Frogs
Green tree frog Frog populations have declined in the last two decades and many species are now extinct. In Queensland, there are about 115 native frog species, 14 of which are endangered and six vulnerable.
The reason for the decline is largely unknown, although habitat loss and fragmentation, urbanisation, pollution, climate changes and disease (e.g. chytrid fungus), as well as natural population fluctuations, are likely to have contributed. It has also been suggested that frogs are “indicator species” that warn of detrimental changes in the environment. Of great concern is that frog numbers have declined in what was previously considered pristine environments.
The decline in frog populations is of serious concern to those involved in conserving biodiversity. It is a worldwide problem, with declining frog populations recorded in North and South America, Europe and South Africa, among others.
Fleay’s barred frog Frogs are commonly found in suburban backyards, so it is quite likely that you will come across an injured one. In the past, little has been done to help injured frogs. However, as the interest in native frogs has increased, frog populations have declined or become extinct and urban areas have encroached on native frog habitats, there has been a growing interest in conserving native frog species. Part of this process has been to try and help injured frogs.
The following section covers some basic information on what to do if you come across an injured frog. One species of frog you should not rehabilitate is the cane toad.
What is a frog?
Frogs belong to the animal class amphibia. The word amphibia comes from the Greek words “amphi” (meaning both) and “bios” (life). It describes animals with backbones that base their life on land and in fresh water. It includes all frogs, toads, and salamanders.
Biology of frogs
The anatomy of the frog is similar, although much simpler, to that of mammals. A frog’s body consists of a head, short neck, front and back limbs and a trunk region, which contains the heart, lungs, stomach, intestine and gonads (reproductive organs). Both liquid and solid wastes leave the body through a single opening, known as the cloaca. A frog’s body is supported and protected by a skeleton.
Frogs do not have ribs. The heart is protected by a panel of cartilage that extends across the “front” of the body from arm to arm. They are covered by a thin, moist skin. The skin has two layers, an outer epidermis and an inner dermis. There is also a thin exterior cuticle layer that the animal regularly sheds and eats. (People often think the frog is convulsing when they see this process for the first time). The skin supplies the frog’s body with oxygen. When under water, dissolved oxygen is able to pass through the frog’s skin directly into blood vessels. From here it is delivered to the tissues and cells of the body. However, frogs do not only breathe through their skin. Air enters the body through the nostrils and/or mouth, passes through the windpipe and into to a pair of simple sac-like lungs.
Frogs have very small teeth on their upper jaw. These are practically useless for chewing. However, their tongue is highly specialised. It is flicked out to grasp at prey. Their eyes are quite basic because they have a fixed lens that is unable to change focus. Their vision appears to be movement-based.
Although frogs have a sense of hearing, they have no external ear. Their ear canal is covered by round, flat skin similar to the head of a drum and is known as the tympanum.
Frogs communicate with each other through calls or croaks. These calls are used to attract a mate and indicate territory or distress. These calls can vary greatly between species and are usually unique to the species and used to identify them.
Life cycle
Most frogs begin their life in water in a larval form known as a tadpole.
During this stage they have a short body, a small mouth opening with rows of “teeth” for grazing and gills for breathing. Tadpoles do not have eyelids, jaws, legs or lungs.
As a frog matures it goes through a transformation process known as metamorphosis. Depending on the particular species and environmental factors (e.g. temperature and humidity), the larval stage can last from a few weeks to a year with one high altitude Australian species taking two years to metamorph. In addition to altering a frog’s body shape, metamorphosis also alters its feeding and breathing methods. Tadpoles have the digestive tract of a plant-eater although they will relish protein sources when available. By the time they metamorph, their intestines have shortened and modified to digest a mostly protein diet. An adult frog primarily eats insects, but will snack on anything that is the right size.
This includes small animals, particularly molluscs, worms, young frogs, tadpoles and, depending on the species, small mammals and birds. The tadpole uses lungs to breathe like a fish but, as metamorphosis approaches, the lungs have formed and will be inflated by a large gulp of air through the mouth when the tadpole leaves the water. The gills may still be present for a few days but are not used once the lungs are activated. (This period can be a confusing. If metamorphs return or fall into the water, the gills of some species continue to function. However, others will drown because the gills have immediately ceased to function once the lungs were inflated.)
The other body changes are to the skin, the structure of the mouth and the formation of bones.
Frog or toad?
The evolution of Australian animals did not include any toad species. Our frog fauna only numbers about 220 species, which is not as diverse as some parts of the world. However, elsewhere on the planet, toad species are very plentiful and some are very attractive and endangered. What we know as the cane toad bufo marinus is also called the marine toad because of its ability to handle low concentrations of salt water.
Giant frog 
Cane Toad 
Toad eggs
Worldwide, the genus bufo has over 200 species and some are rare. The cane toad is an introduced animal in Australia and has an exceptionally high reproductive rate. Some birds and reptiles have made extraordinary strides in learning how to kill and eat toads while avoiding the toxic glands. This makes for a fascinating study on the speed by which animals can evolve.
