Reptiles
Monitor Lizard
Varanidae · also called monitor, goanna
A monitor lizard does not behave the way a reptile is supposed to. It hunts on the move for hours, tracks scent with a forked tongue, breathes through a one-way lung system it shares with birds, and — in the Komodo dragon and several relatives — backs up its teeth with venom. It is, in effect, a lizard built to live like a mammal predator.
Overview
Monitor lizards are the family Varanidae, around 80 species, all in the single genus Varanus, across Africa, Asia, New Guinea, and Australia. They range from the 20 cm pygmy monitor to the Komodo dragon, the largest lizard alive. All are carnivores with a long neck, powerful limbs and claws, a deeply forked tongue, and, unusually for lizards, real stamina.
Built for sustained work
Monitors move air through their lungs in a mostly one-way loop like birds, have a relatively large heart and higher metabolism, and can forage, dig, climb, and fight for long stretches where other lizards would sprint and then stall in oxygen debt. The forked tongue lets them smell in stereo, reading which way a scent trail leads.
Venom and size
The Komodo dragon and other monitors carry venom glands in the lower jaw that drop blood pressure and block clotting, so bitten prey bleeds heavily and weakens — the modern replacement for the old “deadly mouth bacteria” story. The Komodo reaches 80–90 kg; the slender crocodile monitor of New Guinea is longer at over 3.5 m, most of it tail.
Virgin birth
Female Komodo dragons and a few other monitors can lay fertile eggs without a male through parthenogenesis, and in the Komodo these all become sons — so a single stranded female could, in principle, found a colony.
Status
Monitors as a family are Not Evaluated. Many are common and adaptable, but the Komodo dragon is Endangered with a few thousand left, and several Asian and Pacific species are heavily hunted for skins, meat, and the pet trade.
Monitor Lizard: common questions
How big do monitor lizards get, and which is the largest?
The family spans an enormous size range. The smallest, the Australian short-tailed pygmy monitor, is about 20 cm long and weighs under 20 grams. The largest by weight is the Komodo dragon of Indonesia, with big males reaching roughly 3 metres and 80 to 90 kilograms, occasionally more. The crocodile monitor of New Guinea is usually cited as the longest, exceeding 3.5 metres, but much of that is an extremely long thin tail, so it is far lighter than a Komodo. Several other species, including the water monitor of Asia and the lace monitor of Australia, regularly pass 2 metres. Only a few extinct lizards, such as the Australian Megalania, were larger.
Are monitor lizards venomous?
Yes, at least many of them, which was only firmly established in the 2000s. Monitors, along with the Gila monster and some other lizards, belong to a group whose ancestors evolved venom glands. In monitors these glands sit in the lower jaw and secrete proteins that widen blood vessels, lower blood pressure, and stop blood clotting. When a Komodo dragon bites a large animal such as a deer or buffalo, the combination of deep slashing wounds and this anticoagulant venom causes heavy blood loss and shock, so the prey often collapses within a day. This has replaced the older story that the dragon relied on lethal mouth bacteria, for which the evidence turned out to be weak.
What is the difference between a monitor lizard and a Komodo dragon?
A Komodo dragon is a monitor lizard. It is one species, Varanus komodoensis, within the monitor family Varanidae, which contains around 80 species in the single genus Varanus. What sets the Komodo apart is mainly its size, since it is the largest living lizard, and its restricted range on a few islands in eastern Indonesia. In body plan, senses, hunting style, and the forked tongue it is a typical monitor, just scaled up. Other well-known monitors include the widespread Asian water monitor, the African Nile monitor, the Australian goannas, and the arboreal emerald tree monitor of New Guinea.
Can monitor lizards really reproduce without mating?
Some can, through a process called parthenogenesis in which an unfertilised egg develops into a viable embryo. It has been documented in captive Komodo dragons and a few other monitor species, sometimes in females that had been kept alone for years and sometimes even in females with access to a male. In the Komodo dragon's ZW sex system, a parthenogenetic clutch produces only sons, because of how the chromosomes are packaged in the egg. This gives an isolated female a theoretical route to start a new population and then breed with her own male offspring, which may help explain how monitors have colonised so many remote islands.