UPDATE: August 26, 2015, 4:30 p.m. ET: At a news conference this afternoon at the National Zoo, officials including veterinarian Donald Neiffer and panda curator Brandie Smith described the loss of the smaller panda cub as devastating, but said that the opportunity to learn and share knowledge about the experience was "exponential."

As of yesterday at 2 p.m., the smaller cub had been with its mother Mei Xiang, after being successfully rotated out for the larger cub, which was then placed in the care of the panda team.

Previous reports that Mei Xiang preferred one cub over the other were inaccurate. Rather, the mother giant panda was handling each cub well and being a good mother to both cubs, Neiffer said. But Mei Xiang had trouble during the swapback exercise releasing one cub for the other. She was concerned, he said, about giving up the baby.

After the birth of her second cub, the panda team observed that she was having trouble managing the twins. The swapping protocol was developed to help the mother panda handle the two cubs, because it is believed that the animals are largely able to care for just one cub at a time.

Last night, the smaller cub was with its mother, and throughout the night the team watching the mother and cub reported that the baby was nursing and showing no signs of distress. "Mei Xiang was doing everything right," said Neiffer.

However, this morning, after the larger cub was given to Mei Xiang for its turn, zoo officials discovered that the smaller cub had not gained weight, appeared weaker and may have developed a respiratory issue. Despite heroic efforts by the "best medical team" assembled, according to Smith, the cub's condition worsened throughout the afternoon. At the time of death, the Zoo reports, the animal weighed just 2.8 ounces.

"Do we have a feel for why this cub died?" asked Neiffer, who said that a full necropsy and pathology study will be done and analyzed by the team at the National Zoo and its Chinese partners. This information will be shared with other panda keepers and other zoos, along with the experiences of the staff employed to nurture and hand-raise the two twin cubs—including the methods for bottle feeding and for swapping the panda cubs. Neiffer said the entire experience will "increase our knowledge base."

"It is an awesome responsibility to help the mother care for her cubs," said Smith, who noted that the Zoo still has one healthy cub and that Mei Xiang is "the same incredible mother that she was for Bao Bao and Tai Shan," the cubs she delivered in 2013 and 2005.

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The National Zoo reports that one of its newborn panda cubs has died. The smaller cub, which was born on August 23 at 10:07 p.m. ET and died today at 2 p.m. ET weighed just 86 grams at birth, was under the care of veterinarians and keepers. Mei Xiang had refused the staff's attempts to get her to swap out one panda cub for the other. Instead, the 17-year-old female panda had held fast to the larger cub, leaving Zoo staff no choice but to try to rear the animal—keeping it warm inside an incubator, feeding it formula, administering additional fluids and trying to keep it safe from infection with antibiotics.

Zoo officials report that the larger cub is "strong, robust, behaving normally" and is with its mother Mei Xiang.

The Zoo says that it will be holding a press conference at 4 p.m. ET. Watch this space for updates.