Home | Lizards | Monitors (page 4)

Category Archives: Monitors

Feed Subscription

The Monitor Lizards (Family Varanidae) – Family Overview and Species Accounts; – Some Interesting Monitors and Their Care – Part II

Click: The Monitor Lizards (Family Varanidae) – Family Overview and Species Accounts; – Some Interesting Monitors and Their Care – Part I, to read the first part of this article.

Merten’s Water Monitor, Varanus mertensi
The popularity of this most aquatic of the world’s monitor lizards is soaring, and with good reason – it is active, hardy, of manageable size (averages 3 feet in length, occasionally to 4) and very responsive to its surroundings.

Native to northern and western Australia (Northern Territory, Queensland, South Australia and off-shore islands), Merten’s water monitors frequent creeks, rivers, canals, ponds and reservoirs. They bask on the shore or low overhanging branches, and rarely stray far from the water’s edge. Although common at present, this situation may change – water monitors readily consume introduced marine toads, and many are killed by the amphibians’ toxins.

Unique Adaptations
Merten’s water monitors, named after eminent Russian-born herpetologist Robert Mertens (1894-1975) of Frankfurt Germany’s Seckenberg Museum,
are deep brown to black in color, with dark-edged cream or yellow spots. The tail is laterally compressed and the nostrils, which can be sealed during dives, are located high on the snout.

Water monitors require a spacious cage (at least 4’x4’x 3′), a UVB source and a large (preferably drainable) pool. They must be able to bask at 90-95 F and to dry off completely upon leaving the water.

The natural diet consists largely of crabs, crayfish and fish, but frogs, tadpoles, turtle and other reptile eggs, shrimp, snails, nestling birds, planigales and other small mammals, large insects and carrion are also taken. They have been observed foraging in dumps, with discarded sausages apparently being a favored food! Water monitors use their tails to concentrate small fishes when hunting in the shallows.

Captives should not be fed a rodent-based diet, but rather one weighed heavily in favor of whole, fresh-water fish, whole, unshelled prawn, crayfish, roaches, crickets, earthworms, and canned insects/monitor diets . Pink mice are preferable to furred rodents, and should be offered only once each week or two. This species seems to have a quite high metabolism, and does best when fed small meals every 3-4 days.

Blue Tree or Blue-Spotted Tree Monitor, Varanus macraei
This strikingly colored lizard only appeared in zoos in the late 1990’s, and was not named as a distinct species until 2001. Never-the-less, beauty and small size (to 40 inches) has propelled it to the top of the “most desired species lists” of many monitor fanciers.

These slender monitors are jet black in color and variably patterned with blue spots and bands. The prehensile tail comprises 2/3 of the animal’s length.

Blue tree monitors are found only on Batanta Island, Indonesia (off Irian Jaya, New Guinea). Almost entirely arboreal, they inhabit trees and shrubs within rainforests and estuarine swamps. Their wild status is unknown, but the species is listed on CITES Appendix II due to the extremely limited natural range.

Blue tree monitors require a spacious cage with provisions for climbing, UVB light and a basking spot of 90-95 F. The several pairs I’ve kept have co-existed without incident, and frequently rested in close contact with one another. Quite shy and quick to take flight, they do best in a well-planted cage provided with numerous hideaways and located in a quiet area. This is a species to observe, not handle.

The natural diet is likely composed largely of invertebrates such as tree crickets, katydids, snails, caterpillars and spiders, along with occasional treefrogs, lizards, small birds and their eggs, nestling mice and other small mammals.

As with other small monitors, I prefer not to feed too many rodents to this species. Mine thrive upon waxworms, crickets, crayfish, super mealworms, roaches, chicks and quail eggs. Pink mice are given every two weeks or so. Canned insects and monitor diets are often accepted and should be included as a source of dietary variety.

Females lay 2-6 eggs at a time, and may produce up to 6 clutches per year (this may be a function of food availability in captivity). The young average 5 inches in length and hatch after an incubation period of 200-225 days. Captive females will bury eggs within moist substrates on the ground but seem to prefer arboreal nest boxes.

An Escapee that Made the Most of His “Vacation”
The long, slender tail of this monitor is prehensile and adds to its climbing abilities and agility. Alert and quick moving, blue tree monitors can catch even the swiftest of prey in their treetop homes. One individual that escaped from an exhibit at the Bronx Zoo was in the process of swallowing a house sparrow when I recovered him from atop a vine-covered fence!

In depth information about the family Varanidae and its individual species is posted at:

The Monitor Lizards (Family Varanidae) – Family Overview and Species Accounts; – Some Interesting Monitors and Their Care – Part I

Note: Please see The Monitor Lizards (Family Varanidae) – Family Overview and Species Accounts
for information on the natural history of monitor lizards.

Storr’s Pygmy Monitor, Varanus storri
This is the smallest monitor regularly available in the pet trade.   Also nearly the tiniest member of the family Varanidae, this species has much to recommend it as a “first monitor”.  Reaching just 14 inches in length, Storr’s monitor is one of the only monitors that can be housed within an aquarium-sized (see below) enclosure when adult.

A Popular Pet
Storr’s monitors are immensely popular because they combine all of the typical monitor traits in a small package – indeed, they act exactly like tiny versions of their larger cousins.  What they lack in size is more than made up for in attitude, and they are quite bold as captives.  In fact, keeping more than one in a cage, even a mated pair, is difficult due to their aggressive natures.

This plucky, brown little lizard hails from northeastern Australia.  A subspecies, reddish in color but otherwise similar (V. s. ocreatus) ranges further west.  In the eastern part of its range, Storr’s monitor is threatened by the introduced marine toad, which both preys upon and competes with it.  Populations have all but disappeared from some areas already.

