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Varanus bengalensis Print E-mail

Bengal Monitor
Clouded Monitor

Extract from A Little Book of Monitor Lizards © D. Bennett 1995. Viper Press, UK

Varanus bengalensis bengalensis Daudin 1802
Varanus bengalensis nebulosus Grey 1831


The name of this monitor lizard is misleading, for it is one of the most widely distributed of the living varanids. The Bengal monitor inhabits river valleys in eastern Iran, Afghanistan and western Pakistan (Mertens 1942, 1959; Leviton & Anderson 1970; Luxmoore & Groombridge 1990). Elsewhere in Pakistan it is widespread in many different habitats, but reaches greatest abundance in agricultural areas (Auffenberg et al 1991). It is similarly ubiquitous in India, Nepal, Sri Lanka, Bangladesh and Burma, found from desert fringes to rainforests, but is most common in farmlands and dry, open forests (Smith 1930; Deraniyagala 1931; Khan 1989). A different form of the Bengal monitor is found from southern parts of Burma and Vietnam, throughout Kampuchea and Peninsular Malaysia, Sumatra, Java and the Sunda Islands. Clouded monitors are not found in Singapore and although they are present on some islands in the South China Sea they are absent from Borneo. They are found in a similar variety of habitats as the nominate race, including pristine rainforest, but in the extreme south-east of their range they are absent from wetter areas (Mertens 1942, 1959; Lekagul 1969; Brygoo 1987, 1990; Auffenberg 1988; Jasmi 1988). The Bengal monitor is distinguished from the clouded monitor by having larger scales above the eyes and fewer scales around the body (Mertens 1942). The skull of V.b.bengalensis is described in detail by Bhal (1937). In pattern these lizards show enormous variation. Adult Bengal monitors are black, dark grey or brown, with varying amounts of lighter pattern over the back. Hatchlings are said to be dull orange or light brown and boldly banded with black and yellow over the body and tail (Minton 1966; Khan 1988). Adult clouded monitors are much lighter in colour, ranging from light grey to dirty-yellow, with varying amounts of speckled pattern. Juveniles are sometimes darker than older specimens (Smith 1930; Nutphand). As in other monitors, the amount of pattern seems determined largely by the level of rainfall, with the plainest animals found in drier areas. These criteria alone (i.e. scalation and pattern) may be insufficient to assign many individuals to either bengalensis or nebulosus, but it seems possible that the races are separate species (Auffenberg, pers. comm; Bohme 1988). However Gorman (1993) records captive breeding between the subspecies and Deriniyagala (1957) notes that hatchlings show great variation in appearance. The Bengal monitor is often referred to incorrectly as Varanus monitor (Sprackland 1982). Yang & Simin (1987) and Yang & Liu (1994) described Varanus irriwadicus from around the Irriwady river in southern China and Varanus vietnamensis from northern Vietnam on the basis of slight differences in scalation and pattern compared with V.monitor and V.nebulosus. Unfortunately no specimens are available for examination and the descriptions do not make reference to Robert Mertens' comprehensive accounts of varanid taxonomy, relying mainly on much older works. These animals are certainly very closely related to V.b.bengalensis and V.b.nebulosus and suggest that the range of this species extends into China.

Image
Varanus bengalensis


Bengal/clouded monitors reach a maximum length of about 100cm TL in Bangladesh (Khan 1988), 140cm in Sri Lanka (Deraniyagala 1931) and 160cm in Malaysia, where large specimens can weigh over 10kg (Jasmi 1988). Nutphand gives maximum size in Thailand as only 100cm TL and Smith (1930) suggested that specimens of over 2.4m occurred in the forests of northern Burma! Males of both varieties grow larger than females; Auffenberg (1979b) records average size of adult males as about 58cm SVL (150cm TL) and 2.7kg, females 46cm SVL (120cm TL) and 1.5kg. Average size of adults in southern Sri Lanka is 50cm SVL and 2.5kg, but specimens of almost 11kg have been recorded (Wikramanayake & Green 1989; Deraniyagala 1931). A wild specimen in Malaysia caught by Harrison (1955) increased in size from 20cm SVL, 109g, to 40.5cm SVL, 1850g after 22 months and contained 15 eggs. In captivity sexual maturity is attained within three or four years but the animals may not mature for five years or more in less favourable habitats (Auffenberg 1983c; 1986).

