Breeding monitor lizards in captivity is still an exceptional occurrence. Many species have
never reproduced outside their natural range and consistent reproduction over several years
is still almost unknown amongst the larger members of the family. Luckily many of the
people who succeed in breeding their animals take the time to write down their experiences
for the benefit of others. Unluckily (for the linguistically-inept inhabitants of the Englishspeaking
world) the vast majority of the literature on breeding varanids is in German.
Translations of some works can be obtained from the SSAR.
Obtaining a male and female
Obviously in order to attempt to breed monitor lizards it is necessary to have at least one
male and one female. Unfortunately they are notoriously difficult animals to sex. In a few
dwarf species males have spines at the sides of the vent which are only weakly developed or
absent in females, but in at least one species (Storr's goanna) the spines are present in both
sexes. Sex determination by differing scale characteristics around the vent has been
for the Komodo dragon and the Bengal monitor (Auffenberg 1981, Yadov & Rana 1988). In
both of these species and many others males are reported to grow larger than females and the
males of many African and Asian species develop bulbous snouts towards old age. These
phenomena are of little practical use when trying to determine the sexes of wild caught
animals whose ages are unknown, because obviously a four year old female will be larger
than a two year old male. The males of some species are said to have wider heads or thicker
tail bases than the females and although there is no reason to doubt this, such criteria are of
limited use when animals from different locations and of unknown age are concerned.
Sex of monitor lizards is often determined on the basis of hemipenal eversion (notably in
field studies in which the animals are not sacrificed). Wild animals unaccustomed to being
handled often evert their hemipenes when being manipulated. If the hemipenes are totally
everted then the animal is considered a male. But if eversion is only partial it is impossible to
distinguish the male hemipenes from the similar structures everted by the females', unless
equipped with a very good knowledge of the differences in their morphology.
Furthermore, an animal that does not evert its hemipenes cannot definitely be considered a
female. It might be an uncooperative or not particularly well endowed male. Immature
animals do not evert their hemipenes and once the lizards have been handled a few times they
generally stop displaying this behaviour. Acclimatised adults can sometimes be induced to
evert their sex organs by spraying the vent with tepid water; otherwise hold the monitor
upside down, push the tail down and apply gentle rolling pressure with the thumb over the
vent, from back to front. Again, this may serve to determine the sex of some males with
certainty but great caution must be exercised when interpreting the results from animals that
fail to evert completely.
A number of more intrusive methods to detect hemipenes have been devised. A common
method. used to sex many reptiles, is to probe very gently inside the vent with a lubricated.
blunt. metal instrument (Honneger 1978). In theory, the probe will extend further in males
than in females. The problems with this method are that most monitor lizards are too strong
to allow the probes to be inserted without risk of damage. many females probe almost as
deeply as males and the lizards appear to find the process extremely unpleasant. This is a
method that can only be learned by direct observation of someone with experience. Other
methods that are designed to forcibly evert the hemipenes require professional expertise (e.g.
Balsai 1992). In general it is best not to interfere with your lizards' genitals.
Another method of sexing monitors is to use x-rays to detect mineralisation in the hemipenes
(Shea & Reddacliff 1986. Card & Kluge 1995). A vet can do this by rinsing the vent with a
barium solution or dye and taking an image at about 57.5kV. 125mA for 0.04 seconds or
40kV. 25mA for 0.6 seconds. This method has been shown to be effective in many species
but not in Bosc·s. Bengal, grey, Mertens', Nile, rough-necked, water and Timor monitors.
Nor does it work with young animals. Also it is not clear from the literature whether the
opaque material is definitely missing in females.
The most reliable method of sexing monitor lizards is by endoscopic examination (Schildger et al
1993, Schildger & Wicker 1992). This can only be done by a vet and involves
anaesthetising the animal, making a small hole in its body wall, inflating the body cavity with
gas and viewing the ovaries or testes through an endoscope. In the hands of an expert this
procedure looks very simple. The examination may also reveal internal disorders that would
otherwise go undetected until manifesting themselves as sickness. Again this method may be
ineffective when applied to young animals, but when used on adults it is virtually infallible.
