Monitor lizards are able to conserve water much more efficiently than most other wrtebrates.
Works on the water balance of reptiles usually make very dry reading, but Green & King
1993) provide a particularly engaging, lucid and comprehensive account of this subject. That
monitor lizards vary in their abilities to conserve water is not surprising considering the
variety of habitats they occupy. Species from areas where fresh water is always available
dehydrate very quickly under dry conditions (Cloudsley-Thompson 1967). Monitors living in
areas where freshwater is scarce, such as coastal regions and deserts must take measures to
prevent excessive water loss. Physiological adaptations (discussed in Green & King 1993, see
abo Braysher & Green 1970, Green I 972a&b) ensure that very little water is lost as urine. In
arid areas the need to conserve water may influence behavioural patterns as much as
thermoregulatory considerations. The retreats used by monitor lizards to ameliorate
temperature changes also help them to conserve water. The humidity inside burrows, tree
hollows and plant thickets can be much higher than in open spaces. Some monitors remain
underground for several months during the driest part of the year (e.g. Rosenberg's goanna
(Green and King 1978), Bosc's monitor and the Nile monitor (Cisse 1971) even though
suitabk temperatures exi st and at least some food is available. The ability to survive on very
little water may have been the crucial factor that allowed monitor lizards to persist in
Australia when all other large carnivores perished.
The animals that are eaten by monitors usually contain plenty of water, allowing the lizards
go for long periods without drinking. However those that feed on marine animals ingest large
amounts of salt which prevent water uptake and must therefore be eliminated from the body.
Many species of monitor lizard possess glands in the nostril that contain ionocyte cells which
actively transport salts out of the blood and secrete them as very concentrated brine.
In the case of the aptly-named rusty goanna these glands allow them to live in saltwater areas and
feed on salty foods without ever having to drink freshwater (Dunson 1974). Many monitor
lizards can inhabit saltwater areas and are often seen swimming in the sea (e.g. Neill 1958).
This allows them to disperse rapidly through groups of islands and even to reach new
land masses shortly after their formation. Their unfussy diets enable them to survive in areas
that would not support most other large vertebrates.