Tuatara: Volume 8, Issue 3, April 1961
Illustrations of the New Zealand Frog Fauna
Illustrations of the New Zealand Frog Fauna
The accompanying full-colour plate shows the six species of frogs found in New Zealand. Although single species have previously been illustrated in colour, e.g. Leiopelma hamiltoni in McCulloch (1919), no one illustration showing all the known species has yet appeared.
In the illustration the frogs are drawn to show as clearly as possible the main distinguishing features, even if this results in slightly unnatural postures. In some cases a hind leg is arranged to display either the intense colour of the thigh, normally concealed when the frog is crouching, or the nature of the webbing between the toes.
Except where otherwise stated below, the illustrations are of recently-caught, live specimens. This is a necessary precaution, for in captivity frogs may undergo colour changes and lose condition, becoming dull and drab coloured. The water-colour painting here is an attempt to illustrate as nearly as possible typical specimens of each species when they have page 96 assumed a relatively light colour-phase. However, it is still beyond any graphic technique to portray adequately the delicate and subtle colouring and patterning of the living frog.
Some frogs have an extremely wide range of colour forms; of the indigenous New Zealand frogs this applies particularly to Leiopelma archeyi (Stephenson and Stephenson, 1957, p. 875). Individuals of the other species are more constant in their colour form but may vary from the illustrated example in that their overall colouring becomes darker, even to the extent that the pattern of colours present in the light phase is obscured (e.g. Hyla aurea). To achieve standard conditions for the purpose of the illustrations, each live frog was placed on a light background. This ensured that the animal maintained the lightest possible colour phase.
The three species of the genus Hyla, introduced into New Zealand from Australia, displayed considerable ability in changing their body colouration. Hyla aurea when removed from a dark cage in the morning was blackish-green, often with all colour pattern totally obscured. After thirty minutes on a light-coloured background the frog assumed the intense colours shown. Against a dark background Hyla ewingi tended to assume an overall rich brown colour, similar to that seen mid-dorsally in the frog illustrated. The brown faded to a dusky grey, particularly on the sides of the body, after a period in bright daylight.
1. Hyla ewingi from Orua Downs; body length 38 mm.
2. Hyla aurea from Palmerston North; body length 79 mm.
3. Hyla caerulea from Sydney, Australia; body length 68 mm.
4. Leiopelma hamiltoni from Maud Island, Pelorus Sound; body length 39 mm.
5. Leiopelma hochstetteri from Tokatea Ridge, Coromandel Peninsula; body length 38 mm.
6. Leiopelma archeyi from Tokatea Ridge, Coromandel Peninsula; body length 28 mm.
In contrast, the colouration of Hyla coerulea changed little, the animal becoming a slightly darker green in low light intensities, with little change or loss of body markings.
Frogs of the New Zealand indigenous genus Leiopelma showed no obvious colour changes in comparison to hylids. In no case did the body pattern become obscured; but the frogs did become slightly darker coloured when placed in a dark container. Only one marked change of colour was observed when Leiopelma hamiltoni showed small patches of light green amongst the overall golden brown of the flanks; this change occurred after the frog had been exposed to bright sunlight for about two hours. The green patches on the body and legs of Leiopelma archeyi were seen to become more intense and sharply defined in intense light.
It was noted that typical stances are assumed by the six species of frogs when at rest, and some comparisons can be made. Hyla aurea tends to sit with its hind legs closely adpressed to the body, and the forelimbs extended so that the head is raised high. Hyla coerulea often assumes a crouching posture, all four limbs closely pressed against its body, and with hands and feet tucked underneath chin and stomach. When inside its terrarium, H. coerulea favoured a resting position several inches above the floor, in the right angle formed by two vertical sheets of glass. The large expansions of the tips of the digits and toes of this species allow it to walk with ease up smooth vertical surfaces, even glass. H. ewingi typically assumes one of two positions. In the crouching stance the back is flattened, the limbs are held close to the body, and the fingers and toes are tucked in, so that the animal has a rather pear-shaped outline. In the ‘alert’ position H. ewingi resembles H. aurea except that there is a distinct tendency for the forelimbs to be rotated until the digits lie beneath the body.
