Vienna's Schonbrunn Zoo had decided to "let nature run its course", shunning the artificial insemination procedures by which pandas in captivity are usually born. And this week they got their reward, although it was something of a surprise given that an ultrasound scan just three weeks ago had not picked up on the fact Yang Yang was pregnant.

The proud parents of the cub . . . which is just 10cm long and weighs 100g . . . were loaned to the Austrian zoo by China in 2003. Since the baby was born, Yang Yang (whose name means sunshine) has been seen on CCTV cameras tenderly carrying her new baby in her mouth.

Zoologist Regina Pfistermuller described the birth as a "lovely surprise". Female giant pandas are only fertile for three or four days a year. In April, Yang Yang and her partner were spotted mating but zookeepers didn't want to get their hopes too high.

And when a scan on 6 August failed to find a baby, the zoo's team really began to lose hope.

That was until the keepers heard the muffled sounds early on Thursday morning. "We all clustered round the CCTV screens, everyone was hoping for a glimpse of the baby. None of us wanted to believe it until we'd seen it, " said Pfistermuller.

But joy was tinged with sadness when, many hours later, the tiny lifeless body of a sibling cub was discovered in the pen.