The National Zoo's panda mom, Mei Xiang, is refusing to nurture the smaller of the two cubs to which she gave birth on Aug. 22, meaning that zookeepers have to take care of the smaller twin around the clock, the Washington Post reported.

When pandas have twins in the wild, they often only nurture one of the babies, letting the other one die, the newspaper explained. To circumvent that instinctual behavior, experts at the Washington institution had planned to switch the cubs every four hours, letting Mei Xiang spend one-on-one time with each baby.

But when zookeepers tried to swap the offspring around 11 p.m. on Sunday, the panda mom would not put down the cub she had in her possession, USA Today detailed. The team, which nurtured the other baby throughout the night, was able to make the swap in the early morning.

But in the afternoon, Mei Xiang again refused to switch, keeping only the larger cub, according to the Washington Post.

Experts at the zoo, meanwhile, are monitoring the situation very closely after disappointments in 2012, when Mei Xiang's first cub died from lung problems after just six days, and in 2013, when the panda gave birth to a stillborn cub, CBS News noted. The National Zoo is one of only four institutions across the United States to house pandas, which are on loan from China, the network added.

Pandas give birth to the smallest babies in relation to their mothers of any placental mammals, and Mei Xiang's latest cubs are particularly tiny, the zoo noted on Tuesday, according to the Washington Post. The weight of the smaller cub has also been fluctuating over the first few days of its life, something that should not occur after its first 48 hours, the institution added.

Nevertheless, both babies are behaving as a healthy infant panda should, and the offspring are receiving expert care from additional veterinarians, including a panda keeper from Zoo Atlanta. They are feeding the smaller cub by bottle and tube, and administering antibiotics to prevent infection, the newspaper detailed.