Goanna

Goanna is the name given to any of the various Australian monitor lizards of the genus Varanus, as well as to certain species from Southeast Asia.

There are around 20 species of goanna, 15 of which are endemic to Australia. They are a varied group of carnivorous reptiles that range greatly in size and fill several ecological niches. The Goanna features prominently in Aboriginal mythology and Australian folklore. Traditionally, it formed an important part the diet of many Aboriginal peoples.

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Characteristics

Being predatory lizards, goannas are often quite large, or at least bulky, with sharp teeth and claws. The largest is the Perentie (Varanus giganteus), which can grow over 2m in length. Goannas prey on all manner of small animals; insects, lizards, snakes, mammals, birds, eggs. Meals are often eaten whole, and thus the size of their meal may depend on the size of the animal itself, although all species are scavengers and will readily eat animals as large as cattle and camels. However, the Perentie has been observed killing a young kangaroo, and then biting out chunks of flesh like a dog. Goannas have even been blamed for the death of sheep by farmers, though most likely erroneously, as goannas are also eaters of carrion and are attracted to rotting meat.

Not all goannas are gargantuan. Pygmy goannas may be smaller than a man's arm. The smallest of these, the short-tailed monitor (Varanus brevicuda) reaches only 20 cm in length. They survive on smaller prey such as insects and mice.

Most goannas are dark in coloration, whites, greys, blacks and greens featuring prominently. Many desert dwelling species also feature yellow-red tones. Camouflage ranges from bands and stripes to splotches, speckles and circles, and can change as the creature matures; juveniles sometimes being brighter than adults.

Like most lizards, goannas lay eggs. Most lay eggs in a nest or burrow, but some species lay their eggs inside termite mounds. This offers protection and incubation, additionally the termites may provide a meal for the young as they hatch. Unlike other species of lizards, goannas do not have the ability to regrow limbs or tails.

Habitats

  Goannas are found throughout most of Australia, except for Tasmania, and manage to persist in a variety of environments. Most species are terrestrial, or ground dwelling. Prominent among these is the Sand goanna (Varanus gouldii – also known as the ground goanna or Gould's goanna), the most common of all goannas. They are often found in close proximity to a burrow or den, which may be a hollow log, or if in the plains a dug burrow which can be up to a metre (three feet) deep. They may even take over rabbit warrens. The far end of the burrow is often close to the surface, so if the entrance is blocked off (by a predator, or a collapse) the goanna just needs to break through a thin layer of soil to be free.

As well as sandy plains, some goannas live in rocky outcrops and cliffs, often having special adaptations that aid their survival. The spiny-tailed goanna (Varanus acantharus) of Northern Australia, has blunt spines on its tail that make it virtually immovable from the rockface if in danger.

While some terrestrial goannas may occasionally climb trees or outcrops, there are plenty of primarily arboreal species. The lace monitor (Varanus varius) is probably the best-known amongst these, but is not the most common. The lace monitor is the second largest of all goannas, reaching lengths of up to 2 metres. Other more common tree goannas, such as the Timor tree monitor (Varanus timorensis) and Mournful tree monitor (Varanus tristis) do not grow to quite such lengths, averaging only a few feet nose to tail.

Other goannas are adapted to swampy coastal environments such as the Mangrove goanna (Varanus semiremex). Further still, the Mertens' water monitor (Water goanna – Varanus mertensi), found in lagoons and rivers across northern Australia, is streamlined for swimming, utilising its tail as a paddle. Most other goannas are good swimmers, but tend not to voluntarily venture into the water.

Goannas and humans

Confrontations

Like most native fauna, goannas are rather wary of human intrusions into their habitat, and will most likely run away (into the scrub, up a tree, or into the water, depending on the species). A goanna is a rather swift mover, and when pressed will sprint short distances on its hind legs.

Goannas also rear up when threatened, either chased or cornered, and also inflate flaps of skin around their throats and emit a harsh hissing noise.

Some goannas recover from their initial fear of humans, especially when food is involved (or food has been involved previously). This reinforces the wildlife authority's mantra of not feeding animals while camping or erstwhile adventuring. This said, most authorities doubt that a goanna will actually direct an intentional attack on human unless said human attempts to attack it (or grasp at it) first. Aborigines who hunt goannas for food consider the Perentie as a high-risk (but tasty) prey.

