Giant panda
Giant panda Yang Yang and her twin cubs, which were born on August 7, 2016, are seen in this still frame taken from surveillance camera footage dated August 15, 2016 and released on August 16, 2016, in a breeding box inside their enclosure at Schoenbrunn Zoo in Vienna, Austria. Schoenbrunn Zoo/Handout via REUTERS

A giant panda, that gave birth to a cub last week at Vienna's Schonbrunn Zoo, surprised staff of the sanctuary when they discovered that a second cub was hiding beneath its large paws. Measuring around 6 inches (15 centimetres), the hairless, pink cubs arrived on 7 August. The twins are naturally conceived, which is an incredibly rare event for the endangered species, the zoo said in a statement on Tuesday (16 August).

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Initially, the zoo believed that the mother Yang Yang, on loan from China to Austria, gave birth to just one cub as the delivery happened inside a nesting box and could only be monitored by the zoo by an infrared camera. The zoo keepers soon became suspicious after hearing not one but two baby pandas squeaking from the nesting box. Their doubts were confirmed on 12 August when the mother panda was caught on camera cradling two cubs, the zoo said.

Zoologist Eveline Dungl told AFP, "The cubs have little fat bellies and panda mummy Yang Yang is very relaxed. You rarely see them because Yang Yang constantly warms them between her paws... What you can hear very clearly are their suckling and grunting noises when she feeds or licks them."

In accordance with Chinese tradition, the cubs will only be named after 100 days, since up to 50 percent newborns don't survive. The siblings are doing very well so far and are being monitored around the clock, Dungl added.

Female pandas are fertile only for a couple of days every year. The 16-year-old couple Yang Yang and Long Hui, are already parents of Fu Long, Fu Hu and Fu Bao, born in 2008, 2009 and 2013, respectively.