The giant panda cub that died last month did not survive because of underdeveloped lungs and liver damage, National Zoo officials said Thursday.
Standing in front of the giant panda enclosure, where the zoo's two adult giant pandas lounged in the sun, doctors explained what they had found inside the tiny body of the 4-ounce female cub and what its death could mean for the future of the zoo's panda exchange program with China.
Suzan Murray, chief veterinarian at the zoo, said an investigation of each organ showed "liver necrosis and lungs not fully formed."
Necrosis, or the death of body cells, "was likely caused by not enough oxygen getting to the liver," she explained, adding that underdeveloped lungs might be to blame for the lack of oxygen distribution.
Dr. Murray said not enough is known about newborn pandas to say whether this combination of problems is seen often in cubs. But Don Moore, associate director of animal care sciences at the zoo, delivered sobering news that the mortality rate for panda cubs between 6 months and 12 months old is between 20 percent and 26 percent.
Whether the cub was born prematurely, before its lungs could develop properly, was "certainly a possibility," Dr. Murray said.
The nature of a giant panda's pregnancy cycle made it hard to pinpoint how far along the cub was in its development stage when it was born. A panda's fertilized egg does not attach itself to the mother's uterine wall immediately. In fact, the cub's birth on Sept. 16 was somewhat of a surprise to zoo officials, who said they recognized behavior typical for both a true pregnancy and a false pregnancy.
Weeks before the cub's birth, zookeepers allowed mother panda Mei Xiang to isolate herself in a bamboo den she had built, closing down the indoor viewing area to visitors and using cameras to monitor the pregnant panda and, later, the interaction between her and the cub.
"We don't know very much about what goes on in the den," Mr. Moore said.
Volunteers and panda keepers monitored the cub through the squawks it made at its birth and shortly before its death on Sept. 22.
The investigation into the cub's death did conclude it was not killed by Mei Xiang accidently crushing it.
Mr. Moore said that as of Thursday, Mei Xiang was eating about 85 percent of what she normally consumes daily and was still about 20 pounds lighter than her usual 240 pounds. While preparing for her cub, Mei Xiang had not left her den, but Mr. Moore said she was venturing outside her enclosure. To be sure, Mei Xiang spent some of Thursday morning gnawing on a "fruitsicle" of frozen water, apples and pears and sleeping in the sun.
Panda keepers also had been able to remove her bamboo den during the increasingly longer periods of time when she roamed outdoors, and they removed a hard plastic toy she had taken to cuddling before, during and after her pregnancy.
Despite the cub's death, which zoo officials said was mourned around the world, doctors remained optimistic about the opportunity of impregnating Mei Xiang again.
Dr. Murray said based on 14-year-old Mei Xiang's ability to give birth this year, "we're hopeful to be able to do it again in the future."
Whether Mei Xiang or her male counterpart, Tian Tian, remain in the District for the next attempt is something zoo officials and their Chinese partners will consider in coming months.
Zoo spokeswoman Pamela Baker-Masson said a discussion between the zoo and China would be scheduled to discuss the future of the giant pandas.
Mei Xiang and Tian Tian arrived in Washington in December 2000 as part of a $10 million exchange agreement with the Chinese government that ends in December 2015.
The two replaced the zoo's first pair of pandas, Ling-Ling and Hsing-Hsing, who produced five cubs that did not survive.
In 2005, Mei Xiang was impregnated via artificial insemination and gave birth to Tai Shan, a male cub who became a D.C. celebrity for his outdoor antics before he was moved to a new home in China in 2010.
Ms. Baker-Masson said the five-year exchange agreement is about halfway through, at a point where officials can consider exchanging one or both of the giant pandas if they are "found unsuitable" for the panda breeding program.
Among the many things to consider are the best interests of the pandas, conservation efforts, the population of giant pandas in China and the zoo's breeding and research program, Ms. Baker-Masson said.
"There's been no decision on the future of Mei Xiang and Tian Tian and whether they leave or remain," Ms. Baker-Masson said.
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