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A pile of slimy froglets has made conservation history in New Zealand as ecologists celebrate the first hatching in the wild of Maud Island frogs on the mainland for hundreds of years.

The tiny froglets were discovered in Wellington's Karori sanctuary by Victoria University student Kerri Lukis, who is studying the nationally threatened amphibians.

Sixty Maud Island frogs were released into a mouse-proof enclosure in the sanctuary in 2006.

Later that year 30 frogs were released outside the enclosure so the captive and wild populations could be compared.

When it came time for Ms Lukis to compare the populations she discovered 13 froglets attached to adult males in the enclosure.

"This is extra special because Maud Island frogs have never been found breeding in their natural habitat before, and certainly not on the mainland."

Clustered together to conserve moisture, the 13 fingernail-sized Maud Island froglets were transferred to Victoria University where they will be incubated and released as adults.

The Maud Island froglets differed from most frog species in that they hatch from the egg as fully formed froglets without going through the tadpole stage.

Ms Lukis said the discovery was wonderful timing for 2008, the international Year of the Frog.

"It's rare to get a good news story about frogs. Every year around 35 species of frog become extinct and all of New Zealand's remaining native frog species are on the critical list."

While not the rarest species, the thimble-sized Maud Island frogs were nationally threatened.

They had evolved very little over the last 70 million years, resulting in some distinctive features and behaviours including that they didn't croak.

The last population of the frogs was discovered on Maud Island and they have since been transferred to other islands.