Water Monitor
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.

Image
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.

Image
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.

Water monitors are not exclusively aquatic animals, although they are never found very far from water. Much of their food is taken from the land and they are often seen on the ground or in shrubs and trees. Water monitors maintain a reasonably constant body temperature that is significantly lower than that of terrestrial monitor lizards of a similar size (about 30oC compared with 37oc). The body temperatures of other monitor lizards studied rise during the day and drop significantly at night, but the temperature of the water monitor remains relatively uniform.  They accomplish this by spending most of their time in microhabitats with stable temperatures, particularly in the water and in dense thickets of vegetation, choosing cool places during the warmest parts of the day and warm places at night. Some elegant studies by Wikrananayake and Green (1989) and Dryden and Wikrananayake (1991) showed that in some areas of Sri Lanka the water monitor is able to coexist with the Bengal monitor because their different thermoregulatory behaviours keep them apart (discussed in more detail in my forthcoming article on V.bengalensis).

The water monitor is an extremely prolific animal. Large females can produce up to 40 eggs in a year (e.g. Biswas & Kar 1981; Erleden 1988). Eggs are usually laid in two or more clutches and it appears that in areas without pronounced wet and dry seasons reproduction can take place at any time of the year (Khan 1969). Elsewhere breeding usually occurs around the beginning of the wet season. Mating in the water monitor is a rough affair involving a great deal of biting and scratching. Eggs are laid in termite mounds where these are available (both active and abandoned mounds are used), otherwise they are deposited in burrows well above the water level. Incubation times seem to differ widely, with reports varying between two and a half months  to ten months or more (Auffenberg 1988:172). Eggs from a single clutch incubated artificially under similar conditions took between 241 and 327 days to hatch (Kratzer 1973). Like all monitor lizards, the hatchlings are extremely secretive and rarely seen. They are more brightly coloured than the adults. The extremely elastic breeding cycles of the water monitor, in which eggs can be laid in any season and hatch at a speed determined by incubation conditions may ensure an even release of young monitors throughout the year. Ritual fighting, in which two adults (either males or females) stand bipedally and attempt to wrestle each other to the ground has been reported from several areas (e.g. Honneger & Heusser 1969, Vogel 1979; Daltry 1991).

In captivity the water monitor presents considerable difficulties on account of its voracious temperament and enormous size. There are many reports of captive breeding, but most  appear to be single incidents and examples of specimens which produce viable eggs more than once are rare. The most complete accounts of successful maintenance in captivity is given by Hairston & Burchfield (1990;1992) and Andrews & Gaulke (1990). Water monitors grow rapidly and require a great deal of space. In the wild communities of monitors form hierarchies determined largely by physical contact (Vogel1979; Daltry 1991). In confined spaces this behaviour invariably results in severe injuries being inflicted on the weaker animals. Adults kept singly seem to need a minimum of about six square metres of floor space to do well. Groups of water monitors need to be provided with much more space to reduce the danger of the weaker animals being torn apart by the stronger ones. Despite their affinity for water large pools to not appear to be essential to keep water monitors sucessfully. Because of their aggressive nature these animals should not be kept with any other species of reptile! Diet in captivity should reflect the variety of foods taken in the wild, and food should be offered in small amounts. Water monitors which are overfed quickly become extremely obese (eg Sprackland 1992, p127) and reach weights more than twice as heavy as any recorded for free-living specimens. Obese monitor lizards tend to be highly docile and are probably incapable of reproduction. Enclosures for water monitors should include hot spots of up to 45oC with plenty of cooler retreats. Breeding this animal in captivity is challenging, but by no means impossible, providing the animals are not overfed and adequate space can be provided. Finding a compatible pair can be particularly problematic. The stronger animal (usually the male) will almost always inflict serious injuries on the other. A prospective breeding pair should be of equal size to minimise the danger of fatalities. Usually it is not possible to keep the pair together outside the mating period unless a vast amount of space can be provided. Eggs incubated at about 30oC usually hatch after anything from 180 to 300 days (Hairston & Burchfield 1990; Kratzer 1973). There have been suggestions that the sex of hatchlings is influenced by incubation temperature (Hairston & Burchfield 1990), but this has yet to be clearly demonstrated. Sexing water monitors is not easy and the only certain way of identifying a female (other than waiting for eggs to appear) is by internal examination. Males tend to be bulkier than females of the same age, but the prescence of hemipenal bulges at the base of the tail must be interpereted with some caution.

Large water monitors have the potential to be extremely dangerous. According to AUFFENBERG (1986) bites from these lizards have caused fatalities in humans. Very serious bite injuries have been inflicted on at least one keeper in the U.S.A. However even very large monitors can be docile when accustomed to being handled. A large captive bred male at a Texan zoo takes great delight in being scratched under the chin by its keeper. Nevertheless, great caution is required when working with large monitor lizards, and wild caught adults should be handled in the same manner as crocodilians. LOVERIDGE (1946) reports that water monitor which were tame and docile when kept inside quickly became aggressive when moved into the open air.

