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Varanus exanthematicus Print E-mail

Boscs monitor lizard

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

Varanus exanthematicus  Bosc 1792

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
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.



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Monitoring Individuals 1

butaan1.jpgButaan are so shy they frequently remain in a tree for more than a week after being frightened. A large male we rescued from a trap hid in a tree for 22 days before coming down!* . Most lizards do not appear traumatised by being caught and released by scientists, and resume normal activity very quickly. But we think that butaan, especially older individuals, may permanently alter their activity areas after such an encounter. Because the animals are so shy, and highly vulnerable to human disturbance, we have had to develop a range of techniques that allow us to learn about them with the absolute minimum of interference.





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