A visit to the giant panda nursery at Zoo Atlanta helps explain why brand-new twins Ya Lun and Xi Lun aren't yet making their public bows.

The 9-pound cubs appear to have no more muscle control than a pair of fur-covered bean bags, and are still struggling to get their feet underneath their football-shaped bodies. Curtsying would be difficult.

Siblings being siblings, you can count on Sister Number One to help out Sister Number Two when she tries to stand up.

Just kidding.

Big sister is no help at all. On a recent afternoon, as Xi Lun swims toward a crouching position, Ya Lun drops a WWF suplex on her, squashing the already very-squashable bear cub back to earth.

Using innovative techniques, keepers at Zoo Atlanta have successfully reared two sets of twins, the only zoo in the U.S. to claim that distinction. That’s Xi Lun on the left and Ya Lun on the right. HYOSUB SHIN / HSHIN@AJC.COM

Credit: Hyosub Shin

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Credit: Hyosub Shin

On a back wall of the nursery, a television monitor shows their mother, Lun Lun, who is past a set of double doors, crunching bamboo in her own den. The sound of the branches swishing and crackling over the tinny speakers is punctuated by a grumbling sort of happy growl. Then Lun Lun raises her voice slightly.

“She is most likely looking for a cub,” says Stephanie Braccini, Zoo Atlanta’s curator of mammals, who is getting ready to weigh Ya Lun, the frisky one, before sending her in to see her mother. The keepers weigh the cubs before they nurse and after they nurse, which is the only way they can know how much milk the infants are receiving.

“This is my second set of twins,” said keeper Jen Andrew, lifting Ya Lun into an acrylic bin on top of a digital scale. Andrew first volunteered at the zoo in 2009, and joined the staff in 2011.

Twin giant panda cubs Xi Lun (left) and Ya Lun were born Sept. 3 at Zoo Atlanta, the second set of panda twins born at the zoo and the only panda twins in the U.S. They received their names at their 100 Day Naming Celebration on Dec. 12. HYOSUB SHIN / HSHIN@AJC.COM

Credit: Hyosub Shin

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Credit: Hyosub Shin

Her first set of twins were Mei Lun and Mei Huan, born to Lun Lun and father Yang Yang in 2013. The older twins moved to China last month, in keeping with the zoo's arrangement with the Chengdu Research Base of Giant Panda Breeding, which maintains ownership of panda parents and offspring.

The newest twins, born Sept. 3, are the sixth and seventh offspring of Lun Lun and Yang Yang. Until now, their keepers have been delivering them to their mother, one at a time. As they grow more mobile, they will be able to find Lun Lun on their own, and the 240-pound Lun Lun will be more comfortable introducing them to a new environment, such as the exhibit area.

Giant panda cub Ya Lun yawns before being weighed by her keeper, Jen Andrew, at Zoo Atlanta. HYOSUB SHIN / HSHIN@AJC.COM

Credit: Hyosub Shin

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Credit: Hyosub Shin

Currently the cubs are off limits to all visitors, to reduce their exposure to viruses and bacteria. The Atlanta Journal-Constitution was given unusual access to photograph and observe the cubs. Both reporter and photographer were required to wear sterile gowns over their clothes and booties on their shoes. (The public can get a look at the pandas by way of the Panda Cam at www.zooatlanta.org/1212/panda_cam.)

On Dec. 13 both cubs were introduced to Lun Lun at the same time. This was a first. Lun Lun took it in stride, as if to say, “I knew there were two of them all along.”

Today’s news: Lun Lun sees two Luns, registers zero surprise. #ZAPandas #OnlyZooATL (video courtesy of Stephanie B.)

Posted by Zoo Atlanta on Tuesday, December 13, 2016

Though they haven’t been introduced to solid food, both cubs are cutting new teeth. Braccini, herself the mother of a 2-year-old, says that Lun Lun is not too troubled, despite the fact that she is breast-feeding a bear with a mouthful of teeth.

“She seems to be taking it better than I did,” says Braccini.