Boscs monitor lizard
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 searce. 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.
Mertens (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. Bose'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 live 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
"exanthematieus" 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
. However animals from this region are recorded as eating birds
(Piunan 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
" than to V.exanthematicus
Schmidt (1919) recorded Bose's monitors from near Garamba in Zaire and included a picture
of a specimen which clearly fits this deseription. 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.
Bosc's monitor can reach a total length of over 1000m, 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 enonnous 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.
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
Mayor 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
rel:orded as eating mantids, hymenopterans, lepidopterans, scorpions, snails and the eggs of
hoth 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 (l993) who also
found that crabs were included in the diet.
There are no published records of egglaying sites in this species, but according to trappers in
Ghana, who collect the eggs for artificial incubation, most nests are located under the
ground in elevated areas (especially on hillsides). Eggs are also found in termite mounds, but
it is unclear whether these mounds are active or abandoned. 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 mole crickets, and often take to trees,
especially in wet weather. Where food is abundant most individuals remain in areas of less
for periods of at least five weeks. Their diets consist largely of orthopterans but
they also eat frogs, snails and scorpions, together with lesser quantities of beetles and wasps.
The youngsters grow very quickly'and within four months some are large enough to swallow
their smaller siblings. Between 0900 and 1630 hrs. body temperatures range from 26.5-38°C.
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 monitor burrows. Yeboah (l993) 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 & ms).
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.
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 & Huffaker1994, 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. 17-40°C 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 hi s 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 32°C hatch after 127-132 days, at 29°C after 169-194 days. Hatchlings weigh
about 109 and measure about 12cm in total. in captivity a varied diet should be provided,
which reflects the animals' insectivorous diets. Although they do not appear to eat many
vel!ebrates in the wild they will happily take small mammals in captivity. 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.