There are many species of Australian frogs that bear a superficial likeness to the toad. These animals are often killed. Unfortunately, mistaken identify is one of the factors that is leading to the decline in frog populations.
Some basic generalisations are:
Eggs:
All toad eggs are in long strings, which may be tangled around vegetation and looked clumped from a distance, but closer examination easily shows the string of tadpoles/ "toadpoles":Both appearance and behaviour are used to distinguish these, as some frog species can have black tadpoles.
Toadpoles:
- occur in large numbers;
- swim or bask as a swarm;
- make no attempts to hide from strong sunlight;
- have a diamond-shaped body (when viewed from above);
- “round out” as they mature (the nostrils are visible as clear holes in the snout);
- have a jet black body and thin tail muscle and clear fin (do not identify tadpoles at night);
- have no fin markings;
- have ribs that appear transparent until you are very close;
- grows to about 35mm in length.
Toadpoles
Anything that matches this description but is larger is not a toad.
Any tadpole that has blotches, speckles or stripes is a frog. Frog tadpoles can also grow more than 35mm and may have markings on their tail fin. Tadpole behaviour also tends to be “independent”, with animals resting individually on a similar background or darting under leaf litter when disturbed or when exposed to the sun.
Juveniles/Adults:
Juvenile toad A toad's skin is dry and has warts (smooth for females, sandpapery for males). Their eyes have a football-shaped pupil and the iris is dark with a scattering of yellow speckles. Their eyelids have a ridged hood. Frogs have a consistently coloured iris and no ridged hood on their eyelids.
Other characteristics such as legs, body shape, and lack of webbing or glands behind the eye are not sufficient to positively distinguish toads from frogs. All species in the Australian genus uperoliea have a gland behind their eye.
More information about identification of cane toads can be found (including photos) in the website of the frog decline reversal project, inc./cairns frog hospital at http://www.fdrproject.org.au
Cane toads
Cane toad The cane toad Bufo marinus was introduced in the cane fields of north Queensland in 1935 in an attempt to control the cane beetle. Initially, 102 toads were imported from Hawaii. These toads thrived and successfully bred in the Australian environment. Unfortunately, the toad was not able to eradicate the cane beetle (a pesticide was eventually found to do the job).
These days, the cane toad is considered an environmental pest although it is not a pest in its native Venezuela and Brazil. They are spreading across Australia at a rate of about 30km a year. In Australia, the cane toad has very few natural enemies. Their vast numbers, ability to survive in diverse habitats and eat almost anything means they are they able to out-compete native wildlife for food. Cane toads generally eat anything moving on the ground of the right size including worms, centipedes, roaches, caterpillars and beetles.
Cane toads are toxic to most animals. The toxin is located in two large glands, called paratoid glands, on the sides of its head. In general, cane toads pose little threat to humans unless they are eaten or their toxin or skin secretions come in contact with your eyes, nose or mouth. If this happens, seek medical attention. They can pose a threat to domestic animals like dogs and cats. Over the years, animals such as the water rat, ibis and crow have learned to eat the toads’ non-venomous internal organs by flipping them on to their backs. One species, the keelback snake, appears to have a resistance to the venom toxins of toadpoles and toadlets.
Cane toad showing parotid glands While the cane toad is an environmental pest, it should be disposed in a humane manner. Bashing, stabbing, spraying with chemicals and stomping on them are not humane methods. Currently, the recommended method is to put the toads into a container (with air holes) and place them in the fridge the night of collection. The low temperature sends them into a hibernative-type state. Move the container to the freezer the next morning. After being frozen for several days, you can dispose of the container in your bin before collection or bury the toads in the garden.
Always wear gloves when you are handling a cane toad.
Another advantage of collecting and placing toads in containers is that you will be able to verify that it is indeed a toad and not a ground-dwelling frog. If you are not sure, consult frog identification books or contact the Queensland Parks and Wildlife Service or Queensland Museum for advice.
A Frog Friendly Garden
One thing that you can do to help increase the number of native Australian frogs is to make your garden attractive to them. There are a number of factors that will help.
- Insects to eat. You can attract insects to your garden by keeping a compost heap, mulching garden beds and planting a variety of local native plants.
- Humidity and shade. Your garden should be well vegetated if it is to retain moisture and provide shade for frogs. It should contain ground cover, small plants and bushes and small to large trees.
- Sheltered areas. Vegetation, rocks, logs or artificial garden ornaments are essential for providing frogs with a place to hide.
- Places to breed. Ideally, this should be a shady pond with underwater aquatic plants. However, even a temporary pond that fills after rain will be enough for some species.
- Keep your garden free of chemicals (e.g. insecticides, pesticides, fertilisers). Frogs succumb to environmental poisons because they have very thin skin that absorbs chemicals. They also become sick and die by eating insects that have been poisoned.
- Frog ponds. Frog ponds are an ideal way to encourage frogs to breed in your garden. Just make sure that you keep an eye out for cane toads, as they will also find your pond an attractive place to breed. Make sure you remove any cane toads or their eggs.
Last updated: 03 January 2007
