A giant panda on loan from China to Austria has given birth to two naturally conceived twins, an exceptionally rare event for the endangered species, Vienna's famous Schoenbrunn Zoo said Tuesday. Measuring around 15 centimetres (six inches), the pink, hairless cubs arrived on August 7, it announced in a statement. Initially the zoo thought mother Yang Yang had only given birth to one panda because the delivery happened inside a dark nesting box and was only observed via an infrared camera. More than a week passed before zookeepers realised there was a second one. "The cubs have little round bellies and panda mummy Yang Yang is very relaxed," zoologist Eveline Dungl said. "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 because up to 50 percent of newborns do not survive, Dungl explained. But so far the siblings, which are being monitored around the clock, are doing very well, she added. Female pandas are only fertile for a couple of days every year. Yang Yang and her partner Long Hui, both aged 16, are already the proud parents of Fu Long, Fu Hu and Fu Bao, born in 2007, 2010 and 2013 respectively -- and all conceived naturally. The twins are expected to have their first public outing in four months' time. Panda fans can follow their progress via videos and updates on the zoo's website -- www.zoovienna.at