Fascinating frog fauna

Press, 26 October 1984, Page 18

Fascinating frog fauna

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New Zealand’s frog fauna is a small but extremely interesting one. Only six species occur here; three native and three introduced from Australia. The native species are typically frog-like in general appearance but their structure and life histories differ markedly from most other frogs. The introduced species, too, are of great interest as they have had only a hundred years or so to adapt to New Zealand conditions; the native frogs — or their ancestors — have been here for more than a hundred million years. Frogs belong to the Amphibia, the oldest and most primitive class of land vertebrates. The native frogs are of tremendous zoological interest as they have a lineage even more ancient than the dinosaurs and tuatara. It is quite possible that many millions of years ago all the frogs that inhabited the world had characteristics similar to our native frogs. What are some of these characteristics? The native species, Archey s, Hochstetter’s and Hamilton’s (see illustration) frogs do not lay eggs in water, nor do their offspring pass through a typical free-swim-ming tadpole stage. Instead, they emerge from their eggs as tailed froglets. Our native species, therefore, like salamanders, are able to breed in terrestrial or semi-terrestrial situations. This explains why they frequent the vicinity of mountain streams or can live under stones and logs on the higher ridges. Another primitive feature is the shape of the individual bones of the backbone or vertebrae; all have concave or hollowed out joint surfaces. The retention in the adults of two “tail-wagging” muscles is also very unusual, as except for one American species, these muscles are absent in the adults of all other frog species. Our native frogs have no eardrums or vocal sacs; the latter structures act as resonating chambers, consequently they are poor at croaking — the best they can

manage is an un-froglike squeak. Their inability to croak (which may explain why the Maori in times gone by was unaware of the existence of native frogs in New Zealand) is more than compensated for by their large eyes. These protrude well above the line of the head, somewhat resembling those of an alligator. The inside structure of the eyes aids night vision, a feature which is in keeping with their nocturnal life style but which also shows they are relatively advanced. Compared with the rare native frogs, all of which are strictly protected, the introduced species are far more common and widespread. They include two species of golden bell frogs, Litoria raniformis and Litoria aurea and the

brown tree or whistling frog, Litoria ewingi. The brown tree frog, which is known from coastal Manawatu, the West Coast, Southland, and Canterbury, is the only species which might be confused with native frogs. It is about the same size in length (42mm) and brown like the native species, but the orangeyellow patches on the inside of its thighs and the ear drum behind the eye make it easy to distinguish. The green bodies, larger size (90mm in length), horizontal and rather thin roundish pupils, and generally conspicuous turquoise blue thighs of the golden bell frogs clearly separate them from native

frogs. The golden bell frogs each have features which help differentiate them. Litoria raniformis has warty green and gold upper parts, with a green centrally located stripe on its back and a large amount of webbing between the toes on the hind feet. Litoria aurea has a smooth green back with irregular gold spots, no stripe on the back and less webbing between the toes of the hind foot. Litoria raniformis is the most common of the two golden bell frogs. It occurs throughout New Zealand, whereas Litoria aurea is confined to the north and northeast of the North Island.

By

GEOFFREY TUNNICLIFFE