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Frogs

by  Paddy Ryan

They don’t croak, and their dull browns and greens keep them well hidden among leaves and rocks. New Zealand’s frogs are unusual little creatures, and they need protecting. Once widespread, these ancient and primitive species are in decline, surviving in only a few locations.


Frogs in New Zealand

New Zealand has four frog species that are endemic – found only in New Zealand. There were three other endemic species, but they have become extinct since humans arrived. A further three species from Australia have formed self-perpetuating populations.

Frogs are known as pekeketua or pepeketua in Māori.

Features of New Zealand endemic frogs

New Zealand endemic frogs are among the world's most ancient. Their ancestors were carried by continental drift from the supercontinent of Gondwana millions of years ago. In a family of their own (Leiopelmatidae), they differ from most other frogs in many respects:

  • They have no eardrums.
  • They have no vocal sac and do not call or croak, although they make quiet squeaks when disturbed.
  • They catch insects with their mouth, not with a long tongue.
  • They lay small numbers of large yolky eggs in moist places, but not in water.
  • Tadpoles grow inside the egg and hatch as tailed froglets – there is no free-swimming tadpole stage.

Spine and muscles

All the New Zealand native frogs have an extra vertebra, and unlike modern vertebrates their vertebrae are concave at both ends. They have long pieces of cartilage in the abdominal muscles. Although the adults have no tail, they have tail-wagging muscles (called caudalipuboischiotibialis muscles).

They share some of these features with ancient fossil frogs as old as 135 million years, and with one other living primitive family (Ascaphidae) in the American Rockies.

Frog paddling

The primitive native frogs swim using alternate legs – like a dog paddle instead of a frog kick. Because their head moves from side to side, this is less energy-efficient than the swimming style of modern frogs.

Distribution of endemic frogs

Two species – Hochstetter’s frog (Leiopelma hochstetteri) and Archey’s frog (L. archeyi) – occur in the North Island. The other two, the Maud Island frog (L. pakeka) and Hamilton’s frog (L. hamiltoni), are restricted to Maud Island in the Marlborough Sounds and Stephens Island in Cook Strait respectively, apart from recent transfers to other suitable islands for back-up populations. There is the intriguing possibility that a South Island frog may yet be found.

Of the three extinct species described by Trevor Worthy in 1987, remains of Leiopelma markhami have been found in the North and South islands. L. waitomoensis is known only from the North Island and L. auroraensis from a single cave in the South Island. L. waitomoensis was a large, robust frog, almost twice the length of any endemic frog alive today, and presumably many times heavier.


Native frogs

Hochstetter’s frog

Hochstetter’s frog (Leiopelma hochstetteri) lives in forested ranges from Northland (south of Whāngārei) to East Cape, usually alongside streams.

The frogs are brown or greenish, with partial webbing on the hind toes. Females grow to 4.7 centimetres (from snout to vent) while males reach 3.8 centimetres. The total population is unknown.

Archey’s frog

There are two main groups of this North Island species (Leiopelma archeyi) – one in Coromandel and the other in the Whareorino Forest, west of Te Kuiti. Archey’s frog is slim, with a glandular ridge running backwards from the eye. Colour is variable and ranges from mainly green (rarely) through combinations of green and brown to mainly brown. Some Whareorino specimens may be quite brightly coloured, with pinks and brick reds against the background colour.

The male body length is up to 3.1 centimetres and the female 3.7 centimetres. The total wild population is unknown but is almost certainly decreasing, in part because of the introduced chytrid fungus disease.

Hamilton’s frog

This species (Leiopelma hamiltoni) is known only from Stephens Island in Cook Strait, where it is found in only two small areas. It is mainly brown, but juveniles are sometimes greenish.

It is similar in appearance to Archey’s frog, with a glandular ridge running from the eye, but on average Hamilton’s frog is larger. These are the largest native frogs, with males up to 4.3 centimetres long and females up to 5 centimetres. The total population may be fewer than 300 individuals.

Maud Island frog

Females may reach 4.9 centimetres in length, and males slightly less. They are long-lived – one survived at least 29 years. The Maud Island population is estimated at over 10,000 frogs.

