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Venomous lizards

There are some 5000 different species of lizards worldwide and, until a few years ago, only two (the Gila Monster and the Mexican Beaded Lizard) were thought to be venomous. Scientists also believed that those two species had evolved venom production independently from snakes.

Indeed, until recently, nasty swelling and excessive bleeding resulting from another lizard's bite was thought to be due to infection from the bacteria in the reptile's mouth.

But a team of searchers from the University of Melbourne, Australia have revolutionized herpetology by showing that venomous lizards are actually much more widespread than thought.

These scientists, under the leadership of Bryan Fry, have demonstrated that both monitor lizards (commonly kept as pets) and iguanas also produce venom. Nine types of lizard toxins are shared with snakes, but some toxins are new and yet to be investigated for medical research.

Furthermore, it is now thought that venom production had, actually, a single early origin for lizards and snake and that the common ancestor to all venomous species lived about 200 million years ago. The evolution of venom would have, thus, coincided with the rapid spread of small mammals.

To date, the toxin-producing oral glands have been identified in species of the anguimorph and iguanian lineages. It is believed that as many as 100 species of living lizards actually use venom.


1) Helodermatidae

Until a few years ago, the only two known species of venomous lizards belonged to the family Helodermatidae and the genus Heloderma. Those two species are the Gila Monster and the Mexican Beaded Lizard.

These lizards are carnivores. Unlike snakes, they don't inject their venom in the wound; venom merely flows into the wound. Luckily, attacks on people are uncommon for these reptiles are known to bite with tenacity. Their venom causes paralysis, difficulty in breathing and sometimes convulsions, but is normally not fatal to humans.

Gila Monsters and Beaded Lizards Both become sexually mature at around 2-3 years of age. They have been known to live for up to 30 years.

They mate in April (Gila Monster) or May/June (Beaded Lizard) and eggs are deposited 2 months later. The eggs take another 5 and 1/2 to 7 months to hatch.

- Gila Monster (Heloderma suspectum)

The Gila Monster is found in southern Arizona, New Mexico and Mexico, in the Mojave, Sonoran and Chihuahuan deserts. As a adaption to its hot environment, it lives in burrows.

Gila Monsters can grow to a full length of 55 cm (22 inches). They have a stout body with a broad head and a stumpy tail and are mostly black with bright patches which vary in color from yellow to orange or salmon. Their skin is tuberculated.

The Gila Monster is listed as an endangered species both in the US and in Mexico. According to Arizona state law, is it illegal to harass them, hunt, shoot, wound, kill, trap, capture or collect them.

- Mexican Beaded Lizard, or Beaded Lizard (Heloderma horridum)

Mexican Beaded Lizards occur in central and western Mexico. They live in thorn scrub land as well as in tropical and pine forests.

They look quite similar to the Gila Monster, but are bigger (they can measure more than 1 meter or 40 inches, females being bigger than males with a broader head) with a brown body and bright yellow to whitish spots.

Like the Gila Monsters, they are considered endangered and are protected by law throughout their natural range.

2) Monitor Lizards (Varanidae)

There are about thirty species in this family of lizards (not to mention the subspecies). They all have an elongated snout, a long, smooth, retractile tongue, well-developed limbs and strong claws, a long tail and a dull grayish color. However, Monitor Lizards vary greatly in size, with some species hardly measuring 20 cm (2/3 foot) when the Komodo Dragon measures up to 3 meters (10 feet) and can weigh over 350 pounds.

The monitor lizards' geographical range is varied too, though they are restricted to the old world, and mostly found in warm areas of Australia (which hosts the greatest diversity: 17 species), Africa (3 species) and Asia. Most of the species are semi-aquatic.

Those carnivorous lizards sometimes use their tongues like snakes do, to detect a prey by picking up scent particles while flicking it in and out.

Though Monitor Lizards have recently been discovered to be venomous, there has never been a recorded death by one of them in the USA (where they are popular pets). There have also been very few injuries reported. Injuries can come from scratches by their well-developed claws or from bites inflicted by powerful jaws, and also, of course, from the venom sipping into the wound.

Incidents are uncommon because monitor lizards tend to avoid confrontation and rather try to escape. Bites are only inflicted when they are manipulated or maintained in an inappropriate manner.