The debate whether goannas are venomous or not is growing. Previously it was thought that incessant bleeding caused by goanna-bites were the result of bacterial infection. Recent studies ^[1] suggest that monitor lizards (including goannas) are venomous and have oral toxin-producing glands.

Other dangers a goanna presents is from its hefty tail. It can swing this much like a crocodile if cornered. Small children and dogs have been knocked down by such attacks. Often victims in goanna attacks are bystanders, watching the person antagonising the goanna. Alarmed goannas can mistake standing humans for trees and attempt to climb off the ground to safety, which is understandably painful, as well distressing for both man and beast.

It should be noted that goannas are a protected species throughout Australia for all non-indigenous people.  

Folklore

European settlers perpetuated several old wives' tales about goanna habits and abilities, some of these have persisted in modern folklore amongst campers and bushmen. This includes the above-mentioned exaggeration of goannas dragging off sheep from shepherds' flocks in the night. Around a campfire these might even be exaggerated into child-snatching, rivalling drop bears (attack koalas) as tourist scarer, probably more convincing due to the reptiles carnivorous nature and fearsome appearance.

A common tale was that the bite of a goanna was infused with a powerful incurable venom. Every year after the bite (or every seven years), the wound would flare up again. For many years it was generally believed by herpetologists that goannas were nonvenomous, and that lingering illness from their bites was due solely to infection and septicaemia as a result of their saliva being rife with bacteria from carrion and other food sources. However, in 2005 researchers at the University of Melbourne announced that oral venom glands had been found in both goannas and iguanas.^[2]

Because the goanna regularly eats snakes (often involving a fierce struggle between the two), they are often said to be immune to snake venom. The goanna does eat venomous snakes, but no evidence found suggests actual poison immunity. Other stories say the lizard eats a legendary plant, or drinks from a healing spring which neutralises the poison. This is immortalised in Banjo Paterson's humorous poem Johnson's Antidote.

Possibly related to the above poison immunity, goanna fat or oil has been anectdotally imbued with mystical healing properties. Aborigines traditionally used goanna oil as an important bush medicine,and it also became a common medicine among whites in Australia's early days. Said to be a cure-all for all sorts of ailments, and possessing amazing powers of penetration (passing through metal as if it were not there), it was sold amongst early settlers like snake oil in the Old West of North America.

Diet

The diet of goannas varies greatly depending on the species and the habitat. Many of the small species feed mostly on insects, with some being small lizard specialists. Many of the medium to large species will feed on whatever prey items they can catch. This includes eggs, fish (V.Mertensi), birds, snails, lizards, snakes, marsupials, and rodents. All species are carrion eaters and will feed on the carcasses of dead animals.

Possible origins of the name

The name Goanna might have been derived from iguana, as early European settlers likened goannas to the South American lizards.

Another possibility is that the name might have been derived from the South African term for a monitor lizard Leguaan (as the Cape of Good Hope was a popular refresher stop for immigrant ships to Australia from Britain).

Species

For the most part, in common names, "goanna" and "monitor" are interchangeable.

1. Perentie – Varanus giganteus
2. Lace monitor – Varanus varius
3. Sand goanna – Varanus gouldii (also Gould's goanna or ground goanna)
4. Mertens' water monitor – Varanus mertensi
5. Spiny-tailed goanna – Varanus acanthurus (also ridge-tailed monitor)
6. Mangrove goanna – Varanus semiremex
7. Timor tree monitor – Varanus timorensis (also Timor monitor)
8. Mournful tree monitor – Varanus tristis
9. Short-tailed monitor – Varanus brevicuda
10. Racehorse goanna
11. Black-tailed goanna
12. Rosenberg's goanna – Varanus rosenbergi

In Popular Culture

A Goanna named Joanna was featured in the movie The Rescuers Down Under as the cohort of the poacher McLeach.

References

1. ^ Young E, Lizards' poisonous secret is revealed, New Scientist.com news service, 16 November 2005
2. ^ Goanna venom rocks the reptile record, UniNews Vol. 14, No. 22 28 November - 12 December 2005, University of Melbourne, Retrieved March 08, 2006

Further reading

• Cogger, H. (1967). Australian Reptiles in Colour. Sydney: A. H. & A. W. Reed, ISBN 0-589-07012-6
• King, Dennis & Green, Brian. 1999. Goannas: The Biology of Varanid Lizards. University of New South Wales Press. ISBN 0-86840-456-X
• Underhill, D. (1993). Australia's Dangerous Creatures. Sydney: Reader's Digest. ISBN 0-86438-018-6