At 7mm (0.27 inches) long, Paedophryne amauensis may be the world's smallest vertebrate - the group that includes mammals, fish, birds and amphibians.

The researchers also found a slightly larger relative, Paedophryne swiftorum.

Presenting the new species in PLoS One journal, they suggest the frogs' tiny scale is linked to their habitat, in leaf litter on the forest floor.
Finding the frogs was not an easy assignment.

They are well camouflaged among leaves on the forest floor, and have evolved calls resembling those of insects, making them hard to spot.

"The New Guinea forests are incredibly loud at night; and we were trying to record frog calls in the forest, and we were curious as to what these other sounds were," said research leader Chris Austin from Louisiana State University in Baton Rouge, US.

"So we triangulated to where these calls were coming from, and looked through the leaf litter.

"It was night, these things are incredibly small; so what we did after several frustrating attempts was to grab a whole handful of leaf litter and throw it inside a clear plastic bag.

"When we did so, we saw these incredibly tiny frogs hopping around," he told BBC News.

bbc.com