Defence

Monitor lizards are good to eat and all but the largest specimens are subject to predation by reptiles, birds, marrunals and even fish. Their patterns provide them with a high degree of camouflage in their natural environments and their ability to "sniff out" potential predators allows them to avoid many confrontations. The literature suggests that snakes are among the most important predators of monitor lizards in many areas.

The defence of juvenile monitor lizards rests with their secretive behaviour, congregational behaviour and cryptic or extraordinary colouration. Very few observations have been made on juvenile monitors in the wild, although they are often present at much higher densities than adults, and this is testament to their ability to live undetected. Small monitor lizards escape predation by being extremely wary and dashing for cover at the first sign of trouble. Larger monitors have far fewer predators and tend to be more relaxed in the open. Often they freeze when they see people, rather than draw attention to themselves by running, and can remain motionless for long periods, keeping a very close watch on their adversary. If the observer is patient enough to remain still, the lizard may resume its activity, ignoring its audience. Many large monitor lizards do not appear to associate cars or bicycles with danger and some individuals have been described as fearless, inquisitive and even provocative. When running they sometimes attain great speeds and when in good form can easily outrun most people over 100 metres. There is little data available on the speeds attained by monitor lizards when running but they can certainly achieve sustainable speeds much higher than those of many other lizards (Lim 1958; Gleeson et al 1980; Auffenberg 1981; Velentic 1994). When running away from trouble monitor lizards head directly for a safe retreat; many select tight burrows or crevices in which to shelter from predators, often with several different exits (see below). Others seek safety in trees and yet others take refuge in water.

The situation becomes very serious when the lizard has been detected by a predator and escape is impossible. At least two species, Bose's monitor and the rough-necked monitor, are reported to feign death with eyes wide open (Schmidt 1919; Horn & Petters 1982). However not one of over 250 Bosc's monitors that we have caught in Ghana has attempted to use this ruse, suggesting that the behaviour occurs only in certain populations or amongst certain age groups. Perhaps playing dead is used only in response to attack by certain predators, most would be as happy to eat a dead lizard as a living one.

When attacked or threatened monitor lizards will almost always attempt to escape, but should confrontation become inevitable they defend themselves admirably. The threat displays of various species have been described and analysed e.g. V.mertensi (Murphy & Lamoreaux 1978) V.komodoensis (Auffenberg 1981), V.olivaceus (Auffenberg 1988), V.griseus (Bels et al 1995). In general the lizards attempt to make themselves look as large as possible by standing upright on all four legs, flattening the back and inflating their lungs to the maximum extent. This effectively quadruples the volume of the body and makes even a very skinny specimen appear extremely robust. With one eye fixed firmly upon its aggressor the lizards emit a malevolent hiss. Some large monitors may stand up on their hind legs at this stage and present a very intimidating sight, especially when swaying from side to side in the manner of a charmed snake. The effect is completed by distending the throat and opening the mouth to its widest extent, greatly exaggerating the size of the head. A monitor Lizard may remain in this posture (bipedal or quadrupedal) almost indefinitely, the hiss being periodically interrupted whilst the lungs are refilled. If the aggressor approaches closer the lizard may lunge with the mouth open and the tongue fully extended. These lunges are almost always bluff however and the lizard falls short of its target. The only physical contact the lizards will entertain with their enemies are sharp blows from the powerful tails. Large specimens are supposed to be able to knock people unconscious or break their Limbs but often the tail is used much more subtly and blows directed at particularly delicate parts of the anatomy. Only if an attempt is made to seize the animal will it resort to outright violence. Even small monitor lizards are very strong and fierce, so would-be predators must be careful that they do not end up worse off after an encounter with their prospective meal. Monitor lizards deliver a particularly savage bite and once they have seized something they may hold onto it like a bulldog. There is an old, but doubtless accurate, account of an Indian snake charmer who whose hand was seized by a Bengal monitor whilst he was trying to extricate it from its burrow. All attempts to free his fingers failed, and eventually his friends were forced to cut the jaws open with scissors (Mahendra 1930). By patiently hanging on to predators they may be able to exhaust them and then make a quick escape (Stevenson-Hamilton 1947). A number of monitor lizards have interesting relationships with cobras, where either animal may prey on the other (e.g. Bharatan 1971). It has been suggested that many monitor lizards are immune to the venom of cobras and vipers (Rjumin 1968; Root 1978) but in the absence of conclusive experiments this is contested by many people.

Attribution / Courtesy: Daniel Bennett. 1995. A Little Book of Monitor Lizards. Viper Press U.K.