Body size in monitor lizards shows greater variation than in any other family of animals
(Pianka I995) with adult animals weighing between 15g and 250,000g. The family includes
both the largest living lizards and the largest lizards that have ever existed, yet about a third
of the living species are dwarfs that seldom exceed 500g in weight. Such massive size
disparity between species makes the group ideal model animals to study the effects of
gigantism on ecology and physiology, but at present little of their potential in this field has
been explored (McNab & Auffenberg 1976; Auffenberg 1981; King 1991; King & Green
1993, Case & Schwaner 1993, Pianka 1995). Obviously the size of an animal's body has a
major bearing on the types of food it can eat and the habitats it can utilise, as is amply
demonstrated by the changes in morphology that occur in the transition from juvenile
monitor lizards to adults (see below). But for reptiles body size is perhaps particularly
important from the point of view of water conservation and thermoregulation. The larger a
monitor lizard is the slower its rates of water loss, heating and cooling are likely to be. Most
big monitor lizards rely on massive numbers of much smaller animals to sustain them, which
they collect by spending long hours foraging over wide areas. Large monitor lizards are not
able to raise their body temperatures as quickly as those of a smaller size, but they can
undertake prolonged activity in open sun that would rapidly fry their diminutive counterparts
and because they loose heat less quickly they can remain active for longer in
conditions. Also their size deters many of the predators that would attack smaller lizards.
With the fonnidable exception of mankind (and his dogs), there are very few records of
mammals preying on large monitor lizards. Their usual enemies are big reptiles (particularly
snakes and other monitor lizards) and birds of prey. As adults, a number of large monitor
lizards have no predators other than man and this, combined with their ability to withstand
inclement temperatures, means that can wander about oblivious to at least some of the usual
pressures of reptilian life.
In almost all species that have been scrutinised male monitors attain greater lengths and
weights than females. This difference is presumed to be due to the need for females to devote
a great deal of energy to egg production. Possibly their smaller size would make them more
vulnerable to predation, but they also tend to be less active than males (spending less time
moving around and covering smaller areas) and thus reduce their exposure to danger.
This is of particular significance to the herpetoculturalist, because it means that females are less
likely to be collected using most trapping methods and are therefore scarcer in captivity.