The tail of a monitor lizard is truly a multi-purpose organ. The base can hold an enormous amount of fat which may be utilised when food is scarce. It is a very efficient weapon when used as a whip and can be deployed with great force. In aquatic monitors the tail is used to propel the animal through the water, in burrowing species it is often used to block the burrow entrance, whilst in arboreal species it aids balance and in several species it is able to grip like an extra limb. Most of the larger monitors can also use the tail as a prop to enable them to stand on their back feet, either to gain a better view of their surroundings or as part of a threat display.
The length of the tail is often expressed as a percentage of the animal's body length measured from the tip of the snout to the vent, referred to as the SVL. So the total length (TL) of an adult short-tailed goanna is about 20cm, made up of l0 cm each of head and body and tail - in other words "the tail is 100% of the SVL", whereas in a Glauert's goanna with 20cm of head and body and 50cm of tail "the tail is 250% of the SVL". This terminology, though sounding ominously mathematical, is a convenient way of comparing the relative tail lengths of different species. Monitor lizards have tails which are between 80 and 250% as long as their bodies. The longest tails belong to the Australian rock monitors (Glauert's, kings' and the twilight goannas) and the mysterious tree crocodile of New Guinea. In the Australian lizards the long tail of the tail probably aids balance when leaping from rock to rock in pursuit of prey . It is often vividly banded or marked at the tip, suggesting that it could serve as a decoy against aerial predators. In tree crocodiles almost 200cm of tail provide a magnificent climbing aid, enabling these massive lizards to be as agile as monkeys in trees. The tail can be wrapped around branches for support and is strong enough to support the entire weight of the lizard during a rapid descent from a tree. The smaller New Guinea tree monitors are able to use their prehensile tails in an even more precise way and may mate in mid air anchored to a branch only by their tails.
Several of the small Australian monitor lizards have spiny scales on their tails. The spines act as armour plating, and are used in at least two ways. When sheltering in a rock crevice the tail is used to wedge the lizard firmly in position. When in a burrow it can be used to block the entrance, in both cases making the animal very difficult for a predator to dislodge. The spines are sharp and tough and deter many animals that would otherwise make a quick meal out of the diminutive lizards. When cornered the spiny tail becomes a spiked club, quite capable of drawing blood from a human hand. Some other large monitors (including the Bengal. Nile and Bosc's monitors) also use the thick tail base to block their burrow entrances. Instead of being covered in spiny scales, however, they are comparatively smooth, but very powerful, and capable of preventing many potential predators (such as snakes) from gaining entry. Desert species and others that face long periods without food are able to store substantial amounts of fat in the tail base as well as within the abdomen. Whether the fat stored in the tail is used in a different way to that of the abdominal reserves is unknown. Even in animals a few months old the circumference of the tail base can vary by over 100% in animals of similar lengths. reflecting the different rates of feeding, and therefore growth, berween siblings (Bennett & Akonnor 1995).
A number of large monitors are able to stand on their back legs alone for short periods of rime , fonning a tripod with the tail , allowing a much better view than is possible from a few centimetres above the ground, especially in tall vegetation. When monitors are threatened, either by predators or by their counterparts, they sometimes adopt this pose to make themselves appear larger or in preparation for ritual combat. Dwarf monitors appear to be unable to stand bipedally.
Species that spend time in the water tend to have a long tail that is laterally compressed. Mertens' goanna provides the most exrreme example of this; its tail is very high with almost vertical, flat sides. The speeds attained by swimming monitor lizards have not been measured, but it is tikely that those with strongly compressed tails would be capable of exerting much more thrust in the water than those with more rounded appendages. With a suitably shaped tail varanids are able to swim very efficiently and many species have been seen in the sea far from the nearest land. Because of their proficiency in water both water monitors and the mangrove monitor have spread over huge areas and inhabit numerous islands. Many monitors that rarely encounter water (e.g. Bosc's monitor, Caspian monitors and perenties) also have compressed tails, which they have presumably inherited from ancestors who lived more amphibious lives (Mertens I 942c).
The tail also has an important use as a weapon of active defence. Stories of a blow from a monitor lizard's tail breaking peoples' bone are probably exaggerations but they are certainly capable of stunning many smaller animals. At least two species (tree crocodiles and Caspian monitors) are recorded as having specifically struck at the eyes of human aggressors with the tail tip (Murphy 1972, Bennett 1992a). A well aimed blow can have devastating results. Eidenmuller (1993) notes that sand goannas will use their tails to flush out prey animals that have secreted themselves in inaccessible crevices.
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