The attitudes at rest of the three species of Leiopelma vary little from those shown in the illustrations, except that I have lowered and slightly rotated the metatarsus and pes to display the nature of the toe-webbing clearly.
In movement H. ewingi displays the characteristic walk of frogs of the mainly arboreal genus Hyla. H. ewingi walks with its body elevated high from the ground and the hind legs extended well behind the body, almost as if they follow unwillingly the progression of the rest of the animal. The walk of H. coerulea is similar but less ungainly in character, probably because the species is stouter limbed. H. aurea is less typical of the genus, for its walk resembles that of the leiopelmid frogs, being confined to short hind limb movements without rearward extension of the hind limbs, the body being held close to the ground.
Tonic immobility is shown well by the leiopelmid frogs. This state can be induced by gently placing a frog on its back so that its limbs are clear of the ground beneath. After a few seconds when the animal has become quiet and ceases to struggle, one's hand may be carefully withdrawn. A frog so positioned may remain immobile for many minutes provided it is not touched or disturbed in any way. Such tonic immobility is often of use when making measurements or noting the colour of the ventral surfaces of a frog, since it allows the animal to be studied at leisure.
Notes on the Individuals Illustrated
|1||Hyla ewingi, female, head and body length 38 mm.; locality, Orua Downs; painted from the single specimen available.|
|2||Hyla aurea, female, head and body length 79 mm.; locality, Palmerston North; example selected from 40 specimens. A typically coloured frog of this species when in the lightest phase, showing the golden tonings from which it gets its name. The commonest New Zealand frog.|
|3||Hyla caerulea, male, head and body length 68mm.; locality, Sydney, Australia; as a live New Zealand specimen could not be obtained the species was painted from a live Australian specimen. The illustration shows the lightest green phase observed. The specific name of this species refers to the colour it assumes after preservation.|
|4||Leiopelma hamiltoni, female, head and body length 39 mm.; locality, Maud Island, Pelorous Sound. Known as the ‘Stephens Island Frog’, it is also called by local inhabitants of the area ‘the golden frog’. A descriptive name indeed, for without the use of metallic pigments it is extremely difficult to show the true colours of this frog. The animal illustrated was selected from eight frogs, found during a brief visit to Maud Island, and was the most prominently marked animal examined.|
|5||Leiopelma hochstetteri, female, head and body length 38 mm.; locality, Tokatea Ridge, Coromandel Peninsula. The ‘biscuit’ tonings of the specimen illustrated are typical of the lighter coloured individuals of the species. The painting was made from a frog received shortly after its death; it was frozen to preserve its colour. The least attractively coloured of the three Leiopelma species.|
|6||Leiopelma archeyi, female, head and body length 28 mm; locality, Tokatea Ridge. Coromandel Peninsula. Single specimen available was an immature frog. A particularly beautiful frog with delicate body markings.|
I wish to thank: Mr. L. Gurr, Massey College, and Mr. A. Gates, Orua Downs, for assistance in obtaining Hyla aurea and H. ewingi; the Robb family, Maud Island, for their hospitality and assistance in the field while collecting Leiopelma hamiltoni; Mr. D. S. Boswell, Coromandel, for his great assistance in the collection of Leiopelma archeyi and L. hochstetteri; Dr. E. M. Stephenson, New South Wales University of Technology, for kindly supplying a live specimen of Hyla coerulea; the Department of Internal Affairs for permission to procure and retain for the Department of Zoology, Victoria University of Wellington, one example of each of the Leiopelma species; and Professor L. R. Richardson for his advice and encouragement during the preparation of the illustration.
McCULLOCH, A. R., 1919-‘A new discoglossid frog from New Zealand.’ Trans. Roy. Soc. N.Z., 51: 447-449, pl. 30, 4 text-figs.
STEPHENSON, E. M., AND STEPHENSON, N. G., 1957-‘Field observations on the New Zealand frog, Leiopelma Fitzinger.’ Trans. Roy. Soc. N.Z., 84(4): 867-882, pls. 56-59.