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In the depths of an Indonesian rainforest, scientists have identified a fanged amphibian that is the first known frog that gives birth to tadpoles, instead of laying spawn.

The species was originally discovered decades ago, but it was only after a chance sighting that a scientist recently stumbled across a female with newborn tadpoles with her.

Upon closer inspection, experts found the Limnonectes larvaepartus is the only frog out of the world's 6,455 species to give birth to tadpoles directly. 

Limnonectes larvaepartus (pictured) is the only frog out of the world's 6,455 frog species to give birth to tadpoles instead of laying spawn. While the fanged species was found decades ago, it was only recently that experts have discovered evidence of the creature's unique birthing ability

The unusual member of the Asian group of fanged frogs was originally found by Indonesian researcher Djoko Iskandar.

Usually grey or brown, the species measures between one and two-inches long (4cm), and weighs less than two-tenths of an ounce (5 grams).

The frogs live in small streams and puddles in rainforest habitats, doing their best to avoid being eaten by larger fanged frog species, as well as predatory snakes and birds.

Males possess two fang-like projections from the lower jaw that are used in fighting.

Scientists have long theorised that this particular species gives birth to tadpoles, but they had never seen the creatures mate or spawn first-hand. 

The latest discovery was made by University of California, Berkeley, herpetologist Jim McGuire. 

He grabbed a frog he thought was male, but instead, he found that it was a female with a dozen newborn tadpoles with her.

To make the discovery, herpetologist Jim McGuire grabbed a frog he thought was male, but instead, he found that it was a female with a dozen newborn tadpoles with her. Pictured are two newborn tadpoles of the newly described fanged frog

The new evidence of the frog's behaviour, witnessed in the Sulawesi Island (marked on the map) rainforests in Indonesia. The unusual member of the Asian group of fanged frogs was originally found by Indonesian researcher Djoko Iskandar decades ago

'Reproduction in most frogs could not be more different from human reproduction. 

'In this case, what is most interesting, ironically, is that the reproductive mode is more similar to our own,' Dr McGuire said.

The new evidence of the frog's behaviour, witnessed in the Sulawesi Island rainforests, is published in the journal Plos One.

'Almost all frogs in the world - more than 6,000 species - have external fertilisation, where the male grips the female in amplexus (the frog mating embrace) and releases sperm as the eggs are released by the female,' Dr McGuire said.

Those eggs, or frogspawn, mature through stages including the aquatic tadpole larval phase, typically limbless with a tail that propels it through the water.

'This new frog is one of only 10 or 12 species that has evolved internal fertilisation, and of those, it is the only one that gives birth to tadpoles as opposed to froglets or laying fertilised eggs,' he added.

Dr McGuire said: 'It's totally unclear why this mode of reproduction has not evolved more frequently.

'My favourite topic when it comes to frog evolution and diversification is the bewildering variation in reproductive modes that occur. Frogs exhibit all sorts of interesting twists.'

Some frogs in Africa that also engage in internal fertilisation are known to give birth to froglets that never go through the tadpole stage.

Others carry eggs in pouches on their back, brood tadpoles in their vocal sac or mouth, or transport tadpoles in pouches on their backs and sides.

Two known species of female gastric brooding frogs, both of which are now extinct, were famous for swallowing their fertilised eggs, brooding them in their stomach, and giving birth out of their mouths to froglets. 

The fanged female frog is pictured right with the live tadpoles offspring behind her. The new evidence of the frog's behaviour, witnessed in the Sulawesi Island rainforests, is published in the journal Plos One 

Almost all frogs in the world - more than 6,000 species - have external fertilisation, where the male grips the female in amplexus - known as the frog mating embrace, illustrated with a stock image - and releases sperm as the eggs are released by the female