Generally when a monitor is cornered, it will first thrash and strike out with its powerful tails to avoid confrontation while hissing and puffing. These tail strikes can result in superficial welts.

All species of monitor lizards are listed as Appendix II (threatened) animals by CITES and therefore require permits for import/export between countries. A number of species are even thought to be endangered.

Following are a quick overview of a few species of monitor lizards.

- Komodo Dragon (Varanus komodoensis)

Komodo Dragons inhabit Komodo Island and a few other tiny islands east of Java in Indonesia.

These reptiles can grown to be 3 meters long (10 feet) and weigh more than 300 pounds: they are the world's largest living lizards.

This powerful predator runs fast and feeds on virtually any kind of meat. It is capable of killing large animals, such as pigs, goats, deer and water buffalo, and will also eat other Komodo Dragons. These huge monitor lizards have also been reported (but rarely) killing inattentive or unlucky humans. The Komodo Dragon will feed on carrion when it is available.

Until very recently, it was believed that a Komodo Dragon's bite contained bacteria from the mouth that would cause a severe infection in the victim's blood. The Komodo Dragon would have to follow the wounded animal, sometimes for days, until the infection eventually killed the prey.

However, according to a study by Bryan Fry, an Australian biologist from Melbourne University and expert on animal venom, it seems that the Komodo Dragon actually kills its victims by injecting them with venom.

Fry's team found that the Komodo Dragon's venom rapidly decreases blood pressure, expedites blood loss, and sends a victim into shock (the body cannot deliver enough blood to the organs to keep functioning), rendering it too weak to fight. Some of the venom's compounds that reduce blood pressure are as toxic as those found in Australia's Inland Taipan, thought to be the most venomous land snake in the world.

When snakes have a single venom duct that leads to their fangs, the Komodo's biting apparatus is much more complex. Indeed, Komodo Dragons posses six venom glands on each side of the lower jaw (which, combined, can hold about 1.2 millimeters of venom) and multiple ducts located between their teeth.

The drawback is that they cannot deliver their venom as efficiently as snakes and have to rely on a bite-and-pull motion to ooze the venom into wounds during a sustained, frenzied attack. The efficiency of the attack comes from that combination of venom and teeth lacerations.

Yet, while the Komodo Dragon is very deadly to its normal prey species, the toxins it produces seem to have a relatively mild effect on humans.

Komodo Dragons are an endangered species (there are barely a few thousand of them) and are protected from hunting by the government of Indonesia. However, the world's biggest lizard is still threatened by the loss of its habitat, and by the competition with humans for its natural foods of deer and pigs.

- Lace Monitor (Varanus varius)

Analyzes of venom components from the Lace Monitor showed potent effects on blood pressure and clotting ability, bioactivities associated with a rapid loss of consciousness and extensive bleeding in prey.

- African Savannah Monitor, or Savannah Monitor, Rock Monitor, White-throated Monitor (Varanus exanthematicus)

This species occurs in southern and eastern Africa in savannas and arid areas.

It typically measures 70 to 110 cm (2 and 1/2 to 3 1/2 feet) sometimes a bit more.

The Savannah Monitor lives in a tunnel that it digs under rock overhangs, or in a disused animal burrow, a hole in a tree or a rock crack. It is usually solitary and feeds mainly on small animals, such as invertebrates, although it will kill any animal small enough to swallow (such as baby tortoises) and also eat carrion.

When threatened, it lashes its tail and holds on like a bulldog when it bites.

8 to 37 eggs are sometimes laid in a termite mound (like for the Nile Monitor), otherwise in soft moist soil. They hatch in 4 months in captivity, but may take as long as a year in the wild, depending on the season where they are laid.

With the Nile Monitor, the Savannah Monitor is the most commonly sold monitor in the pet trade.

- Nile Monitor or Water Monitor (Varanus niloticus)

The Nile Monitor is a widespread species found in Sub-Saharan Africa extending along the Nile valley to Egypt.

With an average length of 100-140 cm (around 4 feet) and a maximum of 200 cm (6 1/2 feet), it is the largest African lizard.

