One of Mei Xiang's tiny newborn panda cubs died Wednesday, four days after its birth at the National Zoo.
The smaller of the twin giant panda cubs died shortly after 2 p.m.
Zoo officials said the larger cub appears to be strong and robust. The surviving cub is behaving normally and is with mother Mei Xiang.
The cubs were born within hours of each other Saturday night. Zookeepers were attempting to swap them out so both got time and attention from Mei Xiang, as is protocol with panda births.
Zoo officials said the panda team had switched the cubs within the past 24 hours. The smaller cub was with Mei Xiang from about 2 p.m. Tuesday through Wednesday morning.
However, when the team switched the cubs Wednesday morning, the smaller cub seemed weaker, was displaying possible respiratory problems, and hadn't gained weight. Zoo vets gave the cub antibiotics, respiratory support, formula and fluids, to no avail.
The mortality rate for panda cubs in their first year in human care is about 26 percent for male cubs and 20 percent for female cubs, the zoo said in a release. The sexes of the newborns are still unknown.
A zoo official said Mei Xiang wasn't showing preference to one cub over another and was reluctant to give up whichever cub was with her any time keepers attempted a swap. When her second cub was born around 10 p.m. Saturday, she had tried to figure out how to hold both, but was unable to.
"The collective scientific knowledge about giant panda mothers is that they are best able to care for one cub at a time," the zoo said in a release. Giant pandas have twins about 50 percent of the time.
Zoo vets had been caring for each cub during swaps, feeding them a mixture of baby formula, puppy formula and water, and keeping them warm in an incubator.
A day before the smaller cub's death, the zoo said that its behaviors were "good," but that it was still a high-risk time, and that the cub's weight was fluctuating. The panda team was bottle- and tube-feeding the cub in an attempt to provide enough fluids and nutrients.
The cub had shown some signs of regurgitation, and vets were administering antibiotics as a precaution, the zoo said Tuesday.
Zoo pathologists will perform a necropsy on the cub. A final report will be available in the coming weeks.
At the time of death, the cub weighed 79.8 grams, or about 2.8 ounces. A few hours after birth, the cub had weighed just over 86 grams, or about three ounces.
The larger cub, believed to be the second born, weighed in at 138 grams (4.86 ounces) shortly after birth. At its most recent weigh-in, the surviving cub weighed 137.7 grams (4.85 ounces), the zoo said Wednesday.
The panda team is continuing to closely monitor Mei Xiang and the surviving cub around the clock, the zoo said. The risks to the surviving cub remain high.
Vets don't know yet who fathered the twin cubs. Mei Xiang was artificially inseminated in late April with semen from the zoo's male giant panda, Tian Tian, as well as from Hui Hui, a giant panda living in China. It's possible the cubs had different fathers.
Tian Tian has fathered all four of Mei Xiang's previous cubs, two of whom survived: Tai Shan, born in 2005, and 2-year-old Bao Bao, who still lives at the National Zoo.