The monitor lizard is not a single species but an entire genus – Varanus – encompassing more than 80 species that together represent one of nature’s most successful reptilian lineages.
From a diminutive Australian species barely 25cm long to the mighty Komodo dragon stretching past 3m, it spans a size range unmatched by any other lizard family. Ancient, adaptable and surprisingly intelligent, it has occupied Africa, Asia and Australia for tens of millions of years.
How many species of monitor lizards are there?
More than 80 species are currently recognised within the genus Varanus, and new ones continue to be described – in 2026 three new species of rock-dwelling monitor lizards were discovered in northern Queensland. They range enormously in size, lifestyle and habitat.
Australia is home to roughly half all species, where they are commonly known as goannas; others are spread across sub-Saharan Africa, South and South-East Asia, and the islands of the south-west Pacific. One species, the Nile monitor (Varanus niloticus), has even established itself as an invasive species in parts of the southern USA.
Why is it called a monitor lizard?
The genus name Varanus derives from the Arabic waral, meaning ‘lizard’ or ‘dragon’. The common name ‘monitor’ reflects an observed habit: many species stand upright on their hind legs, raising their head and neck as if surveying their surroundings.
One folk tradition holds that the name was also inspired by a belief that Nile monitors would warn humans of approaching crocodiles – though there is no evidence they do so intentionally.
What is the largest species?
The Komodo dragon (Varanus komodoensis), endemic to a handful of Indonesian islands, is the world’s largest living lizard, growing to more than 3m and weighing up to 166kg. It is an apex predator capable of bringing down prey as large as deer and water buffalo.
Adults are ambush hunters: following scent trails to where prey congregates, then lunging from cover with a powerful bite. Prey that escapes often succumbs shortly after, and the Komodo dragon tracks the scent trail to its meal.
How does it find prey?
The monitor lizard’s most remarkable sensory adaptation is its forked tongue. Flicked continuously in and out as it moves, the tongue collects scent and taste particles from the air, which are then transferred to the Jacobson’s organ – a sensory structure in the roof of the mouth – allowing it to detect prey, mates and danger with extraordinary precision.
Unlike most lizards, which must pause to breathe when running, the monitor can sustain pursuit by gulping air through its throat, a technique known as gular pumping.
What does it eat?
Almost all species are carnivorous and opportunistic, with diet varying according to size. Smaller species feed on insects, spiders, eggs and small lizards, while larger ones take birds, rodents, larger mammals and carrion. A handful of species, among them Gray’s monitor (Varanus olivaceus) of the Philippines, consume fruit. Excess calories are stored as fat in the base of the tail – a valuable reserve during the dry season.
Is it venomous?
For decades, the dangerous effects of a monitor bite were attributed to bacteria in the mouth. Research published in 2005, however, identified venom glands in monitors, suggesting most – if not all – species may produce venom.
In most species the venom is not lethal to humans but can cause localised swelling and inhibit blood clotting, helping to immobilise prey.
How does it breed?
The monitor lizard lays clutches of between 7 and 35 soft-shelled eggs, depositing them in burrows, hollow trees or rock crevices. Some species have evolved a particularly ingenious nesting strategy: excavating the wall of a termite mound to lay their eggs inside, then allowing the termites to repair the breach – inadvertently creating a naturally temperature- and humidity-controlled incubator.
Hatchlings are independent from birth, but only about one-third are thought to survive their first year. Some species, notably the Komodo dragon, are also capable of parthenogenesis – reproducing without fertilisation by a male.
How intelligent is it?
The monitor lizard is widely regarded as the most cognitively able of all lizards. Research has shown that black-throated monitors (Varanus albigularis) can discriminate between quantities up to six – a rudimentary form of counting.
In laboratory experiments, juvenile black-throated monitors also rapidly solved puzzle boxes to access hidden food, improving their technique in each subsequent trial. Individual Komodo dragons have been observed recognising their keepers – a level of social cognition unusual in reptiles.
Are monitor lizards under threat?
Conservation status varies across the genus, but many species face significant pressure. The Komodo dragon was reclassified from Vulnerable to Endangered by the IUCN in 2021, with modelling suggesting that rising sea levels could reduce its island habitat by more than 30 per cent by 2050; fewer than 1,400 adults remain.
Across the genus more broadly, decades of commercial exploitation for skins and demand from the exotic pet trade have placed additional strain on populations. Most species are listed under CITES, which regulates trade, though enforcement remains uneven.
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