Varanus indicus
Mangrove monitor

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

Varanus indicus indicus       Daudin  1802  Mangrove monitor
Varanus indicus kalabeck      Lesson  1802  Kalabeck monitor
Varanus indicus spinulosus    Mertens  1941 George's Island monitor

The mangrove monitor is a large active lizard with an enormous distribution. Its scientific name is misleading because the species does not occur in India, or anywhere near it. Mangrove monitors are found from northern Australia and New Guinea east to the Solomon Islands, Marshall Islands, Caroline Islands and Marianas Islands. Its distribution through much of Micronesia remains uncertain. Luxmoore et al (1988) give the following locations: Micronesia (Kosrae, Mortlock, Woleai, Ifalak, Yap, Ulithi and several islands in the Truk Atoll), Guam, Indonesia (eastern sector: Irian Jaya and adjacent islands, north to the Talaud Group and west to Halmahera and the Moluccas (Ambon, Buru, Ceram etc.) and Timor, Marshall Islands, Palau (Ngeaur, Oreor, Babeldaob (Babelthaup) Ncheangel (Kayangel) and Solomon Islands (Guadalcanal, Isabel, Malaita and San Christobal). In Australia they are found only in the extreme north; Arnhemland and especially Cape York, Queensland. Reports of V.indicus in Sulawesi appear to be incorrect (Indraganeali, pers. comm.) but they may exist on Obi or Halmahera in broad sympatry with V.salvator. Additional location data can be found in Peters & Doria (1878), Hediger (1934), McCoy (1980); Heatwole (1975), Fisher (1948), Brandenberg (1983); Mertens (1958) and Burt & Burt (1932). Within this massive range there is a huge amount of variation in size, pattern and scalation and major revision of the group is underway. A previously unknown species from New Britain, V.doreanus, has been recently rediscovered (Bohme, Horn & Zeigler 1994). The V.indicus group represent a taxonomist's paradise, providing he or she does not get sea-sick.

V.indicus kalabeck (from the Waigeu Islands off New Guinea) has small widely spaced scales on the neck and was not considered a valid subspecies by Brandenberg (1983). Bohme (1991) notes that the type specimen of V.i.kalabeck is lost and supports Brandenberg's view that the animals on Waigeu are typical mangrove monitors. Live "Kalabeck's monitor" are advertised by animal dealers in North America, but as far as I can ascertain these animals are actually V.doreanus.

V.indicus spinulosus has a shorter, bulkier snout than the typical mangrove monitor and is further distinguished by its spiny scales on the back and neck and nostrils which are situated closer to the tip of the snout than in the nominate race. They are dark in colour, with large dirty-yellow spots over the back. Only one specimen, collected in 1897 on George's Island in the Solomons, was known to science until live specimens were imported to the U.S.A. in the 1980s. These animals are said to come from forests on the larger neighbouring Ysabel Island, where typical mangrove monitors are also found. On this basis Sprackland (1993, 1994) considers it a separate species.

The mangrove monitor attains different sizes in different parts of its range. Swanson's (1976) claim that they reach 200cm TL is probably an overestimate. Schmida (1985) claims they reach 150cm TL and Cogger (1981) gives a total length of 100cm for Australian specimens. In the Solomon islands they average 50cm SVL (125cm TL). The largest from New Britain examined by Hediger (1934) was 124cm TL. On islands and cays of eastern New Guinea they reach 100cm TL whilst on Ceram they reach almost 140cm TL (Edgar, pers. comm.). The largest from Lego, northern Papua, was 34cm SVL (Room 1974). The largest found on Guam by Wikramanayake & Dryden (1988) were 58cm (male) and 44cm (female) SVL with weights of 1900g and 500g respectively. Whether such great size differences between the sexes exist throughout the range is not yet certain, but seems likely.

