Like all animals, monitor lizards breath so that they can get oxygen in, and carbon dioxide out, of their blood. The oxygen is used in a complex chemical pathway that provides energy for the organism by "burning" food; carbon dioxide is a poisonous waste product produced by those reactions. In mammals and birds this process releases a huge amount of heat that keeps us nice and warm and able to function at any time of day or night. In lizards however, the heat produced is negligible, so that it can be seen as a rather more economical system, though not without its drawbacks. Although lizards need much less food than mammals of a similar size, they are dependant on heat from the sun and as a result their activity is smetly limited by the amount heat available from their environment. Monitor lizards have the ability to generate a small amount of metabolic heat and when they are active they can be very active. The metabolism of some monitor lizards has been said to "bridge the gap in metabolism that has generally been assumed to exist between reptiles and mammals" (Bartholomew & Tucker 1964). Most monitor lizards have much higher metabolic rates than other, similarly sized, lizards when they are active, but use no more energy when at rest. They show an number of adaptations that allow the muscles to receive plentiful supplies of oxygen even under difficult conditions. Under normal circumstances muscles get tired because they burn oxygen at a faster rate than it can be supplied from the lungs via the blood. As oxygen consumption rises so does carbon dioxide production. The waste carbon dioxide dissolves in the blood and is carried to the lungs where it is expelled. But because it is acidic it lowers the pH of the blood and thus reduces the blood's ability to carry oxygen. The problem is partly solved by keeping supplies of oxygen stored in the muscles which can be used when the oxygen supply from the lungs becomes insufficient This supply is limited though, and results in the production of acids in the muscles which further deplete the ability of the tissues to take up fresh oxygen. Monitor lizards are able to minimise these problems in a number of ways. They have very large and efficient lungs which are supplied with copious amounts of blood that contains proteins which prevent the build up of acidic wastes. Furthermore the oxygen stores in the muscles are unusually rich and contain similar buffers that keep the acidity of the blood stable (Bennett 1972, 1973a,b). These and other adaptations, such as careful control over the rate of breathing at different temperatures and hearts that limit the mixing of oxygenated and deoxygenated blood (Webb et al 1971, Gleeson et al 1980, Heisler et al 1983) give many monitors lizards phenomenal strength and, most importantly, endurance. These adaptations are not limited to the larger species. The pygmy Gillen's monitor , for example, can run at 1km per hour for many minutes without showing any sign of exhaustion (Bickler & Anderson 1986). Indeed the experimenters found it impossible to tire the lizards out on their treadmill. Gillen's monitor is one of the smallest members of the family, with a weight of about 30g. It needs about O.2ml of oxygen per gramme of body weight per hour to keep itself alive. When strenuous activity is undertaken oxygen uptake can increase more than twenty five fold and the efficient transport systems ensure that it reaches the working muscles. The ecological significance of these adaptations are obvious. Species endowed with a high metabolic capacity have greater stamina than any other lizards; they can run further, swim further , fight longer and dig longer. They are without doubt the athletes of the squamates. But although many monitor lizards have the ability to sustain high levels of activity for long periods and use more energy than lizards of other families (e.g. the desert monitor (Vernet et al 1988 a,b), white-throated monitor (Wood et al 1977, 1978), Bosc's monitor1 (Gleeson et al 1980), Rosenberg's goanna (Green, Dryden & Dryden 1991), sand goanna (Bennett 1972), perenrie (Green el al 1986, see also Thompson 1995), Gillen's goanna (Bickler & Anderson 1986) and the Komodo dragon (Green, King, Braysher & Saim 1991)) it would be misleading to suggest that all monitor lizards are endowed with supernatural strength and stamina, nor that they necessarily engage in lots of vigourous exercise. The spiny-tailed goanna (Dryden et al 1990, Thompson & Withers 1994), Mertens' goanna (Christian and Conley 1994) and possibly the water monitor (Gleeson 1981 but see also Dryden et al 1992) have lower metabolic rates that do not differ from those of "typical" lizards. Monitors that spend time in water maintain lower body temperatures than other varanids, which may be responsible for their less vigourous metabolisms. The spiny-tailed monitor may ca tch more food by ambush than by actively e:: searching for it and may not require as much strength and stamina as its more active relation. Rosenberg's goanna, though capable of sustained activity, may spend more than 23 hours of the day lying about, even during the height of its activity season (Christian & Weavers 1994). Even the most active species are very economical with their energy, reducing metabolism at night, remaining immobile for long periods when food or water are scarce and only hunting energetically when prey are plentiful. Some species are able to store large amounts of fat in their bodies which may be used to sustain them through long periods of inactivity (sometimes more than six months of the year), in seasons when they may be particularly active but have no time to feed and for the production of large numbers of rich, yolky eggs. During periods of inactivity many body functions are greatly reduced to conserve energy. Red blood cell levels may drop to levels that forbid vigourous activity even if favourable conditions return unexpectedly (Banerjee & Banerjee 1969).
Studies of metabolic rates in the field have important implications for the captive care of monitor Lizards. Adequate exercise is essential to keep the animals healthy but because of the seasonal nature of their activity patterns the artimals should be given the option to reduce their activity, sometimes for long periods, if they desire to do so.
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