It's a boy! The panda team at the National Zoo say the surviving giant panda cub born last weekend is a boy, and his father is the National Zoo's own Tian Tian.
The zoo made the announcement at a press conference Friday, revealing details about the cub and the death of his fraternal twin, a boy also sired by Tian Tian.
Zoo officials determined the smaller cub likely died from pneumonia he developed after breathing in food material. The smaller cub was both bottle and tube fed, and that's likely how the food got into his lungs. The smaller cub would have died without the keepers' intervention.
Keepers said the surviving cub "looks great" and is healthy, active and nursing. The cub has gained 27.9 grams in the past 72 hours, zoo officials said Friday.
The cub's mother, Mei Xiang, tried to put him down and leave the den Thursday night, but his squeals kept her running back. Eventually, she was able to leave the den for a few minutes. The zoo says it's normal for Mei Xiang to start spending short periods of time away from her new cub. Her trips outside the den will increase as he grows.
Before the smaller cub's death, keepers had tried to swap the newborns to ensure that they were both receiving adequate care. Panda mothers are usually unable to care for twins on their own. A zoo official said Mei Xiang hadn't been showing preference for one cub over another and was hesitant to give up whichever cub was with her any time keepers attempted a swap.
Mei Xiang was artificially inseminated with semen from two male pandas this spring: the zoo's male giant panda Tian Tian and from Hui Hui, a panda in China that was deemed a good genetic match.
Tian Tian is also the father of Mei Xiang's previous four cubs. Two of those cubs have survived: male Tai Shan, who was born in 2005 and now lives in China, and female Bao Bao, who turned two on Sunday, a day after the twin cubs were born. A female cub born in 2012 died six days after birth. Another female cub, Bao Bao's twin, was stillborn.
While big sister Bao Bao is just a few feet away from the new cub, zoo officials said Friday that she will not be placed in the same enclosure as her younger brother.
Giant pandas are not social in the wild, and the panda team said there would be a chance of aggression or breeding between siblings. However, they may give the two a chance to see one another when the new cub gets older.
Although zookeepers now know the new cub's sex, he is still going without a name -- for now. Panda cubs are traditionally named at 100 days of age. Bao Bao's name was revealed during a naming ceremony Dec. 1, 2013, after the public voted on several choices.