Monitor (lizard)

I

Introduction

Monitor (lizard), common name for a group of lizards that includes the largest living lizard, the Komodo dragon. Monitors live in tropical and desert areas throughout Africa; in Asia from Arabia through southern China and Malaysia; and in the East Indies and Australia, where they are often called goannas. They are among the most adaptable, active, and intelligent of all reptiles.

II

Characteristics of Monitors

Species of monitor lizards range from 20 cm to more than 3 m (8 in to 10 ft) long. They are characterized by a long, forked, snakelike tongue that can detect chemical traces in their environment. They are sleek, fast runners with tapered heads; long necks; strong legs; and long, powerful tails. Although monitors are mainly terrestrial, some species also climb trees and are good swimmers. The large water monitor of East India can swim far from land.

Monitors are skilled and powerful predators, armed with very sharp teeth and claws. Their flexibly constructed skull and jaws allow them to engulf and swallow some prey whole. Monitors feed on insects, birds, reptiles and their eggs, small mammals, and carrion. The Komodo dragon can take on much larger animals such as pigs and goats, and even deer and water buffalo. Recent studies have shown that monitors have venom-secreting glands in their mouths and can produce low levels of venom to help subdue prey.

Like other lizards, monitors are cold-blooded and have bodies that twist from side to side as they run, restricting their ability to breathe and run at the same time. However, monitors are able to maintain higher levels of activity and endurance than other lizards and can run after prey for long distances.

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Monitors overcome their breathing limitations with a large throat pouch that pumps extra air into their large lungs. Their three-chambered hearts can also separate oxygen-rich blood from oxygen-depleted blood during strenuous activity, allowing their hearts to function more like the four-chambered hearts found in crocodiles, birds, and mammals. Monitors are able to combine the reduced food needs of a cold-blooded animal with energy and stamina more similar to those possible in warm-blooded animals. Monitors have become highly successful predators in the harsh conditions such as those found in some regions of Australia where a warm-blooded mammal would not find adequate food.

III

Reproduction and Intelligence

Male monitors often engage in combat for mates during breeding season, sometimes rearing up on their back legs and tails to wrestle belly to belly to determine the stronger contender. After mating, females lay from 3 to 60 soft-shelled eggs, depending on the species. Eggs may be laid in holes dug by the females, in rodent burrows, or in hollows in trees. Some species lay their eggs in termite nests. Australian monitors sometimes use the heaps of rotting vegetation built as nests by brush turkeys (also called megapodes).

Female Komodo dragons kept in zoos have sometimes laid fertile eggs without mating, a form of reproduction called parthenogenesis. Unusual for parthenogenesis, the females laid eggs that hatched into males. Other types of lizards that reproduce by parthenogenesis such as whiptails and some geckos only produce females that are clones of their mothers.

Monitors observed in the wild and in captivity display complex behavior and levels of intelligence that are unusual for reptiles (see Animal Behavior). Females sometimes guard their nests against predators, although they do not protect their young after they hatch. Some species such as lace monitors in Australia break open mud termite mounds to lay their eggs. The termites quickly reseal the mounds, protecting the eggs; the female monitors are thought to return months later to release the young after they have hatched.

Pairs of monitors may cooperate to raid crocodile nests, with one lizard distracting the female crocodile. Monitor lizards in Australia sometimes forage in garbage in populated areas, adapting to the presence of humans. Komodo dragons kept in captivity reportedly can distinguish between keepers and strangers, and sometimes display playlike behavior with nonfood objects.

IV

Evolution of Monitors

Monitors are among the oldest living lizards and are known from fossils dating from the early Cretaceous Period. They are related to the mosasaur, a marine lizard that lived from 136 million to 65 million years ago and ranged up to 10 m (33 ft) and longer. Giant land monitors lived in parts of Australia, New Guinea, and Indonesia until thousands of years ago. Fossils of a giant monitor from Australia suggest it was about twice the size of the Komodo dragon.

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