Panda mom grieves for the death of her cub, clutches plastic toy at night

The mother of the giant panda cub that died on Sunday is mourning her loss.

Officials from the National Zoo in Washington said Mei Xiang spends most of the day huddled in her den, clutching a plastic toy that zookeepers gave her at night.

The Washington Post reported the zoo director, Dennis Kelly, said the cradling behavior is a maternal sign, which suggests she has not transitioned away from her mothering role.

The cub, at just six days old, was so small that the zoo did not yet know its sex. But the zoo's chief veterinarian, Suzan Murray, said today that it appears to have been female.

While the cause of death has not been determined, initial tests revealed that she showed signs of having an abnormal liver and fluid buildup in her abdomen, the Daily Mail reported.

Murray said at a press conference that the cub's liver appeared hard in some places and that there was a small amount of abnormal fluid buildup in her abdomen.

"Judging too much on the initial necropsy can be a little bit dangerous at times. Something that might appear abnormal to you visually, when you look at it under a microscope, it may indeed be normal," quoted the Washington Post.

The distressed cries of the cub's heartbroken mother alerted veterinarians that the tiny cub, who was born on September 16, had died, The Associated Press reported.

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Panda keepers managed to retrieve the cub an hour later with a set of cushioned grabbers.

Zoo veterinarians performed CPR on the cub but were unable to revive it, the Washington Post reported.

More from GlobalPost: World's oldest panda dies

The birth was a huge surprise to zoo officials, who didn't even know Mei Xiang was pregnant. The zoo had artificially inseminated Xiang in April, but it's difficult to know if a giant panda is pregnant or not.

Zoo staff were also feeling the loss – the cub's birth had heralded a new era of giant pandas at the zoo.