WASHINGTON - One of the two giant panda twins born at Washington’s Smithsonian National Zoo over the weekend died on Wednesday, turning unexpected joy to tragedy in a matter of days.

After an ultrasound revealed a possible pregnancy a week ago, zookeepers were greeted with another surprise Saturday when 17-year-old panda Mei Xiang gave birth to not one but two baby cubs.

But the keepers struggled to keep the smaller of the two healthy as Mei Xiang focused her attention on the larger cub. The undersize cub - nearly half the weight of the other - passed away Wednesday, four days after its unexpected birth had made it an Internet sensation. “This is a very hard loss for us,” zoo director Dennis Kelly told a press conference.

“It’s really hard to keep those little creatures thriving.” Zookeepers faced an uphill battle after they had trouble convincing Mei Xiang to swap between caring for the smaller and larger cub. Tiny, blind, nearly hairless and with little ability to regulate their own body temperature, newborn pandas are completely dependent on their mothers.

Keepers had stepped into that maternal role by bottle-feeding the cub and administering antibiotics to stave off infections, but their efforts ultimately fell short. “After the last swap this morning, we immediately noticed that it appeared weaker, with possible respiratory problems,” chief veterinarian Don Neiffer said.

“We did see some improvements but things turned and his condition declined. We weren’t able to turn things.” Prior to its death, the cub had stopped gaining weight and was only 79.8 grams (2.8 ounces) when it died, as opposed to the 137.7 grams of its larger sibling. Neiffer noted that newborn cubs are extremely fragile and that the cause of death would not be known for sure until an autopsy is performed in the next few weeks.

But he also said the larger cub was doing very well. “We have a very healthy panda. We remain very optimistic about the animal,” Neiffer said. In a statement, the zoo reported the larger cub showed “encouraging signs,” but that they are monitoring it and its mother around the clock “as the cub is still vulnerable and the risk remains high.” It is too early to determine the sex of either cub.

The survival of both twins was always a long shot. Only one of the three sets of panda twins born in the United States has survived and zoo officials noted last week that in the wild one twin is usually left to die as it would be too hard on the mother’s body to care for both simultaneously. Neiffer said that Mei Xiang, who had given birth to two other healthy cubs prior to this weekend, is “a very good mother,” but that she had “some challenges handling both cubs.”