The smaller of the two giant panda cubs born last Saturday at the Smithsonian’s National Zoo died at 2:05pm Wednesday. Provided to China Daily

Sad news came on Wednesday afternoon from the Smithsonian's National Zoo when the smaller of the two giant panda cubs born last Saturday died at 2:05 pm.

The zoo's panda team had been rotating both cubs allowing each to benefit from spending time with their mother, Mei Xiang. The smaller cub was with Mei Xiang from about 2 pm on Tuesday until Wednesday morning. When the panda team swapped the cubs in the morning, they found the little cub had not increased in weight, appeared weaker and exhibited possible respiratory issues.

The team immediately began taking action to improve the smaller cub's condition, administering antibiotics, respiratory support, formula and fluids.

"This is a hard loss for us here at the Smithsonian's National Zoo and the Conservation Biology Institute," said Dennis Kelly, director of the zoo, at a 4 pm press conference.

He said the teams were prepared for twins, with both teams in the US and China working very hard at taking care of the cubs.

Kelly added he takes great comfort in the fact that the larger cub is thriving. "When you deal with endangered species such as giant pandas, every birth is a success," he said.

The zoo's pathologists will perform an animal autopsy on the 4-day-old giant panda cub. A final pathology report will provide more information in the next few weeks.

At the time of death, the cub weighed 79.8 grams, about 2.8 ounces. The mortality rate for panda cubs in their first year in human care is 26 percent for males and 20 percent for females.

The panda team continues to monitor Mei Xiang and the larger cub via the 24/7 Panda Cams. At last weigh-in, the cub weighed 137.7 grams. Despite these encouraging signs, the team continues to closely monitor both mother and cub around the clock, as the cub is still vulnerable and the risk remains high.