Monitor Lizard
The Asian water monitor (Varanus salvator) is one of the world's largest lizards, generally ranked second in weight only to the Komodo dragon, and the most familiar monitor lizard of South and Southeast Asia. Adults usually measure 1.5 to 2 meters, and the record, from Sri Lanka, reached 3.21 meters. It lives along rivers, swamps, mangroves and rice paddies from eastern India and Sri Lanka to southern China and Indonesia, and thrives in cities such as Bangkok and Singapore. It eats almost anything it can catch or find, from fish, crabs and birds to eggs, carrion and human food waste. An excellent swimmer, it uses its flattened tail as a paddle and can stay underwater for long periods. Although listed as Least Concern, it is one of the most heavily traded reptiles in the world, hunted for its skin, meat and use in traditional medicine.
What is a monitor lizard?
Monitor lizards are the large, long-necked, forked-tongued lizards of the genus Varanus, a group of about 80 species found in Africa, Asia and Australia. They include the biggest lizards on Earth, from the Komodo dragon to the crocodile monitor of New Guinea, as well as small species barely 20 centimeters long. The species shown on this page is the Asian water monitor (Varanus salvator), also called the common water monitor, the most widespread and familiar monitor lizard in South and Southeast Asia. It was described in 1768 by the naturalist Josephus Nicolaus Laurenti and moved to the genus Varanus in 1847. The water monitor of the Philippines was long treated as part of the same species, but since 2007 several Philippine forms have been recognized as separate species. Widely considered the second heaviest lizard in the world after the Komodo dragon, the Asian water monitor is part of daily life across much of its range, and it has many local names: biawak in Malaysia and Indonesia, hia in Thailand and kabaragoya in Sri Lanka. Like other monitors, it is intelligent, active and curious, with a high metabolism for a reptile and a constantly flicking forked tongue that it uses to smell.
Types of monitor lizards
With around 80 species, monitor lizards come in a remarkable range of sizes and lifestyles. The Komodo dragon of Indonesia is the heaviest lizard alive, and the crocodile monitor of New Guinea, with its enormous tail, is one of the longest. In Australia, where monitors are called goannas, the perentie is the largest native lizard, the lace monitor climbs tall trees to raid nests, the yellow-spotted or argus monitor lives in the tropical north, and the ackie, or spiny-tailed monitor, is a small rock dweller. In Africa, the Nile monitor is a large, water loving species found along rivers across much of the continent, and an invasive population is now established in Florida, while the savannah monitor, also called Bosc's monitor, lives in the grasslands of the west and center. In South Asia, the Bengal monitor is a common land monitor of drier country, often confused with the water monitor. The Asian water monitor stands out among them all for its size, its close ties to water and its ability to live alongside people, from rice paddies and mangroves to city parks and canals.
How big do water monitors get?
The Asian water monitor is a large, powerful lizard, but most adults are smaller than its reputation suggests. Adults rarely exceed 1.5 to 2 meters in total length, more than half of which is tail. In a study of wild water monitors in Sumatra, males averaged about 1.42 meters and 3.4 kilograms and females about 1.5 meters and 3.5 kilograms, and a common weight for large mature animals is around 19.5 kilograms. The largest on record, measured in Sri Lanka, reached 3.21 meters, and some captive animals have exceeded 50 kilograms. That makes it one of the largest lizards in the world, generally ranked second in weight only to the Komodo dragon. The body is long and muscular, with strong legs and sharp claws for digging, climbing and gripping prey, and the long tail is flattened from side to side, working as a paddle in the water and as a whip in defense. Adults are dark gray, brown or almost black, usually with rows of pale yellow spots or bands that fade with age, and a dark band runs back from the eye. The long, narrow head carries a forked tongue and nostrils set near the tip of the snout, useful when swimming.
Where do water monitors live?
