Australia: The Land Where Time Began
Australian Reptiles Photo Gallery
Lizards feature prominently among the Australian wildlife. 1 Ha of grassland in central Australia can hold more than 40 species of lizard. In Australia there are 5 main categories of lizard - dragons, geckos, skinks, goannas and legless lizards.
The largest is the goanna, of which there are more than 20 known species, living in all habitats of the continent. The most widespread species is the ground-living Gould's goanna, the only part of the country it is not found is in the forests of the southeast. It is of variable appearance, from pale yellow with dark spots to almost black.
A common large eastern goanna is the lace monitor, usually dark blue on its back, it grows to about 2 m. It lives in tall forest trees and feeds mostly on birds eggs and young, though it sometimes scavenges on the ground. It lays its eggs in termite mounds, and the termites repair the hole the goanna dug in the side and the eggs are incubated at the constant temperature maintained by the termites. 10 months later the mother returns to the termite mound and digs a hole to let the young escape, they are not strong enough to dig their way out.
Mertens water goanna is found in the tropical north, frequently seen on the banks of wetlands and rivers, feeding on fish and frogs.
At 2.5 m, the perentie is Australia's largest lizard, and is only beaten by the Komodo dragon, from the island of the same name in Indonesia, in length, making it the second largest lizard in the world.
A bush legend says that a wound from a goanna bite never heals. While not completely true, the wound can easily become infected because goannas feed, in part, on carrion, so usually have fragments of rotting meat among their teeth, as is the case with the relative the Komodo dragon. They often become tame around picnic areas, but it is a good idea to not get too close to them as their long claws, as well as their unhygienic teeth, can do some damage if they feel threatened. It has since been found that they produce venom, see below.
For many years it has been believed that the only venomous lizards in the world were the North American Gila monster and the Mexican beaded lizard. The Australian goanna, like its larger relative the Komodo dragon, were believed to produce their poison-like wounds by bighting with teeth that were covered with bacteria picked up by eating carrion. A team from the Australian Venom Research Unit at Melbourne University has now found that they do in fact produce venom.
The possibility was noticed when a keeper in the Singapore Zoo was bitten by a Komodo dragon. The team leader noticed that the effects of the wound were far faster than could be explained by bacteria, particularly as the Komodo dragons in the zoo were fed only on fresh food, never on carrion. The lace monitor, a species of goanna, as the closest living relative to the Komodo dragon, was studied and a large gland was found running along the sides of it jaws. This gland releases venom when squeezed, as in bighting. They have now found venom in a number of lizards, including the bearded dragon and iguanas.
The largest monitor of all time, the Megalania prisca, larger than a Komodo dragon, probably also had the venom glands.
Evolution of venomous reptiles
Venom is now known to have evolved in only 2 lineages of living reptiles, the snakes and the helodermatid lizards - the Gila Monster and the Beaded Lizard, both from North America. Venom was thought to have evolved in the 2 known venomous lizards independently of snakes, now the study by Fry et.al. suggests that venom evolved in a common ancestor of snakes and lizards. The authors report the presence of venom in 2 more lizard lineages, monitors and Iguania, showing that all lineages producing toxin form a single clade, demonstrating a single early origin for the evolution of venom systems in lizards and snakes. They found that there are 9 toxin types that are shared between lizards and snakes. Analysis of Lace Monitor (Varanus varius) venom components showed that it had potent effects on blood pressure and blood clotting, leading to loss of consciousness and extensive bleeding of prey. Iguanian lizard (Pogona barbata) retains the ancestral venom system characteristics - serial, lobular, non-compound venom-secreting glands on lower and upper jaws, while in the advanced snakes and anguimorph lizards (including Monitor Lizards, Gila Monster and Beaded Lizard) have more derived venom systems characterised by the loss of the lower (mandibular) or upper (maxillary) glands.
The demonstration that the snakes, iguanians and anguimorphs form a single clade supports a single, early origin of the venom system in lizards and snakes. This allows insights into the evolution of the venom system in squamate reptiles and opens new avenues for biomedical research and drug design using hitherto unexplored venom proteins.
Nature V439 N 7076
Another feature that distinguishes the monitors from other lizards is that they can breathe while running, allowing them to run much faster and further than other lizards. Perenties, the largest Australian goanna, can reach speeds of more than 20 km and run down rabbits when it is hunting. In other lizards the same muscles that move the legs are used for breathing, so they can't breathe very well when running. The monitors have a muscular throat they use to pump air into their longs as they run, so they are not hindered by their breathing, enabling sustained running over long distances.
Bearded Dragon (Pogon barbata)
It has been found that this inoffensive lizard, found throughout Australia, has small amounts of crotamine, also found in rattlesnake venom. As with goannas, all bearded dragons have poison glands.
Fossils found at 3 Queensland sites have suggested that the Komodo Dragon actually evolved in Australia, spreading to eastern Indonesia about 900,000 years ago.
Kudnu mackinlayi ?Paliguanidae Arcadia Formation, the Crater, Queensland
Dinosaurs of Australia and New Zealand, John A. Long, UNSW Press, 1998
Shingle Back Lizard
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|