Goannas radiated from the north to Africa and Australia during the Miocene epoch, around 15 million years ago and have an important place in our history and culture.
They were an important traditional native food source and are commonly represented in Aboriginal Dreamtime stories.
Perhaps the most famous monitor lizard is Indonesia’s Komodo Dragon, which can grow up to 3m long. Australia was once home to a giant (Varanus priscus) twice this size!
Today there are 27 extant species of these large lizards in Australia, most are carnivorous. All have a similar body shape to their original ancestors and have sharp teeth and claws to help them hunt and eat.
Size is the distinguishing feature of Australian monitors; the largest is the Perentie (V. giganteus), which grows over 2m long, and the smallest: the Short-tailed Monitor (V. brevicuda), grows to just 20cm.
Monitors are commonly dark-coloured or white and orange-yellow in the desert. Most have camouflage bands, speckles or spots relating to their environments, though these differ between species and age groups. They can live up to 40 years in the wild.
Behaviour of goannas
Goannas mostly live on the ground and dig holes for nests or burrows to protect eggs from predators and provide a constant temperature for embryo development. Lace and Heath Monitors will dig holes into the side of termite mounds to lay their eggs.
This is clever as the termites then rebuild the nest around the eggs, keeping them safe and at a constant temperature. When the young hatch the mothers return to help dig them out.
Several species, including the Perentie, hibernate during the coldest months (from about May to August). Ridge-tailed Monitors get their name from the raised and pointed scales on their tails, which they wedge into rock crevices, making it harder for predators to pull them out.
Goannas are surprisingly good climbers. In fact, Lace Monitors are known as Tree Goannas and are thought to eat more bird eggs than other goanna species.
While they may appear slow, goannas are fast runners and will sprint short distances on their hind legs – often to the safety of water or a tree. The Sand Monitor is sometimes called Gould’s Monitor or the Racehorse Goanna for its speed. Goannas are also known to rear up on their hind legs to scare off attackers or fight, but they’ll also do it to look around for threats from a higher vantage point.
Water Monitors are excellent swimmers and can stay underwater for several minutes hunting for food such as fish, frogs, crabs or shrimps.
What do goannas eat?
Monitors eat just about anything they can catch and swallow whole. Prey is dependent on the size of the goanna but includes insects, birds, eggs, small reptiles and mammals.
In northern Australia crocodile eggs are a favourite food. They’ll also scavenge for carrion and are attracted to rotting meat.
As predators and scavengers goannas play an important role in each of the ecosystems they inhabit. They maintain population numbers of prey species and keep disease loads low through the removal of carcasses.
Monitors have adapted to a range of environments and vary in their hunting methods, such as digging, swimming and tree-climbing. They use their long forked tongues as snakes do, flickering to detect prey through scent molecules in the air.
It was only a decade or so ago that goannas were discovered to have venom glands similar to snakes. However, they don’t have enough venom to cause serious harm and lack fangs to inject into prey. Nonetheless, goannas have a great deal of bacteria in their mouths and a bite will often turn into a nasty infection, so caution and a healthy dose of respect are called for if you see a monitor in the bush!
Where do goannas live?
If you do any travelling in outback Australia you’ve got a good chance of seeing goannas. They're most active in the day, and like to spend time basking in the sun. They're mostly solitary, except for mating season in spring and summer.
How are monitors threatened?
Threats to Australian monitors include habitat loss due to land clearing for agriculture and urban space. Monitors prey on Cane Toads and suffer from the poison glands exuded by the toads, so the spread of Cane Toads has caused declines in many areas, most recently the Kimberly.
Habitat degradation and removal of termite mounds and other habitat features such as fallen timber have a significant impact on some goanna species. Invasive mammals and generalist predators such as foxes and cats may also prey on young monitors.
The conservation status of each individual species can be found on the IUCN Monitor Lizard Specialists Group website.