Within the enormous range of this species a number of subspecies exist, some of which may turn out to be separate species; V.s.togianus from Sulawesi and surrounding area, V.s. scutigerulus from Borneo, V.s marmoratus from northern islands of the Philippines, V.s.nuchalis from central islands of the Philippines and  V.s.cummingi from southern parts of the Philippines  (Gaulke 1991b). The pattern and colouration of the water monitor is extremely diverse. Entirely black water monitors are sometimes referred to as V.salvator komaini (Sprackland 1992) but such animals can come from any population of water monitors (I have seen specimens from Malaysia,  Thailand, Philippines, Sulawesi and India). Black specimens tend to be found only in coastal areas, and are associated particularly with small islands and mangrove forests. The reasons for this are not clear. The validity of the subspecies is beyond the scope of this article, but the accompanying pictures illustrate some the diversity of pattern and colouration of this ubiquitous animal. The relationship of the water monitor to other members of the Varanidae is uncertain. Previously they were thought to be closely related to the Komodo dragon, the mangrove monitor and  the large  Australian goannas (Mertens 1942), but on the basis of hemipenal morphology Bohme (1988) considered that it has no close living relatives.

Bones left in a cave in Borneo provide the first record of human predation on monitor lizards, almost 40,000 years ago (KING 1962). Today the water monitor is of enormous economic value. Up to one and a half million skins are legally exported each year  mainly from Indonesia to Europe, Japan and the U.S.A. (LUXMOORE & GROOMBRIDGE 1989). The skins are tanned and turned into handbags, shoes, wallets watchstraps and drums. In addition many monitors are killed for their meat. In Sri Lanka, where water monitors are not eaten, they are often found around human habitations, but where they are hunted they tend to be much more secretive. The flesh of the water monitor is delicious and a bewildering array of potions are made from various parts of its anatomy, ranging from cures for diabetes, through aphrodisiacs to deadly poisons with which to assassinate ones enemies AUFFENBERG 1982; GADOW 1901; BOONRATANA 1989; ANON 1984; DAS 1989). Even the faeces are thought to have therapeutic properties and have been used to combat eye and skin diseases. The use of the monitor lizard for meat and skin has attracted condemnation from a number of well meaning but misinformed people. The poverty in southeast Asia is incomprehendable to people in the Western World, and the water monitor provides many millions of people with an important source of protein. Furthermore the value of their skins make them one of the most profitable and easily sustained resources of the Asian forests. Without adequate control however, large scale exploitation of the monitor will lead to its demise. A number of feasibility studies have been undertaken to investigate the possibility of farming water monitors, but the fact that they still appear to be very common in many areas where they are regularly hunted suggests that such a programme may not yet be profitable.

In Thailand the water monitor is said to be almost universally despised because it is believed to bring bad luck. Specimens with red tails are said to be particularly loathsome. According to Nutphand their vernacular name (Hiah) is often used as a term of abuse. In contrast the Dawa clan of the Garo Hills in India hold the water monitor in great reverence. According to legend t he founder of the clan once kept a young monitor in a cage, but when its parents came to visit it the man became afraid that the large lizards would take revenge on him whilst he was swimming or fishing. He dressed the little lizard in a yellow coat, put earrings in its ears (sic) and promising solemnly never to catch another monitor he released it. Thereafter the unlikely pair became good friends and the lizard took to carrying the man across the river on its back. Ever since, members of the clan never harm the monitors and always call out "I am a son of Dawa" before crossing a river (PARRY 1932).

In summary, water monitors are gregarious creatures that will live virtually anywhere they have access to a body of water. Their ability to cross wide expanses of sea, together with their unfussy diet and enormous reproductive potential have resulted in their wide distribution and great abundance in many areas. Where they are left unmolested they can happily coexist with people and often act as consumers of refuse. Even in places where they are exploited they are capable of sustaining high densities. However habitat destruction has resulted in reductions in numbers and even local extinctions in many areas. The extent to which water monitor populations are dwindling is not well documented and needs urgent investigation. The next article in this series will focus on another monitor lizard that is equally widespread but has a much more restricted diet and has been banned from international trade since 1975, the Bengal or clouded monitor, Varanus bengalensis.

ANDREWS,H.V. 1995. Growth and reproduction in Varanus salvator (LAURENTI 1768), with notes on growth and reproductive effort. Herpetological Journal 5 (1): 189-194.
GADDOW,H. 1901. Cambridge Natural History. Vol VIII:542-547.
SMITH,H.C. 1931. The monitor lizards of Burma. J. Bombay Nat.Hist.Soc. 34:367-373.