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Water Monitor Print E-mail
The Water Monitor, Varanus salvator

First Published in Reptilian 3 (8). Copyright reserved.

The first two articles in this occasional series on the monitor lizards of Asia discussed two rare and enigmatic animals found only in rainforests and mangrove swamps. Virtually nothing is known of their biology and they are only rarely seen in captivity, at least on this side of the Atlantic (Bennett 1993, 1995). In contrast the water monitor is a widespread animal commonly used for its meat and skin. Despite their huge size they are popular with reptile keepers, probably on account of their endearing personalities and intelligence. The water monitor is one of the most heavily exploited lizards in world and in recent years it has been the subject of a number of studies.

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Range of Varanus salvator
The water monitor has a massive distribution. It is found from India to Indonesia, via Bangladesh, Burma, the foothills of the Himalayas, southern China, Vietnam, Laos, Kampuchea, Philippines, Thailand, and Malaysia. There is some evidence  that the water monitor and the mangrove monitor (V.indicus) are sympatric on the Indonesian islands of Halmahera and Obi, which appears to represent the extremes of their easterly and westerly distribution respectively (de Rooij 1915). Records of water monitors in northern Australia (e.g. de Rooij 1915) are incorrect, and probably the result of confusion with the mangrove monitor.

The water monitor is one of the largest lizards in the world, frequently attaining a length of over 2 metres. The largest come from Malaysia, particularly around the Cameron Highlands. A water monitor over 9 feet long was exhibited in Kuala Lumpur in 1958 (Lim 1958) and large water monitors can weigh over 25kg (Jasmi 1988) but such specimens must be very rare. In some parts of their range the lizards do not attain such great lengths; on Flores, for example they rarely if ever exceed 150cm in total (Auffenberg 1981). In most areas the animals become sexually mature at total lengths of around 130m for males and 120cm for females and in most populations males grow faster than females and become longer and heavier. Under good conditions in captivity the animals can become sexually mature after two years (Andrews 1995).

In all but one of the water monitor populations studied to date, males outnumber females by at least two to one (Khan 1969; Vogel 1979; Gaulke 1989). This difference is usually attributed to to more reckless behaviour of the males, who are more likely to be caught than the cautious females. However this explanation may have to be revised in light of Hairston and Burchfield's (1992) claim that hatchlings bred by them showed a strong sex bias (almost seven to one) in favour of males.

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Varanus salvator nuchalis, Panay, Philippines
As its popular name implies, V.salvator is rarely found far from water, either fresh or saline. They are particularly common in mangroves and on the banks of large rivers, but also inhabit farmland, grasslands, forests, river deltas, swamps and beaches. In Malaysia  I have seen them in pristine rainforest and in ponds in the middle of huge cities. Indeed it can be said that this lizard will live wherever there are permanent or temporary bodies of water, providing food is available and they are not exterminated by mankind. Although they tend to be much commoner in low-lying areas they have been found as high as 1100 metres above sea level. Their adeptness in the water has enabled them to spread to many islands, indeed, they were probably the first large vertebrates to colonise the newly formed islands of the Krakatoa group after the infamous volcanic destruction of Krakatoa itself in 1883, and quickly formed breeding colonies so large that by the 1920s people were visiting the islands solely to hunt the monitor lizards (RAWLINSON ET AL 1990). In many areas, however, the number of water monitors has declined sharply over the last hundred years. They have been exterminated over most of mainland India, with large populations now present only in Orissa and Assam (DAS 1989). Similarly in Bangladesh populations have declined sharply in living memory (KHAN 1988). Habitat destruction (especially the clearing of mangroves) and overcollecting for meat and skin seem to have been responsible for its demise.

Water monitors are not territorial and in areas where they are still common they can reach very high densities. MUNSCH (1987) saw seven adults piled on top of each other along a riverbank in Sri Lanka. They are also known to congregate around carrion. AUFFENBERG (1981) reported that in western Flores 15 could be found along a kilometre of riverbank and 8 per kilometre in mangrove forest. ERLEDEN (1991) found that biomass fluctuated in the mangroves of south Sumatra, but could reach as high as 119kg of lizard per kilometre, possibly the highest biomass of any lizard yet recorded. Densities tend to be highest along coasts (especially in mangrove forests) and lowest in inland mountainous areas.

The diet of the water monitor indicates that they are well adapted for surviving in hostile environments; quite simply, they will eat anything they are able to swallow. Crickets, spiders, beetles, crabs, molluscs, snakes, eggs and young of crocodiles and turtles, tortoise. lizards (including other monitor lizards), fish (including eels of a metre in length) birds, rats, flying squirrels, mouse deer, carrion, scraps from human waste dumps and even human faeces are all eaten with relish (Vogel 1979; Losos & Greene 1986; Auffenberg 1980; Gaulke 1991a; Traeholt 1993, 1994a&b).  Adults are particularly fond of carrion and Deraniyagala (1953) reports that they were regularly seen on top of dead cattle floating downriver. The diet of the water monitor varies with its habitat. In mangroves crabs are their usual prey, elsewhere insects and other small invertebrates form the bulk of their food. Young monitors may be entirely insectivorous (Gaulke 1991b, Traeholt 1994b), but larger specimens do not necessarily eat larger prey animals. The water monitor is overindulgent to the point of gluttony. Smith (1931) records that after eating 40 frogs a female was so gorged that it was powerless to prevent itself being dragged out of the water by the tail. Petzold (1967) reports that gastric pellets containing indigestible parts of prey are regurgitated by captive water monitors. Gaulke (1991) found similar pellets in the stomachs of wild lizards. The water monitor is a highly opportunistic feeder, and its catholic diet enables it to survive in areas which would not normally support large carnivores.

Their ability to consume waste scraps and to prey on animals that are a nuisance to man (including pest species of crabs, rats, crocodiles and venomous snakes) endear them to many people, but their habit of taking domestic birds and their eggs together with the flavour of their flesh and quality of their skins means they are ruthlessly hunted by others. In parts of the Philippines they are considered a nuisance because they disinter human corpses (Taylor 1963) and feed from coffins placed in trees (Gadow 1901), but in other parts of their range this necrophagious behaviour is actually encouraged. In 1889 Anderson  wrote of the Selung tribe of the Mergui Archipelago, who did not bury their dead but left them on platforms in the forest where they were devoured by the monitor lizards "... one man reports  that he has seen as many as 15 lizards engaged on a ghastly meal of this kind". Auffenberg (1982) reports that some tribes on Bali disposed of corpses in a similar way, putting them in baskets which prevent monkeys entering but allow access to monitor lizards.


 
 

 

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Bye Bye Butaan

 butaan1.jpg

Butaan start to visit fruiting trees before they are large enough to swallow the fruits. They make repeat journeys to trees, perhaps to reinforce memory of the position of the tree. If the youngster survives it may continue to use this tree for many decades. Fruiting trees like this are a vital resource for entire populations of butaan. Learn more >


 
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The Butaan Project - Background and History
butaan2.jpgThe butaan was first described to science in 1845 from a juvenile specimen collected by Hugh Cuming. It was labelled only "Philippines". It was named Varanus grayi.  No other specimens came to light for over 120 years. In the 1970s Walter Auffenberg found another specimen with a location in Luzon, established that its correct scientific name was Varanus olivaceus, and undertook a 22 month study of the species based in Bicol. His study revealed that butaan occupy a unique ecological niche and have a lifestyle quite unlike any other monitor lizard. Auffenberg used local hunters with dogs to catch the animals. Of 126 butaan caught during his study, 116 animals were killed.
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