A species of fanged frog that lives on Sulawesi in Indonesia has surprised scientists by giving birth to tadpoles, reports BBC News.

An international team of zoologists has studied the fanged frog and described its behavior for the first time, in a study published by the journal Plos One.

The discovery was made by Dr. Jim McGuire, a herpetologist and associate professor from the University of California, Berkeley, who thought he was holding a male frog. He actually held a pregnant female who suddenly gave birth to a clutch of tadpoles.

Most of the world's 6,000 frog species use external fertilization while mating, during which the female lays eggs and the male fertilizes them with sperm.

According to McGuire, there are lots of odd modifications to this standard mode of mating.

"This new frog is one of only 10 or 12 species that has evolved internal fertilization, and of those, it is the only one that gives birth to tadpoles, as opposed to froglets or laying fertilized eggs," he said.

It is unknown how the male frogs fertilize eggs inside the female. Frogs do not have sex organs to transfer the sperm.

In California, scientists found two frog species that have evolved a penis-like "tail" that administers the sperm, but the new Indonesian species does not have that appendage.

Professor Djoko Iskandar from the Bandung Institute of Technology in Indonesia, and a collaborator on the study, first discovered the new species of fanged frog in the 1990s, but it had not been confirmed as a distinct species until now.

Scientists have named the new species Limnonectes larvaepartus. The Limnonectes family have earned the label "fanged frogs" from the twin projections on their lower jaws used in fighting.

It is believed there are up to 25 species of Limnonectes frogs on Sulawesi, but only four species have been described currently.

"Finding a new species is not that rare, but actually discovering a new reproductive mode is," said Ben Tapley, team leader of herpetology at the Zoological Society of London (ZSL).