Veterinary pathologists on Thursday blamed liver disease brought on by insufficient oxygen for the sudden death of a six-day-old baby panda born at the National Zoo in Washington last month.
The unnamed female cub, born to giant panda Mei Xiang under a Sino-American panda breeding and research program, weighed just under 3.5 ounces when she died.
"The immediate cause of death of our panda cub was liver disease," said Suzan Murray, the National Zoo's chief veterinarian, as Mei Xiang returned to public view alongside her male partner Tian Tian.
"The pattern of death in the cells within the liver suggests a lack of oxygen, and our pathologists found underdeveloped lungs," Ms. Murray told reporters, citing the results of a thorough necroscopy.
"Since the lungs are the organs that take oxygen into the body, it's thought that that impeded the proper absorption of oxygen. The liver cells did not get enough oxygen and then they died, and that was the cause of death."
Asked if the cub, the product of artificial insemination of Tian Tian's frozen sperm, might have been born prematurely, Ms. Murray responded: "It's certainly a possibility."
Mei Xiang, whose appetite and behavior are nearly back to normal in the wake of her pregnancy, gave birth on September 16, causing a sensation because of the rarity of panda births in captivity.
She had given birth once before, in July 2005 to a male cub named Tai Shan who has since gone to China.
Zoo officials wanted Mei Xiang to raise her newborn as naturally as possible, while using webcams to keep a close eye on their progress.
Giant pandas are an endangered species. There are only 1,600 in the wild in China and 300 in captivity around the world.
In the wake of the cub's death, the National Zoo said it will be discussing with its Chinese partner, the China Wildlife Conservation Association, the way forward with their joint panda program in the coming weeks and months.
One possibility is for either Mei Xiang or Tian Tian to return to China in exchange for another bear in hopes of greater success in breeding a cub.
The mortality rate for pandas living in human care for the first year of their life is high, running at 26 per cent for males and 20 percent for females, the zoo said.
On Thursday, Mei Xiang, 14, who now is 20 pounds short of her normal weight of 240 pounds (108 kilograms) -- breakfasted on bamboo shoots and frozen fruit juice, sitting upright on a rock with her back to news photographers.
"Her mothering behavior has already gone down," said Don Moore, the zoo's associate director of animal care sciences, explaining that Mei Xiang no longer cuddles a conical Kong toy as a substitute for her lost offspring.
"Mei Xiang is looking almost normal to us at this point," he said.
Tian Tian, 15, similarly took his time Thursday munching on his breakfast in an adjoining pen, expressing total indifference to his human onlookers.