Varanus exanthematicus

Boscs monitor lizard

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

Varanus exanthematicus  Bosc 1792

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Bosc's monitor lizard is the smallest and most poorly known African varanid. Although it is regularly available in the wildlife trade, details of its natural history are scarce. Virtually all that is written about V.exanthematicus in the literature actually refers to the larger white-throated monitor, V.albigularis. They can be distinguished by the number of scales around the body (see under V.albigularis) and by the enlarged, flattened neck scales often present in specimens of Bosc's monitor.

ImageMertens (1942) and other authors thereafter have given the range of Bosc's monitor as extending from Senegal as far as Eritrea and northern Zaire. The species is well known throughout the grasslands of West Africa, but its distribution in central and eastern parts of the continent is less certain. Bosc's monitor does not occur in rainforests nor in deserts, and thus the northern and southern limits of its distribution are restricted by the Sahara and the belt of rainforest that covers much of central Africa. Most of the pet trade specimens come from the isolated belt of savannah along the southern coast of West Africa, particularly from Ghana and Togo. Schmidt (1919) recognised the species V.ocellatus Heyden 1830 for the "exanthematicus" type animals from eastern parts of the range (Sudan and possibly southern Egypt), but Mertens, who cites many locations in Sudan, considered these animals to be typical V.exanthematicus. However animals from this region are recorded as eating birds (Pitman 1962), a habit associated with the larger white-throated monitor rather than its smaller cousin. Anderson (1898) and Muller (1905) also expressed the opinion that V.ocellatus was more similar to V.albigularis or "V.microstictus" than to V.exanthematicus. Schmidt (1919) recorded Bosc's monitors from near Garamba in Zaire and included a picture of a specimen which clearly fits this description. There is no evidence that Bosc's monitor and the white-throated monitor occur anywhere together, but the exact limits of their distribution and the true identity of V.ocellatus have not been determined. I have examined the skins of many Varanus exanthematicus in Egypt that had been imported from Sudan. All are indistinguishable from typical V.exanthematicus skins from Nigeria. I therefore presume that V.occelatus = V.exanthematicus
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Bosc's monitor can reach a total length of over 100cm, but such specimens must be rare in the wild. However Yeboah (1993) records average length of 16 adults in Ghana to be 130cm TL and there is an enormous specimen from Togo in the collection of the  Koenig Museum in Bonn. A rather thin specimen we found in Ghana measured 41cm SVL and weighed only 975g. In good condition this animal may have weighed twice as much. Bosc's monitors can accumulate massive fat reserves and obese specimens are common in captivity, often weighing in excess of 6kg. Females tend to be smaller, but more heavily bodied, than males and may have slightly shorter tails. Cisse (1976) found females as small as 500g in Senegal that contained eggs. Smallest I haev found weighed 390g..

Bosc's monitor is a shy animal. Its small size, highly seasonal patterns of activity, undistinguished colouration and secretive habits mean that they are easily overlooked and often considered rare in areas where they are actually very abundant. Unlike Nile monitors, they avoid human habitations and it is very rare to encounter one above ground, other than to see them dashing across roads.  The most complete study of this species was conducted in Senegal by Cisse (1971, 1972, 1976). Here the animals have a strongly seasonal activity pattern, fasting and remaining in shelters for six months of the year, from December until late May or early June, when the weather gets cooler and then becomes very hot. Where trees are available they are used for shelter, elsewhere they take refuge in burrows (especially those dug by ground squirrels) or abandoned termite mounds. Activity commences with the beginning of the rains and reaches its peak from July to October, when most food is consumed and mating occurs. Females can produce up to 41 eggs in a single clutch, which are laid in October and November and probably hatch in June or July the following year. Beetle larvae, millipedes and centipedes are the most common food early in the wet season and are replaced by increasing numbers of orthopterans later in the year. they are also recorded as eating mantids, hymenopterans, lepidopterans, scorpions, snails and the eggs of both agamids and their own kind. Most prey are found on tree branches, in soft earth or under ruminant dung. During the height of feeding activity ingested prey can account for over 10% of the body weight. A similar diet is reported in Ghana by Yeboah (1993) who also found that crabs were included in the diet.

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)
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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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