The zoo is involved in a joint project with Germany's Leibniz Institute for Zoo and Wildlife Research, the Beauval Zoo in France and Pittsburgh Zoo in the US.

In a statement on its website, Schoenbrunn Zoo says the aim is to expand the gene pool of African elephants in captivity, to help ensure their survival.

There are reckoned to be about 2,000 African and Indian elephants in zoos worldwide and another 15,000 Indian elephants kept privately - mostly in logging companies and temples.

Schoenbrunn Zoo says 40 elephants have been born from artificial insemination in European zoos since 1998, but the use of frozen sperm from a wild bull is innovative. Two previous attempts with frozen sperm were unsuccessful.

Freezing the sperm enables the team to pick the optimum time to fertilise the female and works out much cheaper, the zoo says.

The Leibniz Institute researcher who developed the freezing technique, Thomas Hildebrandt, said the trick was to freeze the sperm in stages, as mammalian cells are very sensitive to temperature changes.