The surviving and thriving giant panda cub twin born Aug 22 to Mei Xiang at the Smithsonian's National Zoo is male and his father is Tian Tian.

Nancy Rotzel McInerney, laboratory manager of the Smithsonian Conservation Biology Institute (SCBI) Center for Conservation and Evolutionary Genetics, announced the findings at a press conference Friday morning.

She also confirmed that the cub that died on Wednesday afternoon was also a male and also sired by Tian Tian at the National Zoo. The twin cubs born nearly five hours apart last Saturday were fraternal twins, meaning they came from two different eggs.

Rob Fleischer, head of the center, said they started the genetic process of determining gender and paternity on Monday.

To determine gender, scientists at the zoo sequenced a short fragment of the zinc finger protein gene. The method was developed by SCBI scientists and veterinarians and previously determined to reliably identify the sex of pandas, according to the zoo.

"What we have learned will greatly add to our body of knowledge about artificial inseminations in pandas," Fleischer said.

"Determining the pedigree relationships of a cub is a key aspect of helping to maintain a genetically diverse population. Our ability to assess the cub's lineage will help our colleagues ensure that he finds a suitable mate," he added.

Don Neiffer, chief veterinarian at the zoo, said the thriving cub weighed 167 grams (about 6 ounces) at 1:40 am Friday, an increase of 27 grams in 42 hours. "We are ecstatic about that," he said.

Neiffer described the most likely cause of the panda cub twin's passing on Wednesday, based on animal autopsy so far, was complications associated with aspiration of food material into the cub's respiratory system resulting in pneumonia.

The smaller cub, which died on Wednesday, was with mother Mei Xiang from about 2 pm, Aug 25, until the morning of Aug 26. When the panda team swapped the cubs, they assessed the cub and had concerns because he had not increased in weight, appeared weaker and exhibited possible respiratory issues. He died shortly after 2 pm.

Developed from the experience and success of Chinese partners and other zoos, the hand-rearing protocol provided the panda team with several strategies to manage twin cubs. Swapping the cubs between mother and hand-rearing is a proven method that enables zoo staff to best care for twins.

Mei Xiang has so far given birth to three cubs that have survived - two males and one female. Taishan, a male, was born in 2005 and now lives in China for breeding purposes, while Bao Bao, born in 2013, just celebrated her second birthday on Aug 23.

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