Monitor Lizard

Quick Take

• Monitor lizards are called the most intelligent reptiles alive, and their behavior in captivity reveals something about them that no one expects from a cold-blooded predator. See the top 5 facts →
• Unlike virtually every other reptile on the planet, monitor lizards share a key biological trait with mammals, one that changes everything about how they hunt. Explore their metabolism →
• The Nile monitor's egg-laying strategy is so clever it almost looks engineered, turning a deadly threat into the perfect nursery. Discover their egg-laying strategy →
• Their scientific name traces back to an Arabic word, and the explanation for their everyday English name is stranger than you'd guess. Trace the name origins →

These reptiles are mainly native to Africa, Asia, and Oceania, but one species can also be found in parts of the southern United States as an invasive species. Monitor lizards are known for their large claws, powerful tails, long necks, and well-developed legs. They can move quickly and powerfully in pursuit of prey. Most monitor lizards are terrestrial and largely carnivorous. Depending on the size of the lizard, they will eat anything from insects and birds to small mammals.

5 Incredible Monitor Lizard Facts

• Monitor lizards are considered to be the most intelligent of all lizards and some of the most intelligent reptiles in general.
• Monitors do not blink, and they have very good vision. Scientists have observed monitors watching planes in the sky.
• Unlike many other reptiles, many monitor lizards have high metabolisms and burn energy like mammals.
• As pets, monitors have been observed seeking human attention and wanting to play.
• Monitors are alert and skilled predators. As dangerous as they are to smaller animals, their bites are not fatal to humans.

Evolution

Experts believe that monitor lizards’ ancestors originated in Asia, based on the discovery of Archaeovaranus, which lived there during the early Eocene approximately 52 to 56 million years ago.

It is, however, worth noting that the Saniwa, which was also a varanid and another close cousin, lived in Wyoming 48 million years ago, during the same era. It shared the same traits common to monitors in the present day, such as a tail which was nearly double its body length and a finely tapered snout.

However, the story of the genus dates as far back as 145 million years ago, or possibly more recently, during the Cretaceous era, when monitor lizards’ evolutionary branch separated from earless monitors, which happen to be their closest cousins.

Classification and Scientific Name

The monitor lizard belongs to the kingdom Animalia, the phylum Chordata, the class Reptilia, the order Squamata, the family Varanidae, and the genus Varanus.

The scientific name Varanus comes from an Arabic word that means dragon or lizard beast. The standard English name for the monitor lizard may come from the reptile’s habit of standing on its hind legs to look around.

Types

The genus Varanus consists of a wide range of species. The earless monitor is a rare species of monitor that lives in Borneo. It is the only species in the monitor family Lanthanotidae. It looks much like other monitors with an elongated body, but has smaller limbs than typical monitors.

Examples of other species include:

• Ackies dwarf monitor (Varanus acanthurus): These varanids live in western Australia and are notable for their reddish coloration with cream-colored dots. They are mostly insectivorous, although they may nibble on cat or dog food when in captivity. They are capable of growing up to 27 inches long.
• Argus monitor (Varanus panoptes): Capable of growing to 5 feet, they are terrestrial and love to burrow. Although kept as pets, they are not above using their sharp claws to scratch while they attempt to squirm free. They are native to Australia and New Guinea.
• Asian water monitor (Varanus salvator): Second only to the Komodo in size, these varanids are semi-aquatic and are powerful swimmers. They live among mangroves, swamps, or wetlands, and are capable of growing to 6½ feet.
• Emerald tree monitor (Varanus prasinus): These arboreal monitors are especially popular among pet owners because of their bright colors. However, they are not above relying on their teeth when they feel frightened. The reptiles are capable of growing to approximately 3 feet (91 centimeters).
• Crocodile monitor (Varanus salvadorii): Dark green with yellow spots, these monitors are rather reclusive and are noted for their especially long tails. They live in the rainforests and mangrove swamps of New Guinea and West Papua.
• Gray’s monitor (Varanus olivaceus): Terracotta colored with wide darker bands about their bodies, and a generous sprinkling of yellow spots, these varanids are capable of growing to 6 feet. Arboreal and reclusive, they prefer to eat the fruit of the pandan tree, although they are also partial to crabs, birds, and spiders.
• Nile monitor (Varanus niloticus): Capable of growing to 8 feet, they are the largest varanids in Africa. They live along the Nile and are capable climbers and swift runners. Florida is also home to a feral population of escaped and abandoned monitors that were once pets.
• Peacock monitor (Varanus auffenbergi): Shy and reclusive, these small varanids are colored with thin paler bands and thicker dark bands flecked with turquoise dots. They are capable of growing to 23.6 inches (60 cm) and are excellent climbers.

Appearance

Types of monitor lizards vary widely in their appearance and size. The smallest species of monitor grows to only eight inches in length. However, several species are quite large and can grow to lengths between seven and ten feet. No matter what size they are, monitor lizards tend to have thick, solid bodies, long necks, well-developed legs, and long tails. Colors vary widely, with mottled shades of brown, black, gray, green, yellow, and blue being most common.

