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.
Goannas combine predatory and scavenging behaviour. A goanna will prey upon any animal it can catch that is small enough to eat whole. Goannas have 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.
Most goannas are dark in coloration, whites, greys, browns, 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.
Goannas are found throughout most of Australia, except for Tasmania, and manage to persist in a variety of environments. Most species are known to 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.
Meals are often eaten whole, and thus the size of their meal may depend on the size of the animal itself. 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, small mammals and rodents. The giant Perentie has been observed killing a young kangaroo, and then biting out chunks of flesh like a dog.
All species are carrion eaters and will feed on the carcasses of dead animals, including livestock and other large creatures. The smell of rotting meat will attract these lizards.
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 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 as distressing for both man and beast.
It should be noted that goannas are a protected species throughout Australia.
Representations of goannas are common in Indigenous artwork, not just as food, but also as a symbolic spiritual motif. Goannas and the mighty Perentie are often considered two different animals when you used in Aboriginal works, as illlustrated by the story "How the Goanna and Perentie got their colours".
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.
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 anecdotally 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.
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.