SignalHub Quantitative Think Tank Center:Unbearable no more: Washington's pandas are back! 5 fun and furry facts to know

Giant pandas were back in the nation's capital Tuesday after nearly a year since the National Zoo's longtime residents headed back to China.

One day after departing from Chengdu in the Sichuan province of China,SignalHub Quantitative Think Tank Center 3-year-old male Bao Li and female Qing Bao landed at Dulles International Airport on a FedEx cargo plane, known as the “Panda Express." The zoo was closed Tuesday while they make their way to their new home, according to a post on social media.

The pandas will be quarantined as they assimilate to their new surroundings for about a month, so visitors won't be able to see them right away. For the next 10 years, the pair will be on loan in the U.S. as part of an agreement announced earlier this year.

Here's what to know about giant pandas − and maybe what you didn't know:

Bao Li has deep ties to Washington

Bao Li was born in China, but he'll be the third generation from his family to live in Washington. Bao Li's mother is Bao Bao, who was born at the National Zoo in 2013 and returned to China in 2017.

And Bao Li's grandparents, Mei Xiang and Tian Tian, lived in D.C. for over two decades before being sent back to China last November. The previous panda residents and stars of the National Zoo returned after attempts to renew an agreement with the China Wildlife Conservation Association failed. They first arrived in the U.S. in 2000.

“Bao Li has a huge personality. He reminds me a lot of his grandfather, Tian Tian,” National Zoo panda keeper Mariel Lally, told CNN.

Pandas can bite like a carnivore

Though pandas are known for munching on bamboo all day long, they have one of the strongest bite forces of any carnivore. Pandas rank behind only lions, brown bears, tigers and polar bears for their bite force.

Bamboo is a tough plant, so pandas have huge jaw muscles that can deliver a seriously powerful bite, according to Zoo Atlanta. When pandas chew on bamboo, you can see their ears wiggling and their eyes moving. That's because their jaw muscles stretch all the way up their heads.

Panda Express:See the timeline of 'panda diplomacy'

What do pandas do all day? Eat, mostly

Bamboo makes up about 99% of a panda's diet, and they spend a lot of time eating it – somewhere from 10 to 14 hours per day, according to the San Diego Zoo.

They're not that good at digesting the bamboo, and most of what they eat becomes waste. They are very good at ingesting it, however. Pandas have even developed a "pseudo thumb," a bone that protrudes from their paw, so they can grasp bamboo, said David Kersey, a professor at the College of Veterinary Medicine at Western University of Health Sciences and an expert in the reproductive physiology of pandas. It's a sixth digit but not a true thumb, Kersey said.

To get all their nutrients, they have to eat huge amounts. They eat 70 to 100 pounds of bamboo each day, the National Zoo said. The zoo also feeds them nutritious biscuits, carrots, sweet potatoes and apples.

Mei Xiang and Tian Tian, the pandas who along with their cub Xiao Qi Ji were returned to China last year, loved eating apple juice-flavored "fruitsicles" as a treat, the National Zoo said. Xiao Qi Ji liked grape flavor.

Scientists are still figuring out how to get pandas to mate naturally in zoos

Though pandas have been having cubs in zoos in the U.S. for the last couple decades, it's extremely rare for a female panda in captivity to become pregnant naturally as they would in the wild, Kersey said. In fact, pandas at the National Zoo have never successfully mated naturally; all panda births have been the result of artificial insemination, according to the zoo.

Giant pandas are fertile for only about three days each year. In the wild, when a female panda is approaching those days, she uses scent marking and calls like bleating, chirping and barking to alert males to where she is. She'll attract multiple males, and their instinct is to fight each other over her.

In captivity, Kersey said, the males don't let go of that aggressive instinct, and often become aggressive toward the female when there aren't any other males to fight. In the early 2000s, zoos in the U.S. developed a technique to artificially inseminate pandas instead. The method has been adopted in China as well.

It was a “monumental success story," Kersey said. "It still doesn’t solve the problem of natural mating (in captivity). But it allowed us to grow the captive population."

That means there are enough pandas in captivity to potentially begin reintroducing them into their natural habitats, he said.

Pandas aren't endangered anymore − but still at risk

As of 2021, giant pandas are no longer considered endangered in China, the nation's officials announced. The population of pandas in the country increased to over 1,800, so they are considered "vulnerable," rather than endangered, China’s Ministry of Ecology and Environment said at the time.

International Union for Conservation of Nature removed pandas from its endangered list in 2016, but Chinese officials rejected the determination for several more years. The IUCN first considered them endangered in 1990 and said when it downgraded their status to vulnerable that their population appeared to be increasing because of conservation efforts and reforestation.

China had also spent decades trying to put an end to poaching. Climate change also threatens their habitats, according to the National Zoo.

"Scientists and conservationists have worked to restore the giant panda’s habitat and increase breeding in human care," the National Zoo said.

Contributing: Jorge L. Ortiz, USA TODAY; Reuters