Because male monitor lizards are more active than females they tend to be caught more
easily, and as a result females are often more difficult to obtain. Because at present there is
no easy way of sexing most monitors with 100% accuracy, the best policy to ensure the
presence of a pair is to obtain about half a dozen young monitors. allow them to grow up
with alternating periods of solitary and group confinement (if they will tolerate each other)
and try to deduce their sex by their behaviour towards one another. Unfortunately there are
few obvious sex-specific behaviours in monitor lizards. Males often attempt to mate with
each other and animals of both sexes will engage in ritual combat
There are many other good reasons why juvenile animals are to be preferred. They adjust
better to life in captivity, are less likely to be loaded with dangerous parasites and if cared for
properly will live for at least a decade. Usually it is not possible to determine where a captive
animal originated. This is unfortunate because some species have enormous ranges and
specimens from very different habitats may prove incompatible. This is especially true of
Bengal, Nile, mangrove, desert, water and white-throated monitors. Reports seem to indicate
that breeding has occurred between some "subspecies" of monitor lizard. but it is not known
whether fertile offspring were produced. Ideally the potential breeding stock will not be very
closely related (e.g. brother and sister).
Conditioning the animals
The second prerequisite for breeding is that the monitors are in reproductive condition. In
nature most monitor lizards are only reproductively active for part of the year. The ovaries
and testes increase in size towards the beginning of the breeding season and shrink
afterwards. The timing of mating, and egglaying are determined by rigourous selection that
ensures that both sexes are sexually active at the same time of year and that the timing of
breeding and length of incubation provide the youngsters with the best possible chance of
survival. Control of the maturation of reproductive organs is under the control of hormones.
The stimuli that cause the release of these hormones is not always clear, but in many animals
changes in daylength are a major factor. A period of inactivity or reduced activity
may be necessary to allow eggs and sperm to develop. By attempting to imitate the
environmental conditions experienced by the lizards in the wild the chances of breeding are
maximised. Females need plenty of top quality food with plenty of vitamin and
mineral supplements in order to produce healthy eggs, but there may be no change in her
outward appearance until after the eggs have been fertilised and the only way to tell if the
lizards are in breeding condition is by their behaviour towards each other.
Housing Monitors together
Once the animals have been conditioned they must be coerced into mating with each other.
Often it is difficult to house the pair of lizards together for long enough to ensure fertilisation
of the eggs without one killing the other in the process. If immature specimens are obtained
and can be reared together they may pair off naturally without undue violence. Often it
proves beneficial to keep the animals separated prior to attempting mating, even if they will
tolerate each other at other times. Absence makes the heart grow fonder, and the sudden
reappearance of a member of the opposite sex may kindle the names of desire. If the lizards
show no sign of interest in each other or the female rejects the male's advances it is necessary
to provide stimuli in order to make both parties co-operative. Triggering breeding behaviour
takes a variety of forms. Often separating the lizards for long periods and then reintroducing
them results in courtship and mating behaviour. Introducing other animals (i.e. rival males or
substitute females) may trigger the indifferent partner into action. Sometimes a male and female are
simply not made for each other and must be found alternative partners. The order of
introduction can be varied, for example a female can be introduced to an enclosure
containing two or more males, an extra male can be introduced to a pair which are already
acquainted, or several males can be introduced to the female's enclosure. It is of utmost
importance that a close watch is kept on the lizards, and any specimens that suffer excessive
violence removed immediately.
Environmental stimuli will often trigger breeding behaviour in monitor lizards. A brief period
of cool weather often results in mating between temperate animals when warmer conditions
return. If the lizards are normally housed apart they should be introduced whilst still torpid.
Many species react almost immediately to an increase in rainfall by starting intense courtship
behaviour which may stop as soon as the water ceases to fall. In order to fully recreate
tropical storms some imaginative herpetoculturalists have been known to provide lightening
(by firing a camera nash) and even thunder (by shaking sheets of metal). Forest monitors
often do not appreciate bright nashes of light; rough-necked monitors in particular appear to
tind them disturbing. Several authors have reported that keeping the lights on 24 hours a day
can stimulate courtship. This method should really be used as a last resort because of the
considerable stress it places on the lizards.
