VIENNA -- The giant panda that gave birth at Vienna's Schoenbrunn Zoo last week was hiding a tiny little secret -- an extra cub.

The zoo originally announced the birth of a one cub Aug. 7. On Tuesday it said mother Yang Yang actually bore twins, her fourth and fifth cubs after Fu Long, Fu Hu and Fu Bao.

The fact that it took a while to discover the second cub is not surprising. The birth was observed by camera only. And newborn pandas are tiny: the zoo said its latest addition was just 10 centimetres (4 inches) long and weighed 100 grams (3.5 ounces).

The Schoenbrunn Zoo says no other European zoo has managed to breed giant pandas by natural means.