This rather aquatic monitor is common in major river valleys where it forages for food on the vegetation growing on the banks. It is an excellent swimmer and likes to bask on rocks and tree stumps near a river. It eats crabs and mussels, but also frogs, fish, birds and their eggs, as well as turtle, terrapin and crocodile eggs.

The Nile Monitor defends itself in the same ay as the Savannah Monitor, but will first try to escape the danger by diving under water.

The Nile Monitor lays its eggs inside termite mounds, so that they are incubated at the right temperature by the termites which also become the young monitors' first meal.

- Asian Water Monitor or Two-banded Monitor (Varanus salvator)

This species ranges from Bengal and Sri Lanka through southeast Asia.
It is a very large semi-aquatic species which can measure 4 to 9 feet (more than 2 meters).

- Bengal Monitor (Varanus bengalensis)

This monitor is widely distributed in southern Asia, where it can be found from Iran and Afghanistan, to Java in Indonesia.
It is a relatively terrestrial species which may become dormant during periods of extended drought, in parts of it range.

- Emerald tree monitor (Varanus prasinus)
This monitor is found in Papua-New Guinea. It is the second largest lizard, after the Komodo Dragon and has relatively large teeth.

- Giant Monitor (Varanus giganteus)
This Australian species can reach a length of 2.4 m(7.9 ft)

Other monitors species include the Desert Monitor (Varanus griseus) and Varanus Lavescens, both listed as endangered under CITES Appendix I and the Endangered Species Act, the moderately large species of Dumeril's Monitor (Varanus dumerilii), Black Rough-necked Monitor (Varanus rudicollis) and Mangrove Monitor (Varanus indicus), the rare Australian monitor species of Varanus prasinus and Varanus timorensis, the Peach Throat monitor (Varanus jobiensis), the Crocodile Monitor (Varanus salvadorii)...

IGUANIANS (some 1440 species)

3) Iguanas

Iguanas are large lizards belonging to the family Iguanidae. They occur mainly in America and, outside the western hemisphere, only in Madagascar, Fiji, and Tonga. They are known to live in a variety of habitats ranging from trees, to the water's edge, or arid areas.

Iguanas have a similar appearance: they have long limbs with five free toes ending in sharp claws, distinct eyelids, large external eardrums and often a throat pouch.

These diurnal reptiles are famous for their impressive courting and defensive displays; for instance they raise their bodies and bob their heads vigorously.

Though Iguanas, unlike most lizards, are vegetarians, recent studies (notably by Bryan Fry) have established that, at least some of their species can deliver small amounts of venom when they bite, thanks to venom-secreting glands situated in their mouths.

A few of the species of Iguanas are:

- Iguanian Lizard (Pogona barbata)

The Iguanian Lizard retains characteristics of the ancestral venom system, having venom-secreting glands on both the upper and lower jaws, whereas the advanced snakes and anguimorph lizards (including Monitor Lizards, Gila Monster and Beaded Lizard) have glands only on one of the jaws, lower or upper.

- Green Iguana (Iguana iguana)

This lizard is abundant throughout tropical America. It lives in trees often overhanging water.

Green Iguanas measure around 1.8 meters (6 feet) and have a row of leathery spines along the back from the neck to the long, powerful tail.
Males vary in color from grayish to orange and have dark bars on the sides of their body and broad black circles ringing their tail. Females are usually greenish.

The Green Iguana is killed for its flesh and its eggs are also eaten.

- Rhinoceros Iguana (Cyclura cornuta)

This species occurs in Haiti and Puerto Rico. It is terrestrial and derives its name from the three horns on its forehead.

- Marine Iguana (Amblyrhynchus cristatus)

One of the two species of Iguanas found on the Galapagos Islands. The Marine Iguana lives on beaches and comes in the sea to forage on seaweed, making it the only lizard in the world that regularly inhabits the sea.

Venomous Lizards Information
World's largest lizard is venomous too
Chris Kegelman
Lizards' poisonous secret is revealed
16 November 2005 by Emma Young
Komodo Dragons Kill With Venom, Researchers Find
Carolyn Barry, for National Geographic News, May 18, 2009
AFH Guidelines For The Keeping Of MONITOR LIZARDS
The Varanidae Family of Lizards - Monitor Lizards

Branch, Bill, Snakes and Other Reptiles of Southern Africa, Struik, Cape Town