Courtship between acquainted animals often progresses very smoothly, but in many species
courtship and mating are renown for their sadistic character. Generally, but not always,
males are more aggressive than the females. and courtship can be so violent that one of the lizards
dies of its injuries. To prevent this it is best to avoid introducing old. unacquainted animals
that are too set in their ways to take kindly to strangers. The animals should be of as similar a
size as possible or else the smaller animal provided with shelters that the larger one is unable
to enter. The danger of injury can be further reduced by clipping the claws of large males
prior to introducing them to females, but the use of muzzles designed to stop the male biting
the female usually results in a cessation of courtship behaviour. In many species violence is
restricted to a bit of back scratching and gentle "nibbling". The first sign of courtship is
usually a minute examination of the female by the male, who will probe all around her body
with his tongue. particularly around the vent, head and neck. It is likely that these areas
contain the scent producing glands that signal the females' readiness to breed . Eventually the
male climbs on top of the female, still probing furiously and often scratching at her back and
neck with his foreclaws and at her tail base with his hindclaws. Eventually he forces his tail
base under her and copulation is achieved. It may last only a few seconds or else it may
continue for several hours a day over a week or more. As soon as either party looses interest
the male should be removed, partly because they are notorious eaters of eggs, but also
because it is now the female's tum to become aggressive. Heavily gravid monitors have a
particularly aggressive disposition. At one well-known zoo a female water monitor was
digging a hole for her eggs in an outside enclosure when a large Bengal monitor happened to
be passing. The two had been housed together for a long time without complications. but the
water monitor attacked her neighbour with such ferocity that she literally tore it to pieces.
whereupon she continued her excavations
Avoid touching the female unless absolutely essential but drain any large volume of water in
the enclosure, because eggs laid there will be destroyed. Gravid females often like to bask at
high temperatures and this should be encouraged. She must be provided with suitable nest
sites. usually a nesting bolt for arboreal species or a deep tray for terrestrial species. The
receptacle should exclude light and be filled with moist, sterilised leaf litter, a moist mixture
of sand and peat or damp venniculite. A depth of 30-5Ocm is usually sufficient for even the
largest species. In outdoor enclosures it may even be possible to establish a colony of
compatible termites to provide the perfect nest (Boylan 1995). An almost certain sign that a
female is going to lay a large clutch of eggs is that she begins to fast. There is considerable
danger of females becoming eggbound and a close watch should be kept on her for any signs
of distress, whereupon the services of a vet are required immediately.
Normally eggs are laid within 28 days of copulation, but Card (1993) suggests a pause of up
to 92 days between copulation and egg1aying in V.flavirufus
. The female must be watched
carefully and eggs removed as soon as they appear. Eggs may be laid in more than one clutch
and over a period of days or weeks. After laying eggs the female should be fed as much food
as she will eat, heavily laced with mineral supplements, until she regains a robust appearance.
Sometimes with good feeding the female can be ready to mate again within a couple of
months. Once a compatible pair has been obtained it may be possible to maximise egg output
by keeping the animals on six month, rather than 12 months cycles. Temperate species should
be kept warm for four or five months and fed abundant food, then the temperature lowered
and feeding halted for a month. For tropical species four months of normal conditions should
be followed by a small rise in ambient temperatures, higher rainfall and an increase in food for
two months. Four month cycles also seems possible, but the effects of continuous egg
production on the female's health must be considered.
As soon as the eggs are laid they should be removed from the enclosure and placed in an
incubator. The eggs should be maintained in the orientation at which they were laid. Marking
the top of the eggs with a pencil ensures that they are not accidentally turned. In order to
turn into baby monitor lizards they need heat, oxygen and water. For many species the
success of incubation may be detennined more by the condition of the parent animals than by
precise environmental conditions surrounding the eggs. Studies have shown that eggs from
properly-fed females will tolerate a remarkably wide range of temperatures and humidities,
which, although they affect the length of incubation and the size of the youngsters, have little
or no effect upon the rate of successful hatchings(Eidenmuller 1992b, Phillips & Packard
1994). In general more humid conditions result in larger neonates and warmer conditions
result in a decrease in incubation time. Most monitor lizard eggs can be incubated safely at
27-29°C with 90-100% humidity. Higher temperatures tend to give slightly less successful
results. It is wise to split large clutches and incubate eggs at a range of slightly different
In order to maintain humidity it is necessary to surround the eggs with water vapour. This is
usually achieved by almost burying the eggs in a mixture (initially around 1: 1 by weight) of
water and sterile vermiculite or perlite (available from horticultural suppliers) and enclosing
the eggs and medium in a container that allows some air circulation without causing undue
evaporation. This is often achieved by keeping the eggs in a sealed container and removing
the top every few days to allow air circulation. In order to maintain humidity extra water may
have to be added to the incubation medium from time to time. Excellent results have also
been obtained using no medium and simply keeping the eggs over water to maintain a high
humidity. If the humidity is too high the eggs will swell up with water and the pressure will
destroy the embryo. Such eggs look puffy and may appear to sweat. If the humidity is too
low the eggs will wrinkle and shrivel. To keep the eggs at the desired temperature they
should be placed in an incubator, which can be bought or made, according to your
inclination. Details of construction can be found in Broer & Horn (1985), but members of a
local herpetological society will be able to provide more practical assistance. The humidity
and temperature around the eggs should be monitored constantly with a thermometer and
hygrometer and water added whenever the humidity begins to drop.
