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Sexing Monitor Lizards Print E-mail
Captive Care of Monitors
Extract from A Little Book of Monitor Lizards © D. Bennett 1995. Viper Press, UK

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 described 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 the genitals of your lizards.

Another method 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.

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