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Dumeril's monitor

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

Varanus dumerilii Schlegel 1839

Dumeril's monitor is a very mysterious animal. It is found in southern parts of Burma, Thailand, Malaysia, Borneo, Sumatra and many smaller neighbouring islands including Natu, Bangka and Bellitung. Locality data can be found in Mertens 1942, Brandenberg (1983), Sprackland (1993), Bennett (1995a) and Bennett & Lim (1996). Smith (1930) said that they were particularly abundant on Kau-Ye-Kyun Island in the Mergui Archipelago. It is hoped that the Myanmar authorities will eventually give their permission to allow a study of this beautiful creature to be conducted in the Mergui Islands. This monitor appears to be the rarest, or at least the most inconspicuous, varanid in all the countries it inhabits. They may be most common in mangrove swamps, but are also found in forests far from the coast. Maximum size is about 135cm TL. A breeding pair maintained in captivity measured 130cm TL, 2950g (male) and 100cm, 2300g (female) (Radford & Paine 1989). Wild animals of 30cm SVL weigh about 1kg (Lim, pers comm.).

Image
Varanus dumerilii


Very little is known about the natural history of this intriguing monitor lizard (although a detailed account of its pelvic muscles can be found in Akita (1992)). Smith (1930) records that they will take refuge in the sea to escape from dogs and Nutphand notes that they will climb trees when disturbed. Stomach contents of animals collected in mangroves are composed largely of crabs (Brandenberg 1983, Losos & Greene 1988) and Krebs (1979) considered them to be specialised crab eaters, noting that the nostrils can be sealed when the animals are underwater and that the teeth are adept at piercing tough shells. According to Auffenberg (1981) they feed largely on insects collected on the ground in inland forests. Ants and birds have also been suggested as prey (Barbour 1921, Pitman 1962). Raven's (1946) record of monitors feeding on turtle eggs on an island off Borneo was interpreted as V.dumerilii by Neill (1962). Typical, bipedal combat has been recorded between males (Davis & Darling 1986. In captivity youngsters may bury themselves when given the opportunity (Horn & Schulz 1977). Nutphand claimed that this lizard was the least active of the monitors found in Thailand, spending most of its time in rock crevices and tree hollows and using the same retreats consistently. Both he and Lekagul (1969) considered it to be more common than the rough-necked monitor. It is quite possible that populations of both species have been seriously depleted by the destruction of forests, but Dumeril's monitor, at least, is known to inhabit areas of human habitation and so they appear to be able to survive away from pristine forest in some circumstances (Bennett & Lim in press). A study of these monitor lizards in their natural environment would be extremely rewarding. In Thailand they are known as Tut-too, or more properly, Hao-chang-kao (whitejungle monitor). In Malaysia they are biawak kudong.

The beautiful appearance of hatchling Dumeril's monitors is well known (first described by Horn & Schulz 1977) but no satisfactory explanation for the purpose of this bright colouring is available (see Chapter 4). In some areas youngsters have yellow, rather than orange heads (Auffenberg, pers. comm.). Dumeril's monitors make hardy captives, known to survive for over 20 years (Grossman, pers. comm.), but reported breeding success with this species is rare (Zimmermann 1986; Radford & Paine 1989; Frost 1995; Connors, pers. comm.). They are not particularly aggressive but in cramped confines the weaker animals are likely to suffer. Groups of three or four specimens can be housed in an enclosure of 8m2, but breeding has occurred in enclosures as small as 2.4m2. Dumeril's monitor is an excellent climber and also shows great fondness for water. They may prefer basking spots in the mid to high 30s rather than 40oC or above, but should be allowed to bask at the highest temperature they find acceptable. An ambient temperature of 25-32oC is suitable.

Their diet should consist of insects, earthworms, crabs, fish, small mammals, birds and eggs together with a good vitamin and mineral supplement. In North America breeding usually occurs in October even when artificial lights cycles are in use. Clutches can contain as many as 23 eggs and may be laid over several days or weeks. More than one clutch can be produced in a year. Eggs incubated at 28oC hatch after 203-215 days, at 26.7-30oC after 215-222 days and at 25-30oC after 234 days. In light of these figures Wong's (in Bennett 1995a) incubation period of 105-115 days seems impossibly short. Egg development does not always proceed smoothly, but the problem can probably be minimised by feeding the female on a rich and varied diet and ensuring that the incubation medium never becomes dry. Good results have been obtained using a vermiculite to water ratio of 5:6 or 5:8. Hatchlings measure about 18cm TL (8cm SVL) and weigh 10-20g, presumably depending on humidity during incubation. The youngsters can loose their bright colours after as little as six weeks and after five months can attain 13cm SVL. Youngsters should be provided with just enough water to immerse themselves and given damp substrate to hide in. Biebl (1995) suggests that Dumeril's monitors mark their territory in captivity.

The subspecies V.dumerilii heteropholis was described by Boulenger in 1892 from animals collected in northern Borneo. It is said to differ from the nominate race by having scales of many different sizes over the back. Both Brandenberg (1983) and Sprackland (1993) suggest that the differences are too slight to warrant use of the name heteropholis.

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