A study in Sri Lanka showed that Bengal monitors spend the nights in burrows, where their body temperature drops below ambient. The following morning they must raise their body temperatures by basking before commencing activity and for this reason they are rarely active early in the morning (although Munsch (1987) found them feeding before 0600 hrs. elsewhere on the island) and most active in the afternoons when temperatures are highest. Mean active body temperature was 34.5oC. In contrast the water monitors that inhabit the same area spend the nights in thickets of vegetation or in the water, where they are able to maintain body temperatures higher than ambient. Mean active body temperature of this species was only 30oC and so the lizards are able to begin activity without a long period of warming up. They are active mainly during the mornings, and as ambient temperatures increase towards midday they move to cooler areas (particularly in water or high on the branches of trees). When temperatures begin to fall in late afternoon the monitors may recommence activity for a while, but as evening sets in they return to the water or to dense vegetation, which remain warm throughout the night. In this way the thermoregulatory preferences (and therefore the behavioural patterns) of the monitor lizards allow them to co-exist in the same area without coming into direct competition (Wikramanayake & Green 1989). Dryden & Wikramanayake (1991) note that only younger Bengal monitors frequent the vicinity of rivers, and that adults are found in drier forest, scrublands and grasslands whilst water monitors do not usually venture far from the water.

Both races of the Bengal monitor feed mainly on the ground, but clouded monitors in particular are excellent climbers (Taylor 1963). Even large adults can ascend vertical tree trunks with ease, and they are reported to be agile enough to stalk and capture roosting bats (Deraniyagala 1931; Smith 1935). However, despite their large size, these lizards get most of their nutrition from tiny prey and feed mainly on beetles, grubs, orthopterans, scorpions, snails, ants and other small invertebrates, which are consumed in enormous numbers. The lizards collect ants and similar sized prey by licking them up with the tongue. Vertebrate prey are comparatively rare, but include frogs, molluscs, fish, lizards, snakes (including cobras) and small mammals. The monitors often use their tongues to collect small insects and find much of their food by rooting the ground, especially under cow pats and in leaf litter. They often forage in human rubbish dumps and occasionally take carrion (Henry 1912; Williams 1935; Brongersoma 1947; Deraniyagala 1953; Harrison 1955; Bharatan 1971; Sharma & Vazirani 1977; Sharma 1982; Losos & Greene 1988; Auffenberg & Ipe 1983; Auffenberg et al 1991). Feeding behaviour in captivity is discussed in Auffenberg (1983c). Around Lake Haliji in Pakistan, Bengal monitors often rest on floating vegetation and spend most of their time close to the water, although they take almost all of their food from dry land (Auffenberg et al 1991). In temperate regions activity is halted or reduced during the cooler months whilst the tropical nebulosus may be active throughout the year. Bengal monitors shelter in burrows or crevices in rocks and buildings, whilst clouded monitors prefer tree hollows. Both races are known to take refuge in abandoned termite mounds and both will frequent human habitations (Annandale 1921; Mahendra 1931; D'Abreu 1933; Auffenberg 1983a; Dryden & Wikramanayake 1991)).

Observations in captivity show that males are more active than females, spending an average of 4.5 hours a day moving about, compared to 2.85 hours for females (Auffenberg 1979b). Males also consume more food (up to 18% of their own body weight) and therefore grow faster than females, but a consequence of their high activity levels may be that they suffer higher rates of predation. Around Lake Haliji home ranges are around 53,000m2 for males and 44,000m2 for females. In less productive areas home ranges can exceed 300,000m2 (Auffenberg et al 1991).