Mangrove monitors are always found close to water. In northern Australia they are restricted to mangroves (Cogger 1981) and in New Guinea they are never found far from the coast. In the Yap group they were found in forests and swamps (Fisher 1948). In the Solomons they are said to prefer more open areas to thick forest and to be common in coconut plantations (McCoy 1980). In mangrove swamps they prey largely on crabs (Dryden 1965, Uchida 1967, McCoy 1980). They  also take small mammals, snails, birds and their eggs, other lizards, the eggs and young of turtles and crocodiles, slugs, worms and a variety of insects (Tanner 1951; Swanson 1976; Webb et al 1977, Dryden & Taylor 1969, Losos & Greene 1988). In captivity they prey avidly on fish and these are probably taken in the wild where conditions permit. Traeholt (1993) noted that water monitors appeared unable to catch fish in deep water, but mangrove monitors appear to have no such problems and in captivity have been known to polish off large (and expensive) shoals of rainbowfish in just a few hours (Behler, pers.comm.). The fact that prey animals are taken from underwater, on and below the ground and from trees testifies to  the ubiquitous behaviour of this monitor lizard. They swim and dive as well as any other goanna, can climb with great agility, leap from high trees or rocks and can dig proficiently (Hediger 1943, Bustard 1970). In some areas they spend most of their time in the water, either resting or looking for food, whilst elsewhere they are more terrestrial in habit. Similarly in many areas they will take refuge in water when alarmed, but in the Solomons, for example, they are more likely to take shelter in a tree. Studies in Guam have shown that at least during the warmer parts of the year activity occurs only in the mornings (Dryden 1965). Here most breeding probably occurs early in the dry season (Wikramanayake & Dryden 1988). In captivity specimens from Guam have laid eggs in most months of the year (Groves pers.comm.) and Brandenberg (1983) examined recently hatched animals collected in almost every month of the year from New Guinea. Mating and combat, performed quadrupedally, are described by McCoid & Hensley (1991).

The clutch size of this species is probably smaller than would be expected of a medium sized monitor lizard, on account of the very small size of the females. A specimen from Guam laid 22 eggs over 3 years in clutches of between 1 and 4 (typically 2) eggs (Groves, pers. comm.). Luxmoore et al (1988) record a clutch of five eggs. McCoid (1993) reports that captive females from Guam lay up to  four eggs in a one week period and a 38cm SVL road killed female contained ten shelled eggs. Kukol (1993) reports that a female of unknown origin and unreported size laid 30 (infertile) eggs between September and January in clutches of up to nine eggs. Wesiak (1993a&b) reports that a captive female laid 25 eggs in five clutches over 26 months, with an average of 88 days between the last four clutches. Each clutch contained 4-6 eggs. Such observations indicate that when food is abundant mangrove monitors may attempt to reproduce continuously, producing large number of relatively small clutches. In this way a small bodied female can maximise her reproductive output.

Mankind appears to have introduced the mangrove monitor to a number of Pacific Islands in the last few decades. According to Uchida (1967) they have been present of Ifaluk in the western Caroline Islands only since the Second World War. Dryden (1965) reports that the Japanese may have introduced the lizards to Japtan in the Marshall Islands before the war. The lizards flourished and soon began to raid the local chicken houses. When the Americans arrived the locals asked them for help in getting rid of the monitor lizards. They responded by introducing the dreaded marine toad (Bufo marinus) which proved toxic to the lizards. As the lizard population dropped, however, the rat population began to rise. The Americans were asked for help once again, this time to get rid of the toads, but unfortunately there appears to be no record of their response (Owen in Dryden 1965). Gressit (1952) notes that marine toads were introduced to the Palau Islands for a similar reason, and suggests that the demise of the monitor lizards may have led to an increase in numbers of beetles known to be coconut pests. McCoid et al (1994) note that numbers have declined on Guam in the Marianas Islands.

The mangrove monitor has a beautiful skin and as a result it is hunted in many places for its leather, which is used for drum heads and other purposes. It seems strange that international trade in this species is so small. Mertens (1942) referred to it as one of the most heavily exploited monitor lizards. In 1980 trade in over 13,000 skins was declared but since then numbers seem to have been minimal (Luxmoore et al 1988). The species is said to be protected in Indonesia and so it seems likely that they are referred to as V.salvator in CITES documentation. In many places they are used as a food source but may also be persecuted because of their reputation for preying on domestic animals.

In captivity the mangrove monitor can be a very timid and nervous animal. They need to be provided with a spacious enclosure that allows them to climb, and provided with plenty of hiding places. Mangrove monitors will bask at temperatures up to 50oC. A large pool of water is essential for these animals' wellbeing. Because males tend to be much larger than females courtship and mating can be violent (Polleck 1979; McCoid & Hensley 1991). The first report of captive breeding comes from the Philadelphia Zoo, where a pair were housed in a 1.5m2 enclosure. Many clutches of eggs were produced and finally two hatched after 198-199 days at 28-29oC (Groves, pers.comm.). Wesiak (1993a&b) housed three adults together in a 3.4m2 enclosure with a large pool of water and records an incubation period of 174 days at 26.5oC. Kok (1995) records eggs hatching after 152-182 days at an unspecified temperature. Hatchlings weigh about 25g, measure 27cm TL and will immediately accept baby mice and insects. Because females produce huge amounts of eggs particular attention needs to be given to their nutritional needs. Suitable diet should include large insects, fish, birds, eggs and mammals. It seems likely that many of the animals sold under the name V.indicus are actually V.doreanus.

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