If Mei Xiang were raising twins in the wild, it would not be in her best interest to keep both alive. The fragility of the cubs is a drain on the mother’s energy, Ms. Baker-Masson said. The behavior is also seen in other animals: Female cheetahs, she said, are likely to abandon a single baby because it’s better to expend the energy on raising a litter.

“They need so much of her resources,“ Ms. Baker-Masson said, when asked if Mei Xiang was favoring one cub over the other. “We have no evidence of favoring — it’s just instinct.”

Zoo officials don’t know whether the cubs were fathered by Hui Hui, a giant panda who lives at a research center in China’s Sichuan Province, or by the National Zoo’s own Tian Tian. Officials hope the cubs were fathered by Hui Hui, a bear that could help diversify the genetics of pandas in captivity, but Dr. Neiffer said that paternity and genetics testing will have to wait until the cubs are stronger. Right now, officials are focused on swapping, feeding and keeping the babies warm.

The giant panda exhibit at the National Zoo is a huge draw for visitors, both in person and online: A “Panda Cam” keeps interested parties abreast of minute-by-minute updates on Mei Xiang’s life, and the zoo issues frequent updates, which have included her ultrasound pictures. Video of the twins’ birth was filmed live with the hashtag #PandaStory.