The Asian water monitor has one of the widest ranges of any large lizard. It lives in eastern India, including the Sundarbans and the Andaman and Nicobar Islands, in Bangladesh and Sri Lanka, and from Myanmar across Thailand, Laos, Cambodia and Vietnam to southern China, including Yunnan, Guangxi, Guangdong and Hainan. To the south it ranges through Peninsular Malaysia and Singapore and across the islands of Indonesia, including Sumatra, Java, Bali, Borneo and Sulawesi, as well as Sabah, Sarawak and Brunei. In Hong Kong the IUCN considers it possibly extinct. It is mainly a lowland animal tied to fresh and brackish water: rivers, streams, lakes, swamps, mangroves, coastal wetlands and rice paddies, usually below 1,000 meters, although it has been recorded up to 1,800 meters. Few large reptiles adapt so well to people. Water monitors live in plantations, villages, fish ponds, rubbish dumps and city canals, and some cities have famous populations, such as Lumpini Park in Bangkok, where large monitors swim in the lakes and bask on the lawns among joggers, and the parks and waterways of Singapore and Kuala Lumpur.
What do monitor lizards eat?
The Asian water monitor is one of the most opportunistic predators among large reptiles: it eats almost any animal it can catch and overpower, and almost anything dead it can find. Its diet includes fish, frogs, crabs, snails, insects, rodents, birds and their eggs, snakes, turtles and turtle eggs, and young crocodiles, and it readily raids nests. It is also an important scavenger, feeding on carrion of all kinds, and it has learned to exploit people: around villages, markets and tourist resorts it eats fish scraps, garbage and other human food waste. In a study on Redang Island in Malaysia, the stomachs of 20 adult monitors contained mostly human food waste from the resorts. Monitors hunt actively, walking, swimming and climbing in search of food, constantly flicking their long forked tongue to pick up scent particles, which are analyzed by a sensory organ in the roof of the mouth, so they can find a carcass or a nest from far away. They usually swallow food whole or tear it into large chunks with their sharp, serrated teeth, which are replaced roughly every two months. By eating carrion and rats, water monitors help keep waterways and villages clean.
Behavior and reproduction
The Asian water monitor is at home both on land and in the water. It is an excellent swimmer that moves by sweeping its flattened tail from side to side, with the legs held against the body, and it often escapes danger by diving and staying underwater for long periods. Adults are mainly ground dwellers, although they climb well, while young monitors spend much of their time in trees, safer from predators. It is active by day, basking in the morning to warm up and then foraging along banks, through vegetation and in the water. Water monitors are mostly solitary, but several may gather at a large carcass or a rich source of food. When threatened, a monitor may hiss, inflate its throat, raise its body and lash its tail, and if grabbed it can bite and scratch hard. Breeding takes place from about April to October. Males sometimes fight by rearing up on their hind legs and wrestling. About a month after mating, the female lays 10 to 40 eggs in a rotting log or a tree stump, and the eggs take six to seven months to hatch. Adults have few natural enemies apart from saltwater crocodiles and people. In captivity they live 11 to 25 years; wild lifespans are shorter.
Is the monitor lizard endangered?
No. The IUCN lists the Asian water monitor as Least Concern, thanks to its huge range, its ability to live in disturbed habitats and its fast breeding. It is nevertheless one of the most heavily traded reptiles in the world. Its skin makes a strong, attractive leather used for shoes, belts, bags and watch straps, and as many as 1.5 million water monitor skins are traded each year; in Peninsular Malaysia alone, 50,000 to 120,000 are taken from the wild annually. Monitors are also hunted for meat, for traditional medicine and for the pet trade. Because of this, the species is listed on CITES Appendix II, which requires export permits, and Indonesia and Malaysia, the main exporters, set yearly harvest quotas. Studies of the harvest suggest that the water monitor tolerates heavy hunting better than most large reptiles because it grows and breeds quickly and thrives around farms, but local declines are common where hunting is intense or wetlands are drained. Several countries give it strong legal protection: it is protected in Thailand, listed under the highest category of India's wildlife law and, since 2021, a first class protected species in China, where it has become rare. In Sri Lanka, many people tolerate it because it eats venomous snakes and carrion.
