Herpetologists and hobbyists alike have long known that there is something “different” about the family of lizards known collectively as “monitors” (or, in Australia, “goannas”). One cannot observe a monitor for long without getting a sense of the animal’s intelligence – they scrutinize the world differently than do other reptiles, and their reactions to new situations are surprisingly quick and complex. Recent studies have confirmed that monitors are unique, and, in many ways, the most intelligent and advanced of the lizards.
In years past pet keepers were limited to a very small number of monitor species from which to choose. An explosion of interest in the group has radically changed that situation, and today animals rarely seen even in zoos are being commercially bred in huge numbers. Happily, there are monitors even for those without much room – and the very smallest types still exhibit true monitor behavior, intelligence and “attitude”.
Today I’ll present an overview of the group as a whole.
Sixty eight species of monitor lizards, all classified within the family Varanidae and the genus Varanus, range across Asia, Africa and Australia. Although generally associated with warm climates, one species, the desert monitor (V. griseus) may be found as far north as Kazakhstan – at roughly the same latitude as southern Vermont.
In Australia, where over 2/3 of the world’s species are found, monitor lizards have evolved to fill a wide range of ecological roles held elsewhere by other lizards and by large, predatory birds and mammals. Huge Australian species such as 8 foot long parentie monitor, V. giganteus, and the lace monitor, V. varius, are the dominant predators in their habitats, as are Komodo dragons, V. komodoensis, on the islands of Komodo, Padav, Rinca and Flores.
At 8 inches long, the short-tailed monitor, or pygmy goanna,
V. brevicauda, is the smallest member of the group. The infamous Komodo dragon is the world’s largest lizard. It occasionally tops 10 feet in length, but is dwarfed by extinct monitor species which may have measured over 20 feet long.
General Physical Characteristics
All monitors share a similar body plan – a long neck and relatively small head, sturdy body and limbs and a long, powerful tail, and most measure in the range of 2 to 5 feet in length.
The tongue is deeply forked and is flicked out repeatedly. As with snakes, the tongue carries airborne chemical cues to the Jacobson’s organ, thus conveying information about the environment and other animals.
Male monitor lizards compete for females by grappling, often rising onto their hind legs during tests of strength. All species lay eggs.
All monitors are alert, effective predators, with the various species taking an incredibly huge array of prey – termites and other insects, snails, spiders, crayfish and other invertebrates, birds and their eggs, frogs, turtles, snakes, hatchling crocodiles and other reptiles and amphibians, and rodents, weasels, tree kangaroos and other mammals to the size of adult deer. Large monitors living near developed areas also prey upon domestic dogs, cats, chickens, ducks, pigs and goats, and consume carrion and offal as well. The Philippine monitor lizard, or bataans (V. olivaceus), is unique in feeding upon fruit at certain times of the year.
On rare occasions, the Komodo dragons have attacked and killed people. In fact, current restrictions on the long-held tradition of feeding goats to these huge lizards (a tourist attraction with possible links to local religious beliefs) is possibly at the root of the recent rise in attacks on people and livestock.
Monitor lizards have evolved to occupy a wide variety of habitats – there are arboreal specialists, such as the green tree monitor, V. prasinus, aquatic species such as the mangrove monitor, V. indicus, and grassland dwellers such as the savanna monitor, V. exanthematicus. There are also many generalists – Gould’s monitor, V. gouldii, a large lizard that occupies nearly all of Australia, is equally at home in grasslands, open forests, river valleys, cliff-sides, semi-deserts and nearly all other habitats within its huge range.
The Nile monitor, V. niloticus, is native to sub-Saharan Africa but is now thriving in southern Florida, where released pets have established breeding populations. Approaching 7 feet in length, this aggressive predator is severely impacting the local ecology by out-competing and preying upon a wide variety of native species.
Wide-Ranging and Isolated Species
The size of the ranges of the different species varies greatly in extent. For example, the blue tree monitor, V. macraei is limited in distribution to Batantan Island off Papua New Guinea while the 9 foot long Asian water monitor, V. salvator, is found from India through Bangladesh, Myanmar, southern China, Vietnam, Laos, Cambodia, Thailand, the Philippines, Malaysia and Indonesia.