Giant pandas Mei Xiang and Tian Tian have another son, Washington's National Zoo has announced.

The zoo said the surviving panda cub is male and the son of its male panda Tian Tian.

Mei Xiang gave birth to twins on Saturday but the smaller cub, also a male fathered by Tian Tian, died on Wednesday.

The most likely cause of death was complications from food getting into its respiratory system resulting in the development of pneumonia, according to officials.

"Once an animal becomes ill, things become more difficult," said chief veterinarian Don Neiffer. "It's hard to come back."

Pink, hairless and blind, newborn cubs weigh three to five ounces at birth. Mei Xiang weighs more than 700 times as much.

National Zoo's associate director for Animal Care Sciences, Brandie Smith, said the surviving cub appears to be healthy and is gaining weight.

Tian Tian is the father of Mei Xiang's other cubs, daughter Bao Bao and son Tai Shan. During this year's panda breeding, Mei Xiang was artificially inseminated with semen from Tian Tian and a panda in China that was deemed a good genetic match.

The National Zoo is one of only four zoos nationwide to have pandas, which are on loan from China. But the Washington pandas have a history that makes them closely watched.

The zoo's first pair of pandas, Ling-Ling and Hsing-Hsing, were a gift from China following former president Richard Nixon's historic 1972 visit to the country. The pair had five cubs while living at the zoo but none survived.

The zoo's current adult pandas, Mei Xiang and Tian Tian arrived in 2000. The pandas belong to China as do any cubs they have. Tai Shan, 10, returned to China in 2010. Bao Bao, who turned two on Sunday, still lives at the National Zoo.

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