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Varanus exanthematicus Print E-mail

Bosc's monitor is predominantly a creature of grasslands and in Ghana they are most common in grasslands subjected to some agricultural use. However the species also occurs in small numbers in the woodlands of the forest-grassland transition zone at Nwanta in the Kyabobo National Park. Unusually, the diet of monitors in this area appear to consist of small mammals as well as invertebrates. Although they are absent from rainforests, genetic analysis (using randomly amplified DNA polymorphisms) suggests that the rainforest belt does not act a barrier to gene flow between the populations in Guinea and coastal grasslands. The remaining "forest belt" is a mosiac of forest patches and farmland that the lizards may have been able to colonise (Bennett 1997)
In Ghana this species builds shallow nests (15-30cm deep) in fields that contain up to 15 eggs. The three nests I have examined showed an unusually high hatch rate of 100%. According to trappers in Ghana, who collect the eggs for artificial incubation, nests are also located under the ground in elevated areas (especially on hillsides). Eggs may be found in termite mounds, but it is unclear whether these mounds are active or abandoned. Hatching occurs in March and April. In the coastal grasslands of Ghana young Bosc's monitors reach their greatest abundance in farmlands with sandy soils, where up to 55 specimens can be found in less than 150,000m2 during August and September, equivalent to a biomass of 36kg of baby monitor lizards per km2! The animals shelter in shallow burrows, particularly those of large crickets, and often take to trees, especially in wet weather. Where food is abundant most individuals remain in areas of less than 500m2 for periods of at least five weeks. Their diets consist largely of slugs and orthopterans together with lesser quantities of beetles and wasps, but within a few months they also eat frogs, snails and  scorpions. The youngsters grow very quickly and within two months some are large enough to swallow their smaller siblings. Between 0900 and 1630 hrs. body temperatures range from 26.5-38oC and tend to be lower than the body temperatures of adults. According to Cisse (1971) the animals may emerge from their retreats and commence activity without basking, but in undisturbed grasslands in Ghana we found what appeared to be cleared basking sites located very close to burrows used by juvenile monitor lizards. Yeboah (1993) found that Bosc's monitors commenced activity about an hour before Nile monitors where the animals were sympatric. In August and September, when Bosc's monitors in Senegal are said to be most active, adult lizards in Ghana appear be largely immobile and do very little feeding (Bennett & Akonnor 1995, Bennett 1997).

Schmidt (1919) stated that in Zaire Bosc's monitors may be inactive during the dry season.  He also provided a picture of a specimen feigning death with one of its hind feet in its mouth. Of 250 specimens caught by us, none attempted to play dead, but one that was presumed to have been injured during capture assumed an identical posture to that in Schmidt's photograph, from which it had failed to recover after 4 days.

A recent article about the diet of this species (Good 1998) was very misleading on the subject of the diet of this species and on other aspects of their ecology. Most of the referenced information in the article is confused or in error.  I have examined the stomach contents and fecal samples from over 200 of these animals in the wild. Only one specimen was recorded as having eaten a vertebrate, the rest had fed only on invertebrates.

In captivity Bosc's monitor has a reputation for being a lethargic, if not boring, animal which can be attributed to their need for a reduction in activity during part of the year. The easy availability, small size and comparatively docile disposition of this species makes it the most suitable monitor lizard for keeping at home. The animals are generally very tolerant of each other and colonies can be maintained providing none are so small that they may be regarded as food by others. Many of the animals exported from West Africa are described as captive bred, but the usual way of collecting them is to excavate fertile eggs from the wild and hatch them artificially before shipping them to Europe and the USA. Captive breeding or successful incubation of eggs laid by wild-caught females has been described by several authors (Linley in Bennett 1992d, Bayless & Huffaker 1992, Rowell 1994, Roder & Horn 1994, Bayless 1994). An enclosure as small as 1m2 is sufficient to house a pair, providing that a suitable thermal gradient can be established (i.e. 17-43oC during the day). Cisse (1971) reported that even in captivity Bosc's monitors would refuse food and water throughout the dry season. Rowell (1994) noted that his specimens went off food from November to February but many specimens will happily gorge themselves year-round and can become extremely obese. Eggs incubated at  32oC hatch after 127-132 days, at 29oC after 169-194 days. Hatchlings weigh about 10g and measure about 12cm in total. In captivity a varied diet should be provided, which reflects the animals' insectivorous diets. Juveniles should be fed as much food as they will eat, adults should be fed with care and intake reduced when they begin to look too fat.


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