Monitor lizards include 60 living species of large lizards in the genus Varanus, family Varanidae. Monitors inhabit tropical and subtropical regions of Africa, Asia, and Australia. Monitors are among the most advanced of the lizards, in terms of achieving an active, predaceous lifestyle.
The largest species of monitor, and the world’s largest lizard, is the extremely impressive Komodo dragon (Varanus komodoensis ), which can achieve a length of 9.8 ft (3 m) and a weight of 300 lb (135 kg). The Komodo dragon is an endangered species, occurring only on a few, small Indonesian islands. An even larger monitor lizard, known as Megalania, was about twice as long as the Komodo dragon, and is known from fossils collected in Australia. The smallest species is the short-tailed monitor (Varanus brevicauda ) of western Australia, only 7.9 in (20 cm) long and weighing 0.7 oz (20 g).
Monitor lizards have a massive body and powerful legs. Most species have strong claws on their feet, and all but the largest monitors can climb well. The tail is long and powerful, usually about twice as long as the body, and can be flailed as a potent weapon. Monitors grow throughout their life, so the oldest individuals in a population are also the largest ones.
Monitors have a long, specialized tongue with a bifurcated tip that is highly sensitive to smell and taste. The tongue is extended to pick up scent chemicals, and is then retracted into the mouth where the scents are analyzed using an organ on the roof of the mouth.
Monitor lizards grow replacement teeth in the gaps between their mature teeth. They have at least 29 vertebrae above their hips. Nine of these are neck vertebrae, supporting the unusually long neck of these lizards. Their powerful jaws are hinged in the middle, allowing them to swallow large prey. The head of monitors is tapered, and there are distinct ear holes.
Monitors are active predators, hunting during the day. They stalk a wide range of animals and eat carrion and eggs as well. Monitors ingest their prey whole if it is small enough, but they can also dismember large prey items so they can be swallowed.
Monitors, like all lizards, are poikilothermic or “cold-blooded.” They are most energetic after they have been heated by the morning sun, since their muscles work much more efficiently and easily when they are warm. Monitors can run quickly to chase down prey. When doing so they lift their body and tail clear off the ground.
Monitors also swim well, and may seek water as a refuge when threatened. They can walk underwater, and can use their tongue to smell underwater.
When threatened, monitors can be formidably aggressive animals. They can inflict painful bites and scratches, and the largest species are capable of killing a human. However, monitors can be readily tamed in captivity.
The Nile monitor (Varanus niloticus ) is a widespread, rather aquatic species found in Africa. The Bengal monitor (V. bengalensis ) is widely distributed in southern Asia, occurring from Iran and Afghanistan, to Java in Indonesia. This species is a relatively
terrestrial monitor, and in some parts of its range it may become dormant during periods of extended drought. The two-banded monitor (V. salvator ) is a large, highly aquatic species that can attain a length of almost 10 ft (3m), and ranges from Bengal and Ceylon through southeast Asia. The giant monitor (V. giganteus ) can reach a length of 7.9 ft (2.4 m), and is one of seventeen species of monitors that occur in Australia.
The most impressive species of monitor, and the largest living lizard, is the Komodo dragon (V. komodoensis ) of Komodo Island and a few other tiny islands east of Java in Indonesia. This powerful predator is capable of killing large animals, such as pigs, goats, and deer. The Komodo dragon has rarely been known to kill inattentive or unlucky humans. The Komodo dragon feeds on carrion when it is available. It is an endangered species, with a population of only a few thousand individuals. The Komodo dragon has been protected from hunting by the government of Indonesia. However, this animal is still significantly threatened by loss of its habitat, and by diminishment of its natural foods of deer and pigs by human hunters.
Monitors are hunted in many places for their meat, skin, and eggs. A preparation of the fat of monitors is used in traditional Chinese medicine, and monitors may be hunted for this trade anywhere that they occur. Monitors are also threatened by losses of their natural habitat in many places. Some populations of the Indian monitor (V. indicus ) have been decimated by poisoning when they attempt to eat the cane toad (Bufo marinus ). The cane toad excretes a highly toxic chemical from large glands on the sides of its neck, which poisons native predators that attempt to eat the toad. The cane toad has been widely introduced in the tropics in misguided attempts to achieve a measure of biological control over some types of insects that are agricultural pests. Some species of predatory birds have also been decimated by the cane toad, and so likely have other species of monitors in addition to the Indian monitor.
Carroll, R.L. Vertebrate Paleontology and Evolution. New York: Freeman, 1988.
King, Dennis, and Brian Green. Monitors: The Biology of Varanid Lizards. 2nd ed. Melbourne, FL: Krieger, 1999.
Murphy, James B., et al, eds. Komodo Dragons: Biology and Conservation. Washington, DC: Smithsonian Institution Press, 2002.
Zug, George R., Laurie J. Vitt, and Janalee P. Caldwell. Herpetology: An Introductory Biology of Amphibians and Reptiles. 2nd ed. New York: Academic Press, 2001.
"Monitor Lizards." The Gale Encyclopedia of Science. . Encyclopedia.com. (February 20, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/monitor-lizards-0
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