Frogs of Northern Australia

Water seekers 

Anyone who has ever sat outside in the evening after fresh rain in many parts of northern Australia will have heard a frog chorus, sometimes so loud it drowns out conversation. Northern Australia is not short of the odd frog or two — or to use the scientists’ term frogs are ‘abundant’ in parts of Australia’s tropical savannas. There is also a reasonable diversity of frogs in northern Australia — around half of the 216 species of frogs native to Australia can be found in the north.

It is perhaps not surprising that there are reasonable numbers of frogs in Australia’s north. Frogs are amphibians, a term which comes from the Greek for 'double life' referring to the fact that many frogs start life as eggs, and then become tadpoles which live in water, and subsequently sprout legs, lose their tails and live on land as adults. While not all frogs need to lay their eggs in water, they all need moist environments.

Far northern Australia, like parts of far southern Australia, has very low variability in rainfall, i.e. reliable rainfall. However, this rain tends to fall in the wet season with large areas of the tropical savannas being very dry for several months of the year and this is a challenge for frogs.

Why do frogs need moist conditions?

This reliance can be traced back to the ancestry of amphibians. Amphibians were one of the first groups of animals to evolve from fish and develop adaptations to living on land, but they generally retained features that made them dependent on water such as the need to reproduce by laying eggs in water.

While fossil evidence indicates some early amphibians were quite large with scaly skins that may have retained water well, modern frogs appear to have adopted a different solution to survival: they are small with a skin that is more permeable to water and air and that assists with breathing. Indeed the skin must remain reasonably moist so that oxygen can pass through the skin and enter the blood.

This means frogs are not found where it is too dry (the Sahara desert in Africa and Saudi Arabia) or too cold (Siberia, Greenland and Antarctica) as shown below.

Distribution of frogs (including toads) — red areas support frog populations, yellow areas are too cold or dry for frogs (from Tree of Life website www.tolweb.org/tree)

How do frogs survive in the drier parts of northern Australia?

Despite their dependence on moisture, frogs are found through much of arid Australia due to their remarkable ability to exploit what little water there is in these environments. These adaptations also prove useful in northern Australia which has severe dry seasons where little, if any, rains falls for months at a time.

Frogs cope with this seasonal ‘desert’ quite well: many live in areas with permanent water, while others can lay their eggs in temporary ponds. Northern Australia has many such ponds that form in the wet season and then evaporate in the long dry season and which are good homes for frog eggs as they have few predators. They are often to be found teeming with tadpoles. By the time the ponds dry out the tadpoles have developed into small frogs which then find refuge in cracks in the soil, crevices in timber and trees. Others use refuges provided by people — a familiar example being the green tree frog (Litoria caerulea) which lives in down-pipes and toilet bowls.

However some frogs do not need toilets or even ponds to survive. Around one-third of Australian frog species can burrow into the ground to escape dehydration. Burrowing frogs typically have round bodies and short limbs adapted to digging. Some burrowing frogs can form a cocoon of dead skin cells around their bodies when they are in the ground to limit water loss.

The giant frog Cyclorana australis is found in western Queensland, across the Top End of the NT and in the Kimberley. Like some other members of this genus, it can spend much of the long dry season buried underground. It stores water in its bladder and in pockets under the skin and evaporation is reduced by a cocoon of shed skin. Once the rains arrive this frog breaks through the cocoon and emerges from the ground.

Are all tadpoles free-swimming?

No. Several frog species raise their tadpoles in more protected environments. For example, the northern gastric-brooding frog Rheobatrachus vitellinus which was found in the rainforests of north Queensland raised its tadpoles in the female’s stomach. When they were further developed, the froglets were regurgitated. Unfortunately, gastric brooding frogs have not been seen in the wild since 1985.

What’s in a croak?

With rain comes the opportunity for frogs to mate and lay eggs in water, and for most frog species the mating season involves advertising. When conditions are good for mating male frogs will sit around and call to prospective females letting them know that here is a healthy single male frog sitting next to some nice water for egg-laying and who is interested in having a good time.

Each species has a distinct advertising call — presumably to better target appropriate partners. Calls range from the more conventional "crawk crawk" of  the common green tree frog Litoria caerulea to the barking sounds of the white-lipped tree frog Litoria infrafrenata of Cape York Peninsula, the growling call of the growling green eyed frog Litoria eucnemis of Cape York Peninsula and the chuckling of the northern laughing tree frog Litoria rothii. A call that is increasingly being heard across the north is the continuous mechanical “nock nock nock nock” mating call of the cane toad.

There are also reports of some frogs calling for other purposes, such as a 'rain call' in response to rainfall, and the 'release call' issued by males when other males attempt to mate with them.

Most frog calls are made with the mouth closed by drawing air through the larynx. Often the call may then be amplified by a sac of skin under or around the throat. Some frogs, however, will make distress calls with the mouth open.

What do frogs eat?

Most adult frogs eat live invertebrates of various kinds and will usually need moving prey. Often frogs are not fussy and the diet varies with the availability of prey, for example winged termites will form a much larger part of the diet when they are swarming. The cane toad is particularly unfussy and appears to eat most small things than move and many things that don’t move — like pet food.

Go to the navigation bar at left to keep reading about the types and diversity of frogs and the threats to frogs.

References

Department of Environment and Heritage (2006) Infection of Amphibians with Chytrid Fungus Resulting in Chytridiomycosis – Background Document for the Threat Abatement Plan, Commonwealth of Australia, Canberra

Hero J.-M., Morrison C., Gillespie G., Roberts J. D., Newell D., Meyer, E. McDonald, Lemckert F., Mahony M., Osborne W., Hines H., Richards S., Hoskin C., Clarke J., Doak N. and Shoo L. (2007) Overview of the conservation status of Australian frogs. Pacific Conservation Biology 12, pp 313-320.

Roleants K., Gower D.J., Wilkinson M., Loader S.P., Biju S.D., Guillaume K., Moriau L., and Bossuyt F. (2007) Global patterns of diversification in the history of modern amphibians. Proceedings of the National Academy of Sciences (DOI:10.1073/pnas.0608378104)

Tyler M. J. (1994) Australian Frogs – a natural history. Reed Books, Chatswood NSW