Behavior

Monitor lizards are generally solitary reptiles, but in some areas with limited water resources, monitors will gather in groups of up to 25. Otherwise, monitor lizards typically lead solitary lives until they come together to breed. Monitors are mostly terrestrial and move about most comfortably on land — especially the larger species. There are a few arboreal and aquatic monitor species as well.

Most monitors are completely carnivorous. Monitors generally avoid humans but can be dangerous if provoked. They are faster than they appear and have strong jaws and tails. Monitors can be very aggressive in protecting their territory or food. Many monitors are active hunters that stalk and chase down their prey. Other types of monitors are more interested in scavenging.

Habitat

These lizards are readily found in Africa and Asia as well as islands in Oceania. They can now be found in parts of the Americas, but they are considered an invasive species due to their non-native status. Monitor lizards in the Americas originated from the exotic pet trade. Monitor lizards adapt to many environments. They can be found in jungles and rainforests, but also in aquatic areas and hot and dry areas.

Water monitors like the Asian water monitor live in coastal areas with semiaquatic habitats such as swamps and mangroves. They also adapt to places with canal systems, such as those found in Sri Lanka.

Arboreal monitors like the Timor tree monitor spend most of their time in trees. They have long tails and sharp claws that are well-suited to climbing through branches and trees.

Diet

These lizards’ diet depends on where the reptile lives as well as its size and type. Tree monitors tend to eat insects and birds. Some species of tree monitor eat mostly fruit. Water monitors tend to eat anything they can find in the water. This includes fish, water birds, insects, eggs, and water mammals or reptiles.

The lizards that largely live on land, such as the Komodo dragon, may eat a wide variety of live prey as well as carrion. The diet of the Komodo dragon consists of about 50% deer native to its island. Komodo dragons are also known to eat pigs and cattle if they can get them. These lizards are opportunistic predators.

What does a monitor lizard eat?

A wide variety of food including fruit, insects, birds, fish, mammals, and reptiles.

Predators and Threats

For the largest species of these lizards, the main predator is humans. Some types of monitors are hunted by humans for their skin in the leather trade. The only exception to this is the Komodo dragon, which has rough and rugged skin not suitable for leather. For all types of monitors, the young lizards are preyed on by other lizards, snakes, large birds, fish, and big cats. Smaller lizards are also preyed on by these animals both when they are young and when they are adult-sized.

The conservation status of these lizards depends on the species. For example, the Komodo dragon is considered Endangered. On the other hand, the Asian water monitor has a conservation status of Least Concern. The Panay monitor lizard is considered Endangered. These lizards mainly decline in numbers due to habitat loss, decreasing water resources, and hunting by humans.

What eats monitor lizards?

Large adult monitors have few predators. Smaller monitors and baby lizards may be eaten by birds, other reptiles and lizards, wild cats, and humans.

Pets

Five different species of monitor lizards can be owned as pets:

• The Ackie Monitor
• Blackthroat Monitors
• Whitethroat Monitors
• Water Monitors
• Savannah Monitors

The Ackie is one of the best monitor lizard species to own as a pet. Ackies are a relatively small species by monitor standards, and that alone has its advantages. Ackies will usually measure about 2–3 ft as adults and come in shades of yellow and red.

There is no personality difference between the two color variations.

Although these pets are exotic and rare, they are becoming more popular with time.

Reproduction, Babies, and Lifespan

Males will often fight to mate with females. The Komodo dragon usually mates between May and August, and eggs are then laid around September. Female lizards are often antagonistic towards the male, which means the male has to fully restrain her during mating or she may get injured.

These lizards typically lay their eggs in a concealed nest in the ground. They might also lay them in hollow tree stumps. The exact method differs depending on the species. For example, the Komodo dragon might lay its eggs in abandoned ground bird nests, or they might lay them in holes in the ground or in hills. Some monitor species can lay as many as 30 eggs. Komodo dragons lay about 20. Nile monitors have a clever way of laying eggs where they dig their eggs into termite mounds. The termites will naturally cover the hole, thus providing a secure and warm place for the eggs. When the eggs hatch, the baby lizards often eat the termites before leaving the mound.

Because young lizards and unhatched eggs attract many predators, few young lizards survive from the original clutch. Young land monitors may spend much of their early life in trees to avoid predators.

Most of these lizards can live at least 20 years. Komodo dragons may take as long as nine years to reach full maturity. They can live up to 30 years. Larger monitors tend to live longer than smaller species.

Population

There are roughly 80 species of these lizards in the world. The smallest species live in trees or water, and population data is unknown for most of them. The rare earless monitor lizard is an example of a species whose population data is unknown. It’s considered vulnerable due to its small range. However, the earless monitor is nocturnal and is not often seen by locals. Population data for many of the smallest monitors is unknown. Large species like the Komodo dragon are protected in some areas to keep their numbers from dwindling. Loss of food sources and habitat are the main reasons for declining monitor populations. If food and habitat are plentiful, then monitor populations in these areas tend to increase.

In Captivity

There are plenty of zoos across the country where you can see Komodo dragons and other monitor lizards, including the following:

• Bronx Zoo
• Louisville Zoo
• Fort Worth Zoo
• Toledo Zoo and Aquarium
• Smithsonian National Zoo
• Pittsburgh Zoo