New Zealand's four matchbox-sized native frog species all lack ears, don't croak and hatch directly into froglets without going through a tadpole stage.

"These are living fossil frogs...They were alive before there were dinosaurs roaming. These frogs - not necessarily the same species but frogs that morphologically are very, very similar - lived 200 million years ago."

Three of the species live entirely on land and all have little or no webbing between their toes.

New Zealand's rarest species, the Hamilton's frog, numbers less than 300 and is found only on a few hundred square metres (yards) of rocky ground on the summit of a single islet between the South Pacific country's main North and South Islands.

Populations of the most widely found native frog, the Hochstetter's frog, were also in decline and research was under way to discover if the fungus was responsible, Waldman said. The chytrid fungus, identified in Australia and central America in 1998, was discovered in New Zealand in 1999. The fungus kills most of the frogs it infects. Frog populations around the world have been in decline in recent years, with researchers variously blaming the chytrid fungus, habitat loss, viruses and pollution for the reduced numbers.

U.S. researchers, earlier this week, reported they had found that very low levels of a popular weedkiller can cause male frogs to grow female sex organs and curtail their croaks.

Some scientists view frogs as an ecological barometer, but Waldman disputes that claim.

"It's very hard to kill a frog, they have very robust immune systems - it much harder to get a frog sick than a human being sick. They're not all that sensitive, which makes this story even more worrisome."