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.