[Photo] All change for the Papua New Guinea frog (left: a young frog and right: an older member of the same species)

A new species of frog undergoes a remarkable transformation as it grows into an adult, report scientists.

Shiny black juvenile frogs with yellow spots dramatically change into peach coloured adults with bright blue eyes.

Scientists discovered the unique frog in a remote part of south-eastern Papua New Guinea.

The bright pattern of the young frog could act as a warning to predators, they say, but it is a mystery why the adult then loses this colour.

The scientists from Bishop Museum in Honolulu, Hawaii, US, report their findings in the journal Copeia.

Amphibian species come in a range of colours and patterns, from the brightly patterned poison dart frogs to the plainer greens of the common toad.

After metamorphosising from a tadpole, some frogs change in colour as they get older.

However, it is unknown for juveniles and adults of a species to have strikingly different colour and pattern schemes.

The research team came across the new species of frog Oreophryne ezra while on a expedition to find new species on Sudest Island, Louisiade Archipelago, off the south-eastern tip of New Guinea.

Of the new species they found, the frog particularly caught their attention.

"It's always exciting to discover a species you know to be new. However, the obviously unusual biology of this frog made its discovery especially exciting," says Dr Fred Kraus who along with Dr Allen Allison undertook the study.

"The remarkable thing about this frog is the drastic nature of its change in colour pattern as it matures from a tiny froglet into adulthood," Dr Kraus says.

As a juvenile the frog is dark black with yellow spots and black eyes but then switches to a uniform peach colour with blues eyes.

"This raises the question of what possible function the striking colours of the juveniles might serve," says Dr Kraus.

Juveniles closely resemble the general appearance of some of the poison dart frogs from the tropics.

Like these frogs, the colouration could serve as a warning to potential predators.
Although untested, the frog may also have harmful toxins in its skin like those present in poison dart frogs.

Poison dart frogs have skin that contains harmful alkaloids acting as a chemical defence against predation.

"If this is the case this would make this species another instance of the independent evolution of such a system," says Dr Kraus.

The behaviour of the frog also points to the idea that its colour advertises that it is toxic.

The researchers write how the juvenile frogs perch in conspicuous places during daylight hours and also demonstrated a lack of a well developed escape behaviour, indicating that they have another form of defence.

One aspect that cannot be explained is if the colour offers protection to the juvenile, why does the frog then change its colour scheme as it ages to one that offers no protection.

For now this poses further questions for the researchers.

"No other such instance is known in frogs," Dr Kraus says.

"If it does serve as protective warning colouration, the reason for its loss remains a mystery."