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.
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