Frogs Types of the Tropical Rainforest
Hylids are very attractive and photogenic frogs and they are quite often the types of frogs that are featured in books, magazines and T-shirts. They are found in many different environments all over the world. There are more than 790 species in 40 genera (Cogger and Zweifel 1998). Australia has 3 genera and over 70 species. The highest diversity of species is found in the wetter forests of Australia, New Guinea and most especially tropical South and Central America, where about two thirds of the species of this family occur. This is most likely where then tree frogs originated, with a fossil record going back to more than 50 million years (Cogger and Zweifel 1998). They are generally absent from the polar regions and much of Africa (EOTAW 1977, Cogger and Zweifel 1996). However, in Africa there is another closely related family that replaces them.
Tree frogs generally have long limbs, rounded toe discs and large mouths. The eyes tend to be laterally placed on the head and directed forward (Cogger and Zweifel 1998); presumably to be effective in judging distances when leaping and catching their prey. Like other Tree Frogs in other families, there is often a flattened area of round skin called a disc on the end of the digits. The discs secrete a sticky substance from mucus glands in the skin (EOTAW 1977). If the toe pads are examined under a microscope one can see interlocking irregularly shaped cells with narrow gaps (Martin 199-) It is these gaps that catch onto tiny irregulations on the climbing surface, even glass. They have extra disc-shaped piece of cartilage in each digit of the fingers and toes which allows the disc to be pressed up against a surface even when the finger or toe is at an angle (EOTAW 1977, Cogger and Zweifel 1998). The detailed pattern of irregulations can also occur on the skin of the underside of the stomach (Martin 199-), which explains why many frogs squeeze their stomach against a window; they are increasing the surface area to enable themselves to cling to the smooth glass.
Like most frogs, they need to return to water to breed, with nearly all of them laying eggs in water. Subsequently they have a free swimming aquatic tadpole stage. Some may lay their eggs on vegetation overhanging water, so when tadpoles hatch they plop into the water below (Cogger and Zweifel 1998). Others may breed in holes or vegetation that holds water, such as the bromeliads of the Neotropics (Cogger and Zweifel 1998). The tadpoles tend to have laterally placed eyes, to see below and above the waterline (Cogger Beonos and Zweifel 1998) and feed by filtering microscopic food particles (Cogger and Zweifel 1998).
The most distinctive feature of these frogs are their big beautiful black eyes that are so photogenic. The eyes of this group of frogs have another diagnostic feature in the striking lace-like venation on the eyelid that is visible when the eyes are shut; this pattern is called ‘palpebral venation’. Its appearance thus results in the common name of ‘Lacelid’ for this genus. Barker et al (1993) suggests this probably help in camouflage when the frog has its eyes closed when asleep or resting. Unlike many other frogs, when the pupil contacts it forms a vertical rather than horizontal slit. Most of the 20 or so species of Lacelids are found only in New Guinea; another one or two (depending on classification), are found in the Cape York region of Australia. They are normally found in vegetation and on rocks around fast flowing streams in tropical rainforest. As most Lacelids live near rainforest streams, they breed in fast flowing water (Nightingale 1992). The eggs are large and have the tendency to clump together (Nightingale 1992) and thus not get washed away. Tadpoles have a strong sucker mouth to cling to rocks and vegetation and avoid being swept away downstream (Nightingale 1992). They are also thus strong swimmers, with a flattened body, low tail fins and dorsally placed eyes (Cogger and Zweifel 1998) and a muscular tail (Freeman 1998).
Green Tree Frog, Litoria caerulea
Great Barred Frogs White Lipped Tree Frog
These are large leaf brown frog with striking barring on the legs and large dark eyes. The beautifully subdued browns of their skin enable these frogs to blend in with the leaf strewn floor of their habitat. This habitat tends to be the rainforest and wetter sclerophyll forests of the east coast. 3 species live in northern NSW and southern Queensland, 1 species lives in North Queensland, and 1 in New Guinea.
The most cryptic group of frogs in the Australian tropical rainforest are the 'Nursery frogs' or 'Narrow mouthed frogs'. Most members of this family are tiny and dull coloured, and thus very rarely seen. However, their calls are commonly heard on warm, wet nights in the Australian tropical rainforest, although many people may not immediately recognize these often strange calls as coming from frogs.
The 'Ornate Frog' has a high, and quite loud 'meeeep', like a little toy car. It is difficult to find, but is usually calling from perched on a tree trunk, often only a few metres from the ground.