The zoo's director says photos of the twins have shown for the first time "that they are a boy and a girl." That's good news for Schönbrunn zoo - as all three cubs born previously at the zoo were males.
The twins' birth, one month ago, was only observed by camera so the fact that it took so long to establish the cubs' genders is not surprising.
The newborns were tiny, weighing only 100 grams and with a length of around 10 centimetres. They now weigh around a kilogram and are around 35 centimetres long.
The baby pandas have fat milk tummies and are getting more and more fur. They don't have to be warmed by their mother 24 hours a day anymore; they can now sleep next to Yang Yang in the bamboo nest and can stay alone for short periods of time while their mum leaves the breeding box to eat.
Zoo director Dagmar Schratter says it's now possible to tell the twins apart by their black shoulder markings. “The male has a uniformly wide shoulder marking, whereas the shoulder marking of the female tapers in the middle.”
She added that brother and sister also have noticeably different characters - the boy tends to get impatient if he isn't able to drink his mum's milk right away but his sister is much quieter. Both pups suckle for an average of four hours a day.
Zoo officials at first thought mum Yang Yang had just given birth to one newborn but became suspicious after hearing sounds coming from the breeding box. Their suspicions were confirmed when the camera filming the mother looking after the newborns caught both of them on film at the same time.
The zoo says that it's quite common that Giant Pandas give birth to twins but the mother usually only rears the stronger of the two.
Because the survival rate for pandas in the first few weeks is only 50 percent, the Chinese tradition is to only name the newborns after 100 days. The zoo will decide on names for the twins with its Chinese partner, who the pandas belong to.