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Water Monitor Print E-mail

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.

 
 

 

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