Smell and taste mean the same thing to the monitor lizards and play a very important role in social behaviour and in feeding. Their tongues are more highly adapted to detect scent than in any other lizard family and, like snakes, it has lost all functions other than its sensory one (although some species use the tongue to lick up small insects) (Mertens 1942a, Smith 1986). Taste buds are present on the tongues of at least some monitor lizards (Schwenk 1985), but by far the most important olfactory organs are the well developed Jacobson's organs, located near the tip of the snout. Active monitor lizards continually flick out their magnificently long tongues whilst active, and in doing so take samples of the air to the Jacobson's organs, which are paired so that each receives information from one fork of the tongue. Equipped with this very sensitive apparatus monitor lizards can detect faint smells. So faint in fact, that one species can smell carrion from as far as 11km away when conditions are suitable. It is believed that the lizards are able locate the source of the smell by determining whether the scent is stronger on the left or right fork of the tongue and moving accordingly (Oelofsen & Van Den Heever 1979; Auffenberg 1981). In many species eyesight is relied upon to find most living food. but it is never eaten without first being investigated with the tongue. ill many species this sense is relied upon to the extent that the lizards will swallow almost anything of a manageable size, providing it smells right. Aluminium foil and porcelain eggs (used to encourage chickens to lay) have been found inside wildcaught monitors, presumably because the former had been used to wrap meat, and the latter had been sat on by a hen (Luckhoff in Anon 1937). Wesiak (1992) reports that spiny-tailed monitors become so excited by the smell of blood that they will attempt to consume their own injured limbs. Monitor lizards can follow a prey animal's scent for considerable distances, and also hunt for food underground and underwater using their remarkable olfactory powers.
The social behaviour of monitor lizards is very highly scent orientated. Many keepers will deny it, but monitor lizards have a very distinctive smell. Male water monitors will attempt to engage in ritual combat with a gloved arm that bears the odour of another male (Groves, pers. comm & pers.obs). Male white-throated monitors have been seen attempting to mate with dead females that still smell fresh (Lambiris 1966). Most monitors show very little external sexual dimorphism. This means that males and females look alike, so that often we can only determine their sex by internal examination. But the lizards recognise each other instantly by smell. In some areas they may be acquainted with the unique scents of many of their counterparts, and know which ones to avoid and which to follow. At least one author has reported a noticeable smell emanating from mating couples in the wild (Tasoulis 1983) and it is often possible to sniff out burrows that monitor lizards have slept in, even if the animal are no longer there. Several authors report that males follow the precise paths taken by rival males and females. A group of Bengal monitor lizards that escaped their captor in Florida hid themselves in the same tree that other escapees had used months before, although there were hundreds of other trees in the area (Auffenberg 1983a). Presumably the first monitors had marked the trees with long lasting chemical signals that the later escapes recognised and followed for safety. Studying olfactory behaviour in the wild presents enormous difficulties which have yet to be overcome. It is not yet clear by what mechanisms the lizards release their pheromones. In some species scents seem to be left in faeces, which may be deposited strategically (Auffenberg 1981). Larger, dominant, animals probably leave faeces in more prominent places than their weaker counterparts. They may be used to warn off other monitors from strategic areas such as favourite basking places and strong burrows. More usually scent signals are produced in skin glands and rubbed onto the ground or onto strategic objects such as trees or termite mounds. At present little is known about these glands and nothing about the chemicals they produce. Monitor lizards spend a lot of time investigating each other with their tongues, particularly around the head, shoulders and the pelvic girdle, so it can be presumed that the glands are most common in these areas. Some species, such as mournful goannas. drag their tail base along the ground while walking. This presumably leaves a trail of scent which "an be detected by other monitor lizards. Lace goannas leave scent signals by rubbing their vents on the ground or by wiping their heads on hard surfaces (Christie 1984: Carter 1990). Most scent deposition is doubtless done in subtle ways that have so far eluded the eyes of researchers'.
Having a distinctive smell can be dangerous however. because they reveal the animals' presence to larger predators. especially other monitor lizards. Presumably mechanisms exist which help disguise strong smelling lizards from predators with a keen sense of smell. Juvenile Komodo dragons have been seen to disguise their scent by rubbing themselves in unappetising bits of carrion (Auffenberg 1981). It is likely that monitor lizards are able to avoid an number of potential predators by recognising their scent. Experiments have shown that young captive born V.aLbigu/aris avoid foods that smell of cobra or viper, but happily take food smelling of harmless snakes. Furthermore the smell of venomous snakes initiates defensive behaviour, even if the lizards have never seen a snake in their lives (phillips & Albert 1992).
The extent to which olfaction regulates the behaviour of monitor lizards, particularly in their social lives, is probably much underrated . Humans have an extremely poor sense of smell, which, of course, is why we know so little about it.
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