Based on measurement and appearance, the Maud Island frog is identical to Hamilton’s frog. But in 1998 it was described as a new species, Leiopelma pakeka, because of genetic differences. More recent study based on mitochondrial DNA has questioned this, and it is possible the two frog groups will once again be treated as one species.

Even so, the fact that Maud Island and Hamilton’s frogs live on separate islands means that over time they are evolving differently. From a conservation point of view, two separate populations, with some genetic differences, should be managed as if they are different species, because such diversity is valued.


Introduced frogs

The three introduced frog species are Australian, and belong to the hylid tree frog genus Litoria.

Unlike the native frogs they have a visible external eardrum (tympanum), and a horizontal, not rounded, pupil. Only the whistling frog is similar enough in size or colour to be mistaken for a native frog. They all have loud calls, and an aquatic tadpole stage.

Whistling frog or brown tree frog

This frog (Litoria ewingii) was brought to Greymouth in 1875. It is widespread in both main islands and may still be extending its range. It is less reliant on water than its cousins. Its ‘weeeep eeeep eeeep’ call sounds like a cricket. Mainly light brown on the back, it can change from light to dark. It has a white stripe from armpit to jawline, and orange thighs. It grows to 5 centimetres long (snout to vent).

Green and golden bell frog

Introduced to the Auckland area in the late 1860s, this frog (Litorea aurea) has not moved far. It is currently found only in the North Island north of Gisborne. Its call is a drawn-out croak, and it is mainly green with gold or bronze patches. The back of the thighs and groin are bright blue, and the belly is smooth and white. It grows to about 9 centimetres.

Frogs and fungus

The lethal chytrid fungus disease, widespread in Australia, means the southern bell frog faces extinction there, and the green and golden bell frog may also be at risk. New Zealand could have been a refuge for them – but now the fungus has arrived. It was identified in the southern bell frog in Christchurch over the summer of 1999–2000 by researcher Bruce Waldman.

Southern bell frog

Introduced into Canterbury from Tasmania in the late 1860s, the Southern bell frog (Litoria raniformis) is the most widespread frog in New Zealand, found throughout the country. Its call is a series of short staccato croaks. It has a similar colour pattern to the green and golden bell frog but often has a warty back. A distinct cream-coloured fold runs backwards from the eye, and there is a pale stripe down the midline of the back. Like the green and golden bell frog, the groin and back of thigh are bright blue. It grows to about 10 centimetres.


Ecology and reproduction

Native frog habitats

The native frogs are terrestrial (living on land) or semi-terrestrial, and none lay their eggs in water. They are all well camouflaged and stay in retreats under logs, stones or in vegetation during the day. This helps retain moisture and reduce the risk from predators.

Some live on misty ridge-tops far from surface water.

Only Hochstetter’s frog is semi-aquatic, tending to stay close to streams. It is also the only one with partial webbing between the hind toes. The other three species are forest dwellers, although Hamilton’s frog is mainly restricted to a 600-square-metre boulder bank on Stephens Island – surely the smallest known habitat for any frog. The Maud Island frog reaches densities of five frogs per square metre in suitable places.

Archey’s frog is terrestrial but may climb several metres into trees and bushes during the night, before returning to its retreats at dawn. The Whareorino Forest population in the King Country, discovered in 1991, may spend more time in trees than the Coromandel groups.

Humidity

All frogs need a moist environment to keep their skin damp, as they breathe through their skin as well as their weak lungs. The native frogs are more likely to be seen at night when the humidity is high, or after rain. Archey’s frog is able to withstand substantial drying. One study showed that frogs that were dehydrated to 92% of their body weight increased hydration to 99% over four hours when placed on wet leaves. Climate change could have a profound effect on the native frog populations if it leads to dryer conditions in some areas.

Habitats of the introduced frogs

The introduced Australian bell frogs usually live near water, but the whistling frog is not as restricted. Whistling frogs are nocturnal, but both the bell frogs may be active during the day or night, and sometimes sunbathe in the afternoon.

Feeding

All the frogs feed on any invertebrate prey that they can take hold of and ingest. The native frogs take prey directly into their mouths, sometimes using their arms. Introduced frogs tend to use their highly extensible fast-flicking tongues to capture prey.

Finding a mate

The native Leiopelma frogs do not make loud mating calls like other frogs, but may find each other by using visual cues or through pheromones (chemical signals between animals). Research has shown that the Maud Island frog can respond to the scent cues in frog faeces. Males of the introduced species call or croak to attract females.

