Heat

Like all reptiles, monitors have very little control over their body temperatures and depend on heat from the environment. Activity is only possible within a narrow range of temperatures and when conditions are too hot or too cold the lizards must remain in insulated shelters. Some species never encounter adverse temperatures (i.e. those from moist tropical forests) but others only rarely experience conditions that permit activity. The need to alter behaviour according to thermoregulatory considerations restricts the Lizards' activity to a large extent but the freedom from having to use energy to generate body heat allows the lizards to be vastly more energy efficient than other vertebrates of the same size. The parietal eye, situated on the top of the head, is an important organ in thermoregulation (Kulshreshtha & Khan 1988).

Temperatures of a number of monitors lizards have been recorded in the field. Activity temperatures usually lie between 22 -38°C with aquatic monitors having lower temperatures than other species. In some monitor lizards' body temperature drops sharply at night but others attempt to keep their body temperature relatively stable. At temperatures below 20°C monitor lizards become slow and at 5°C they become completely torpid (Spellerberg 1972). Exposure to temperatures above 42°C for more than half an hour results in death. "Critical Thermal Maximums" have been recorded for several species of monitor lizard but much less is known about the lower range of temperatures tolerated by species that must survive cold winters. From the point of view of captive husbandry this is very unfortunate. It would be expected that desert monitors, who have to wntend with much higher and lower temperatures than their relatives in tropical areas, would be better able to tolerate fluctuations in temperature. Spellerberg (1972) suggests that lace goannas can withstand temperatures as low as ^-8.5°C for short periods. Rates of heating and cooling have been established for a number of species; Komodo dragons (McNab & Auffenberg 1976, Green et al 1991), lace goannas (Brattstrom 1973), grey monitors (Francaz et al 1976, Franl:az & Vernet 1978), Nile and white-throated monitors (Bowker 1984), Bengal monitors (Meek 1978, Earll 1982), Gould's or sand goannas (Johnson 1972) and water monitors (Meek 1978, Traeholt 1995),

Monitor lizards usually fall into a deep sleep in the evenings and wake up at about the same time each morning, regardless of the weather. During the night body temperatures may have fallen too low to permit activity and so if conditions outside are not extreme they emerge from their retreats shortly after waking and move to a basking area nearby where they lie in the sun until warmed sufficiently. This can take between a few minutes to several hours. Monitors like to bask on surfaces that retain heat well. Some individuals of several species bask all slightly elevated patches of earth which they keep clear of debris. Other basking sites include tree branches, elevated rocks or mounds and road surfaces, which are often their downfall (e.g. Bush et al 1991). If the weather is cool the lizards may bask for several hours and then return to their burrows, but if suitable body temperature are achieved they can commence activity. However they cannot spend long in places where the temperature is unduly hot or cold before having to seek out more equable areas. Burrows, tree hollows, bodies of water and thickets of vegetation usually provide conditions that are cooler than ambient during hot weather and warmer than ambient in cold weather. Monitor lizards that like to maintain a reasonably constant body temperature can shuttle between open ground , dense vegetation, burrows, trees and water according to their heat requirements, coUecting food from each area. Monitors living in overgrown habitats such as rainforests, where there is little direct sunlight but air temperatures are always suitable for activity may do very little basking. Monitor lizards in the extreme northern and southern parts of their range (i.e. northern and southern Africa, southern Australia and central Asia) may spend more than half of the year in a donnant state. In contrast monitors that live in the tropics, where the temperatures are much more constant both from day to night and from season to season are active throughout the year, but expend most of their energy during the breeding season.

Attribution / Courtesy: Daniel Bennett. 1995. A Little Book of Monitor Lizards. Viper Press U.K.