Image Credit: Sarah L. Voisin/The Washington Post via Getty Images

The National Zoo's giant panda mother, Mei Xiang, spent the night cradling a cub-sized toy as panda lovers from D.C. to China mourned the death of her week-old cub.

Zookeepers at Washington's Smithsonian National Zoo discovered Mei Xiang's cub dead Sunday morning. In keeping with Chinese tradition, it had not yet been given a name. Preliminary tests ruled out the possibility that the 4-ounce cub had been crushed by her 216-pound mother.

"Yes indeed, Mei Xiang was a good mom who protected her cub and did not crush her," said Dr. Suzan Murray, the zoo's chief veterinarian. "We all felt that Mei Xiang had been a very good mother and had been very gentle with the cub, and indeed we have found that to be so in the initial necropsy."

While it could take up to two weeks to determine the cause of the cub's death, Murray said the initial animal autopsy showed it may have been related to problems with her liver, which was barely the size of a lima bean.

Unlike other baby pandas' livers, which are soft and the same color throughout, this cub's liver had patches of hard tissue and had splotches of different coloring, Murray said. Scientists also found excess fluid in the cub's abdomen.

"There is great sadness amongst our staff and that's to be expected," National Zoo Director Dennis Kelly said. "Every loss is hard. This one is especially devastating."

Before giving birth last Sunday, Mei Xiang had endured five pseudo-pregnancies. Each time, Murray said the giant panda held her pear-shaped toy day and night awaiting a cub.

Murray said the cub's father is "blissfully unaware" that his daughter died. In the wild, pandas are solitary animals and the father is usually not involved in raising cubs.

Mei Xiang is slowly returning to her normal routine, Kelly said. After not eating for a week while nursing her newborn cub, the panda mother began drinking water and eating bamboo and biscuits Sunday afternoon.

Kelly said he is "cautiously optimistic" that Mei Xiang will return to normal.

"It's a hard time for us but with your support we'll get through it and we'll learn more about how to protect the species," he said.

The Zoo's PandaCam, showing a live stream view of Mei Xiang's den, is back up and running today. It was turned off on Sunday following the cub's death.