Feature

New, Rare, Old, Odd Frogs

One of New Zealand's native frog species has been found to be extremely rare --  only 300 survivors are to be found on Stephens Island in the Marlborough Sounds. The rarity of this species, and the existence of a new species on Maud Island in Pelorus Sound, has been revealed by genetic testing carried out by Dr Ben Bell of Victoria University's School of Biological Sciences.

It had been thought that New Zealand had only three living species of native frog -- all distinct from their overseas counterparts because they do not develop from tadpoles but grow directly from eggs.

These three are Leiopelma hochstetteri, a larger more robust frog which is semi-aquatic, L. archeyi, the smallest species, and L. hamiltoni, both of which are terrestrial. Another three species, evident from fossil records, are now extinct, possibly because of the introduced Polynesian rat or kiore. Bell has managed to breed all three living species in captivity, but he is concerned about the effects of predators, exotic afforestation schemes and mining activities which could seriously threaten the frogs' limited ecological zones.

The most dramatic recent finding is that populations on Maud and Stephens Islands -- long believed to be the same species -- are actually two distinct species.

"Since the frogs looked very similar and have similar habitats and lifestyles, it was thought that they were the same species," Bell says. "Until now we couldn't do the genetic tests to determine this definitively, because to get the tissue you usually have to sacrifice a few frogs -- something we weren't willing to do to a threatened species. However recently we managed to use toe tissue from a small sample which have been held in captivity, which was enough to show that though the frogs look similar outwardly, they are in fact separate species."

L. pakeka Brett Robertson, VUW

The new species -- the Maud Island frog -- has been named Leiopelma pakeka, after the original Ngati Kuia name for Maud Island. It had previously been categorized as L. hamiltoni which is found on Stephens Island and in subfossils around the North and South Islands.

Genetic data now indicate that L hamiltoni and L. archeyi are sister taxa, closer to each other than L. pakeka, which is surprising as L. hamiltoni and L. pakeka are geographically closer to each other. Another extraordinary aspect of differentiation is the discovery that the different species have different chromosomal mechanisms of sex determination.

No Tadpoles

The lack of a tadpole phase makes New Zealand's native frogs unique. They undergo early development in eggs and then hatch as tailed larvae which continue to receive nourishment from stored yolk reserves.

The parenting behaviour of L. archeyi and L. hamiltoni is unusual in that once the eggs are laid the male takes over parental duties, sitting on the eggs until they hatch. The young then climb onto the male's back and stay there until they have developed into tiny frogs. This takes up to a month or more, during which the male must remain more or less immobile. This behaviour protects the young from predators and other sources of danger such as microbial contamination, and also provides the moist environment necessary for larval survival.

"The other species, L. hochstetteri, does not exhibit this behaviour -- the larvae hatch at an earlier stage and swim out directly into the water."

The frogs have an unusually long life span, often living for more than 30 years.

Conservation Issues

"The discovery that the Maud Island and Stephens Island species are distinct increases the conservation significance of both as they are the single known populations of the species," Bell says.

Bell and colleagues Charles Daugherty and Jennifer Hay recommend that since L. hamiltoni is one of world's rarest frogs, it warrants the highest level of conservation protection. It is confined to a small rock pile near the summit of Stephens Island and, while available habitat is regenerating as a result of intensive management, the tiny population numbers less than 300 individuals.

They suggest it be put on the International Union for the Conservation of Nature "endangered species" list. L. pakeka is less at risk, numbering around 10,000, but should be considered "vulnerable", they say. The researchers also suggest a programme to encourage new populations on other predator-free islands, to reduce the risk of extinction. Three hundred L. pakeka have been transferred to Motuara Island already.

The latest finding differentiating the two species calls into question the categorisation of some of the fossil frogs which were previously identified as L. hamiltoni.

New Zealand's native frogs are generally brown and smooth skinned, with some having coloured bands along their abdomens or legs. L. archeyi has some beautifully coloured cryptic patters of pink, green or brown. The females are generally larger than males. L. pakeka is a little a larger and generally darker than Stephens Island frogs. Both live in vegetated scree habitats and it is likely physical similarities which gave rise to their being classified as the same species may be the result of selection under broadly similar environments.

"All New Zealand's native frogs live in cool, moist forest or ridge-top situations, retiring by day beneath cover such as stones and fallen logs," Bell says. "They usually only emerge at night, and in wet weather. Since they only make limited chirping calls they are not usually noticed, unless you are specifically looking for them. They are thought to be quite archaic, like other New Zealand fauna, with relatives in Argentina dating from the Jurassic period."

These may be descendants of ancient frogs which inhabited Gondwana before it fragmented. Because of this antiquity, the frogs are of considerable scientific interest.