo notes, and "ever since their arrival, the pandas have symbolized cross-cultural collaboration between the United States and China."

The bears were a delight for more than two decades, drawing tens of thousands of visitors, and putting the animal on par with cherry blossoms as a symbol of Washington.

Ling-Ling died in 1992, and Hsing-Hsing died in 1999, leaving the zoo's giant panda house empty for the first time in 27 years.

In 2000, China sent the zoo Mei Xiang and Tian Tian on a 10-year, $10 million lease.

In 2011, an almost five-year agreement lowered the annual lease price from $1 million to $500,000.

And in 2015, another deal was reached to keep the giant pandas in Washington until next year.

This article was written by Michael E. Ruane, a reporter for The Washington Post.