Storr’s monitors are quite active and should be housed in a tightly secured aquarium of at least 30 – 55 gallons (“long models are best”).  They require a source of UVB light  and a basking site that reaches 90 F or so.

Although they are often fed pink mice and small rodents in captivity, the natural diet of this species consists largely of insects.  The bulk of their food in captivity should be roaches, crickets, waxworms, super mealworms, mealworm beetles and wild-caught insects.  Canned insects, such as silkworms and grasshoppers, offer an excellent source of dietary variety.  Many individuals also accept canned monitor diets.

Captive breeding is possible, but pairs must be watched closely for aggression.  The 2-5 eggs hatch in 100 days at 82F.

Crocodile Monitor, Varanus salvadorii

Crocodile Monitor

At the other end of the spectrum from Storr’s monitor is southern New Guinea’s massive crocodile monitor – it approaches 9 feet in length (nearly as long as, but much lighter than, the Komodo dragon), and there are unverified reports of much longer individuals.  The prehensile tail is twice as long as the snout-vent length.  Crocodile monitors are black in color and boldly marked with yellow spots, and sport long claws and a bulbous snout (particularly so in mature males).

Highly arboreal despite their size, crocodile monitors dwell in the canopies of lowland forests in the vicinity of rivers and mangrove swamps.  Little is known of their status in the wild, but there is concern given the limited range; the species is listed on CITES Appendix II.

In contrast to the other lizards described here, crocodile monitors are suitable only for very advanced hobbyists, or as zoo animals.  Those I worked with learned to tolerate my presence, but remained wary and agressive even after many years.

In contrast to the usual structure of monitor teeth, those of the crocodile monitor are serrated and over-lapping – possibly an adaptation to holding onto and carrying large prey high above the forest floor.  These animals are formidable predators, and capable of inflicting potentially life-threatening wounds.

Captive-bred animals are available and adjust moderately well (please see above) if given huge, high-topped enclosures that accommodate their arboreal lifestyles – forcing crocodile monitors to remain on the ground is, in my experience, quite stressful to them.

A cage or room of 10’ x 10’ by 10’ would be required for a single adult.  UVB light and a basking spot of 90-95 F are needed.  A humid environment is preferred, but they must be able to dry off as well.

Wild crocodile monitors feed upon cuscus, young tree kangaroos, naked-tailed rats, bandicoots, bats and other mammals, birds and their eggs, frogs, snakes, lizards and carrion.  Captives do well on rats, mice, chicks, crayfish, large roaches and hard-boiled eggs.

Image referenced from Wikipedia.

Check back on Friday for more Monitor Species Profiles.


The Monitor Lizards (Family Varanidae) – Family Overview and Species Accounts, Part II

Click here: The Monitor Lizards (Family Varanidae) – Family Overview and Species Accounts, Part I, to read the first part of this article.

Monitor Intelligence
Monitor lizards exhibit an extraordinary degree of intelligence and retain what they have learned for long periods.  Gould’s monitors (V. gouldii) have been observed taking indirect routes when chasing rabbits.   Rather than running directly after the rabbit, they veer off in a direction that takes them away from the animal, but leaves the lizard in a position to intercept the rabbit at the mouth of its burrow!

Parentie monitors (V. giganteus) seeking animals hidden within burrows do not dig away at the burrow entrance (as would a foolish dog!) but rather sniff the ground several feet away.  Once they locate the underground position of their prey, they dig directly down to reach it, keeping an eye on the entrance as well.

An Experience with a Bright Monitor
I have often had the good fortune to observe monitor intelligence in action.  While working at the Bronx Zoo, I once looked up from a phone conversation to see a 6 foot long crocodile monitor (V. salvadorii) go shooting by, followed by an equally fast-moving coworker of mine.  We cornered the animal behind a large cage, where he remained as long as we were in view.  As soon as we moved off to either side, he cautiously peered around the cage and looked both right and left, to fix our new positions.  The lizard most definitely did not want either of us sneaking up on him, and made sure he saw both of us at all times.

Mangrove MonitorThe stout fellow was recaptured – with more wear and tear to us than he!  Monitors are incredibly strong – a 7 foot long water monitor (V. salvator) I worked with was able to move along with myself and 2 other strong men trying to pin him down – this despite being rather seriously ill.

Monitor Venom
In 2005, Dr. Bryan Frye and researchers at Australia’s Melbourne University discovered that several species of monitor lizards, including the Komodo dragon, V. komodoensis and the lace monitor V. varius, produce venoms of varying strengths.  Lace monitor venom was subsequently shown to cause the lizard’s prey to rapidly loss consciousness by affecting the blood’s pressure and clotting ability.

Until this discovery, bacteria in the mouth of the Komodo dragon were thought to be responsible for the quick onset of death seen in deer, goats and other large animals bitten by these lizards.  While such bacteria no doubt add to the trauma associated with a bite, it now seems certain that venom delivers the knockout blow.  A combination of venom and bacterial infection is also the likely source of the strong reaction often associated with bites inflicted by monitor lizards upon people.

The Bearded Dragon, Pagona vitticeps, a popular pet species not related to the monitor lizards, was also shown to produce mild venom – other members of the lizard family Agamidae are being studied.

An interesting article on Florida’s introduced Nile monitor lizards is posted at:

The Monitor Lizards (Family Varanidae) – Family Overview and Species Accounts, Part I

Larry the Nile Monitor from Forgotten Friend Reptile SanctuaryHerpetologists 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.

Species Diversity
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
Water Monitor Eating a FishAll 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.


Scroll To Top