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Malayan Water
Monitor Lizard

Varanus salvator

Asian/Water/Common Water Monitor, Two-banded Monitor, Rice/Ring/Plain/No-Mark Lizard

beautifully spotted monitor lizard
Among the largest lizards in the world, Malayan Water Monitors can survive in habitats that wouldn't be able to support other large carnivores.
Mangrove and wetland wildlife at
Sungei Buloh Nature Park
Main features: Large greyish lizards, adults grow to over 2m long and weigh up to 25kg. Males are larger than females. Juveniles are more colourful.



monitor lizard face and clawsStatus in Singapore: Quite common, particularly in habitats near water.


World distribution: Asian subcontinent from India (and Sri Lanka) to China, down Southeast Asia to Indonesia, the Philippines, New Guinea islands in the Indian Ocean and South China Sea.


Classification: Family Varanidae, Suborder Sauria, Order Squamata.
They are so successful because they are cold blooded and hence make more efficient use of food.

monitor lizard eating a dead fishIn addition, they eat anything that they can swallow. From tiny insects, to crabs, molluscs, snakes, eggs (of birds and crocodiles), fish including eels up to 1m long. They also eat birds, rodents, small mouse deer, even other monitor lizards. They are particularly fond of carrion. They even eat rubbish, human faeces, and even dead bodies. They eat prey almost as big as themselves: one 1.2m long ate a snake 1.3m long.
Cold blooded creatures do not have cold blood. Instead, their blood tends to match the temperature of their surroundings.

Warm blooded creatures have to burn fuel constantly to keep their temperature constant. Cold blooded creatures don't and thus can get by with less food.

Water Monitors are more active than reptiles of their size because they are able to maintain an almost constant body temperature. They do this by choosing appropriate micro-climates in their habitat; hiding when it's hot and in warm places when it's cool at night.
The Water Monitor's main hunting technique is to run after prey that it has spotted, rather than stalking and ambushing. Like snakes, they have a forked tongue that they stick in and out regularly to "smell" their prey and other tasty titbits.

Water Monitor Lizards are highly mobile. They swim well (keeping their limbs to the side of the body, and propelling themselves through sinuous undulations of the flattened tail). They have even been seen swimming far out at sea. They can remain underwater for up to half an hour.

monitor lizardThey run fast for their size as they have powerful leg muscles. In fact, they are faster than most of us can run.

They also climb well, to search for food as well as to escape predators, using their strong curved claws. The young usually stay in trees for safety. If cornered up a tree, they will jump into the safety of a stream or river.

They usually hide in a burrow built in a river bank. The entrance starts on a downward slope but then increases forming a shallow pool of water. The average length of the burrow is about 9.5 m, the average depth is about 2m. In the burrow, the average temperature is around 26 degrees Celsius.

The name "monitor" probably originated from the superstitious belief that Nile monitors warned of the presence of crocodiles. Nile monitors eat crocodile eggs and were therefore often seen near crocodile nesting sites.
Water Monitors are rarely found far from water. Both fresh and saltwater. They are particularly common in mangroves, banks of large rivers. Also found in grasslands, forests, swamps, beaches and even cultivated land. From sealevel up to 1,100m high. They are among the first large vertebrates to colonise new islands.

Breeding:
Water Monitor Lizards breed rapidly. Larger females produce a larger clutch than smaller ones, up to 40 eggs a year in 2 or more clutches. Mating involves a lot of biting and scratching. Females lay their eggs 4 to 6 weeks after breeding. 3 to 25 white, soft-shelled eggs are laid, with an average of 15 per clutch. Eggs are laid in termite mounds (both active or abandoned mounds), along rotting logs or hollow stumps or in burrows. Eggs take 2.5-10 months or more to incubate.

Juveniles are more brightly coloured with bright yellow markings on the body and yellow bands on the tail contrasting against a darker body. But they are more secretive and less commonly seen. Males grow faster than females, become longer and heavier. In ideal conditions, they reach maturity in 2 years at 1-1.3m for males and 0.5-1.2m for females. Water Monitors can live for up to 15 years.

Role in the habitat: As scavengers, Water Monitors keep the habitat neat and tidy, and also control populations of their prey. They in turn provide food for larger carnivores such as crocodiles and birds of prey. Small young Water Monitors are particularly vulnerable even to large birds such as herons.
When attacked, Water Monitors try to intimidate predators by lashing out with their tails, inflating their throats, hissing loudly, turning sideways and compressing their bodies. When cornered, they will bite and claw. Unlike other lizards, they do not drop their tails in self defence.
monitor lizard inflating its throat
Inflating the throat to intimidate
Water Monitors are a source of protein and income to poor rural people. Sustainable harvesting is possible because even in places where they are hunted, they are still rather common.

Status and threats: Water Monitors are not considered endangered although they are commonly hunted for their meat and skin and have been exterminated over most of mainland India. Elsewhere, populations have declined sharply. Habitat destruction also affects them. Up to 1.5 million skins are legally exported each year mainly from Indonesia to Europe, Japan and the US to be made into fashion goods. One explanation why they remain plentiful despite this is becuase the skins of medium-sized Monitors are preferred. Those of larger Monitors are too thick and tough, thus possibly sparing large females who lay more eggs. Their meat is considered delicious and a bewildering array of potions are made from various parts of their bodies, ranging from cures for diabetes to aphrodisiacs and deadly poisons used in assassinations. The gall bladder is brewed for a medicinal tea to treat heart and liver problems. Skin ointments are made from the rendered fat. In Sri Lanka, the locals protect them because they eat the crabs that would otherwise undermine the banks of the rice fields.

LINKS

REFERENCES
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  • Merel J. Cox, Peter Paul van Dijk, Jarujin Nabhitabhata and Kumthorn Thirakhupt, "A Photographic Guide to Snakes and Other Reptiles of Peninsular Malaysia, Singapore and Thailand", New Holland, 1998 (p. 107: habits, habitat, photo).
  • Kelvin K P Lim and Francis L K Lim, "A Guide to The Amphibians and Reptiles of Singapore", BP Science Centre, 1992 (p. 97-98: habits, habitat, photo).
 
By Ria Tan, 2001