Mating

Frog mating is called amplexus. In the native Leiopelma species, amplexus is inguinal: males clasp the females around the ‘waist’. In the three introduced species amplexus is axillary: males grasp the females around their armpits.

Egg laying and care

Native frogs

All four native species produce very large yolky eggs (between 1 and 22, depending on species). Female Hochstetter’s frogs lay their eggs under stones or fallen vegetation. Researcher Joan Robb has also found clutches laid in tunnels bored by the larvae of the large Uropetala carovei dragonfly.

Hochstetter’s frogs give no parental care – the tailed froglets hatch out, absorb the yolk, and resorb (assimilate) their tail.

The other three Leiopelma have very similar reproductive behaviour. Females lay small clutches of large eggs, which the males sit over until they hatch. Young froglets, complete with yolk sac and tails, climb onto the male’s back and stay there for a month or longer until they become independent. Such care may keep the youngsters moist, reduce predation and possibly reduce fungal or microbial infections.

Introduced frogs

The introduced frogs lay large numbers of eggs in water and leave them to fend for themselves. Tadpoles stay in the water until metamorphosis approaches: the tiny, tailed froglets leave the water for long periods, and eventually become frogs.


Threats and conservation

Chytrid fungus

The New Zealand native species are threatened by the chytrid fungus Batrachochytrium dendrobatidis, discovered in Archey’s frog in 2001 after their numbers had plummeted. The fungus is thought to have arrived with imported pet frogs. It damages the frogs’ skin, through which they breathe and take in moisture, and may also release toxins.

Predators

Five dead Archey’s frogs found in the Whareorino Forest showed signs of attack by rats. Interestingly, the rats left the heads of their victims intact. The frogs have defensive glands around the head and backs that secrete compounds that may be bitter or toxic. Other evidence, largely anecdotal, suggests that native predators as varied as the giant kōkopu (a fish), tuatara (reptile) and weka (bird) may avoid the native Leiopelma frogs.

Little terrors

When tiny Archey’s and Hochstetter’s frogs were picked up by a weka (a flightless bird), the frogs gave off high-pitched squeaks until the weka dropped them. The Archey’s frogs then stood their ground with a head-butting posture, which further discouraged the large bird. Unfortunately, introduced rats and other predators are not so easily put off.

Changing environment

Habitat change affects frogs, and their range has been greatly reduced in the last few centuries. Forest clearance, animals browsing and even routine road work will further deplete native frog populations. Climate change may present other challenges to their survival, despite their 100-million-year survival to date.

Conservation

In 1921, legislation was passed making it an offence to harm frogs or remove them from their environment. However, to slow their decline, more active measures will be needed – protecting habitat, removing or reducing nearby mammalian predators, and preventing the spread of fungal disease.

Making safer habitats

The highly endangered Stephens Island frog population was restricted to one site on one island. A new habitat was then set up on the island, and 12 adult frogs were transferred there in 1992. They appear to have settled into the new area. In May 2004, another 40 Stephens Island frogs were moved to Nukuwaiata Island in the Marlborough Sounds.

After a trial move to a second site on Maud Island, 300 Maud Island frogs were taken to Motuara Island in May 1997, and in 2006 the population there seemed to be doing well.

Emergency measures

The Carter Holt Harvey Native Frog Research Centre at Auckland Zoo is a captive breeding facility for Archey’s frogs, free of the chytrid fungus. The aim is to breed back-up populations in case the chytrid fungus or another ecological catastrophe wipes out the species in the wild. The centre will work with the Department of Conservation to find a cure for the fungal disease that threatens frogs around the world.

Unwanted frog species

It is vital that new species of frog do not become established in New Zealand as they may out-compete native frogs. In 1999, tadpoles of the Australian banjo frog (Limnodynastes dumerilii) were reported from the Waitakeres. The area was thoroughly searched, and several thousand tadpoles, found at a person’s home, were destroyed. Fortunately in this case, the aggressive species does not seem to have become established.


External links and sources

More suggestions and sources


How to cite this page: Paddy Ryan, 'Frogs', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/frogs/print (accessed 21 October 2019)

Story by Paddy Ryan, published 24 Sep 2007