When monitor lizards lay eggs it often comes as a surprise. Once more the advantages of
being a member of the Herpetological Society are manifest If you are unprepared for the
event a fellow herpetoculturalist will hopefully be happy to provide your eggs with a safe
Eggs that are infertile or die during development should be removed from the incubator
immediately. For the first few weeks development of the eggs can be monitored by viewing
them against a strong light shone through a small hole in a dark cloth or piece of card. In
fertile eggs a visible network of blood vessels appears before they finally turn opaque.
Embryos can often be felt to move when the eggs are disturbed, but handling should be kept
to a minimum. The eggs should increase substantially in weight and slightly in size as they
develop and may begin to loose weight in the final weeks and then shrink and collapse just
prior to hatching.
Length of incubation shows great variation and may be determined genetically as well as
being influenced by temperature . In the wild the eggs may experience great fluctuations in
temperature and humidity and will hatch at a time when food for the youngsters is abundant.
Thus development is not necessarily continuous and may halt altogether during very cold
weather, although this is not documented. Under artificial conditions the eggs should be
incubated at a temperature that does not vary by more than about 2°C. Even under relatively
constant temperature the eggs may hatch after an unexpectedly long or short period.
The effect of temperature on the sex of hatchling monitor lizards is not fully understood (see
Branch 1989 for a digestible account). Each fertilised egg cell contains genetic information
on one of the chromosomes that is believed to determine whether the hatchling will be male
or female (King et al
1982). Therefore the gender of each egg ought to be determined before
it is laid. Some breeders have noted a marked difference in the sex ratios of eggs incubated a
different temperatures (i.e. at lower temperatures a higher proportion of males is recorc1ed, at
higher temperatures more females) but no hard data has been published.
After the monitor has developed fully it should poke its head out of the egg by making a slit
with a sharp ridge on its snout and, after a rest of a day or so, leave the egg altogether. The
eggs do not always hatch at the same time; some may even take twice as long as others, but
generally they should all slit within a fortnight. Unfortunately this does not always happen
with captive reared eggs, and a common problem is for the little monitor to develop fully die
in the egg. The reasons for this are probably connected with the condition of the female prior
to egglaying. The young of a poorly nourished female presumably lack the strength to break
out of their shells. As long as the eggs look healthy they should be left alone, and only if dead
eggs are found to contain fully developed youngsters and other eggs look poorly should any
attempt be made to assist hatching. The assistance of an experienced member of the
herpetological society is indispensable for this operation. Ideally, just the head of the lizard
should be freed from the egg, but in practise it is very difficult to determine the position of
the head until after the egg has been opened. A small window should be made in the shell and
if the lizard looks fully developed it can be expanded just enough to release the head. Many
lizards that lack the strength to hatch unaided die within a few weeks, but some may survive.
In all cases high humidity must be maintained around the hatching egg, to prevent the
youngsters becoming stuck to their shells. Often the hatchling lizards still have large yolk
sacs attached to their bellies which will soon shrink and drop off unaided.
Care of the Young
When the youngster eventually leaves its shell it should put into a clean terrarium with a
sunken, shallow water dish. Hatchling monitors without yolk sacs can often be housed
together, but if any of the youngsters appear weak or are much smaller than their siblings
they should be housed separately. Very young monitor lizards have a much larger ratio of
skin area to volume than adults and are much more rapidly influenced by changes in heat and
humidity. If they are allowed to dehydrate or overheat at this stage they are unlikely to
recover. They should be handled as little as possible. We know very little about the behaviour
of newly hatched monitor lizards in the wild. They are certainly very secretive animals that
spend a lot of time hidden below ground or in trees and frequent shady humid microclimates.
Great care must be taken to ensure that baby monitor lizards receive adequate and suitable
nutrition. Newly born monitors normally refuse food for the first few days because they still
have egg yolk in their bodies. Thereafter they need small quantities of foods at frequent
intervaL,. Invertebrates make the best foods for young monitors. but a variety of types may
have to be offered to induce the animals to stan feeding. Even young monitors can become
obese if they are fed incorrectly. so very small mammals (or chopped up pieces of larger
specimens) should only be used as an occasional food. All food should be dusted with a
vitamin and mineral supplement. The youngsters grow at very different rates and it will soon
be necessary to separate them. Some grow very quickly and can double or triple in weight
within a month.