Ritual combat is described by Ali (1944) and R.Y. Deraniyagala (1953) and takes place in typical bipedal fashion. Courtship and mating behaviour is described in Auffenberg (1983b). In order to mate the male must completely immobilise the female, who may struggle vigourous to escape, and so only the strongest males are likely to succeed. According to Auffenberg et al (1991) pair bonding occurs in this species and fecal signals play an important role in social behaviour. Annandale (1922) reports that pairs of monitors are often found together. In northern parts of their range Bengal monitors reproduce during the wet season (Minton 1966). In Sri Lanka most eggs are laid between January and April, but some may be produced at other times of the year (Deraniyagala 1953). In Thailand eggs are laid through the year and Nutpand suggests that three clutches may be produced by a female over this period. In the northern parts of the range eggs may be laid in burrows, termitaria or rotten logs (Deraniyagala 1958, Auffenberg 1983a) but in Thailand an active termite mound is the usual choice of nest site (Nabhitabhata, pers.comm.). Auffenberg (1983a) suggests that females may return to the same places each year to lay eggs. Maximum clutch size is usually considered to be 30 (e.g. Smith 1930, 1935, Deraniyagala 1953). Incubation times in the wild are not well documented; Deraniyagala (1958) reports that eggs laid in December had developed after five months. Vijaya (1985) found eggs in September that hatched in July. Young Bengal monitors are very secretive. According to Auffenberg (1983a) the hatchlings remain in a group with their brothers and sisters for at least several months after hatching. They spend almost all of their time in trees and are entirely insectivorous.

For many years Bengal and clouded monitors have been collected on a large scale for their meat and skins. The meat of this species is reputed to be particularly sweet and easy to digest and for this reason it is often given to invalids (Deraniyagala 1953). They are not easily attracted to carrion, as is the water monitor, and are usually collected from their burrows (D'Abreu 1932) In India and Sri Lanka, where water monitors are considered inedible, the Bengal monitor is consumed with delight. Around Kerala in southern India it forms a major part of many peoples diet (Luxmoore & Groombridge 1990) and in Thailand and Vietnam the flesh is often considered superior to that of the water monitor (Deraniyagala 1931, Darevsky pers,comm.). Their skins are also highly prized, particularly the dark skins of specimens from Bengal. The adverse effect of this heavy exploitation on populations was first documented more than 50 years ago (Deraniyagala 1931; Champion 1935) and the animals have now been exterminated in parts of Sri Lanka, India and Bangladesh. (Auffenberg 1986; Sarker 1987). Saxena (1993) noted that the number of road-killed monitor lizards decreased sharply during the 1980's. However they are still common in many areas: Khan (1988) estimated densities of Bengal monitors in suitable habitats within Bangladesh as being around 13 per km2, Das (1988) saw up to 20 specimens in an hour in parts of Bengal and in Thailand and Malaysia the species is still very widespread (Nabhitabhata, pers.comm; pers.obs.). When commercial trade in the species was outlawed by CITES in 1975 many countries ignored the ban. Ten years later Japan was still importing hundreds of thousands of skins from Bangladesh, Pakistan Thailand and Malaysia (Luxmoore & Groombridge 1990). In Indonesian they are known as biawak nyambak (wandering monitor) and in Malay as biawak puru (warty monitor). In Sri Lanka they are called talagoya or undumbu, in Burma kon-phut.

These monitors are very hardy in captivity but reports of captive breeding are rare (McKeown, pers.comm., Klag & Kantz 1988; Koop in Horn & Visser 1989; Gorman 1993; Auffenberg, pers.comm.). Most youngsters die before they hatch or shortly afterwards, a problem indicative of poorly nourished mothers. In nature these animals are largely insectivorous but in captivity they often refuse to take small prey items if they are also fed larger animals (Subba Rao & Kameswra Rao 1986). Therefore in captivity they should be fed on large numbers of different types of invertebrates, and this diet supplemented with less regular meals of birds, fish and mammals. All like to bask at high temperatures, but the clouded monitor can be expected to be less tolerant of cold nights than the Bengal monitor. Large amounts of space and furnishings that allow the lizards to search for food (particularly a soft substrate) are essential. Best results were obtained by Auffenberg, who maintained a group of monitors in greenhouses 18-150m2. Males were regularly moved from one group of females to another in order to stimulate courtship behaviour. Eggs were either left in piles of compost provided for nesting or incubated over water, and over 90% hatched successfully. At 29-30oC they hatch after 235-254 days, at 30-34oC after 172-173 days. Clouded monitors have the potential to be extremely prolific in captivity, producing over 70 eggs in three clutches over a year (Brysott, pers. comm). According to Yadov & Rana (1988) females have shorter tails than males and can further be distinguished by looking for patches of scales arranged as pre-anal flaps in males that are absent in females (clearly illustrated in Auffenberg et al 1991).

For more information on the Bengal monitor, the reader is referred to Auffenberg (1994). This work represents 17 years of research including 43 months of field work by Walter Auffenberg, whose contribution to the understanding of monitor lizard ecology is unsurpassed.



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