Monitor lizards appear to show great variation in their thermoregulatory behaviour. Some species like to maintain a relatively stable body temperature whilst others like to get hot during the day and cool down at night. Even within small populations, individuals show a great deal of variation in their preferred temperatures. Monitors regulate their temperature largely by moving to warmer or cooler microhabitats and so during the day the enclosure must accommodate a thermal gradient that includes both hot basking areas and much cooler retreats. Ideally the temperature of the enclosure should reflect the range experienced by the lizards in the wild. This varies enormously between species. In many parts of the tropics the nights are scarcely cooler than the days and there is little seasonal fluctuation. Elsewhere temperatures may drop sharply at rtight and for most of the year the weather may be too hot or too cold to allow much activity. By installing an efficient heating system in the terrarium the lizards are given the opportunity to thermoregulate naturally and seasonal changes can be approximated. Of course all heating elements, thermostat sensors and wires must be positioned where the lizards cannot touch them. Any connections that could conceivably get damp should be sealed in epoxy resin.

Heat Gradients
We have seen that monitor lizards' behaviour is determined largely by thermoregulatory considerations and that animals with no control over their own body temperature tend to be lethargic. The enclosure therefore must be heated in a way that provides a variety of temperatures in a relatively small space. They must have access to substantial heat (above 40°C, sometimes as high as 50°C) for part of the day and always have plenty of cooler retreats available. High temperatures enable the lizards to digest their food quickly and thoroughly and promote rapid growth and good health. However, the basking area should account for only a small part of the enclosure and burrows or other shelters, situated as far from the heat sources as possible, must be provided. This is most important, because although low temperatures kill reptiles slowly, excess heat is rapidly fatal. All the monitors studied to date die when their body temperatures reach about 41 °C. Monitors that are too hot show it by panting (a normal behaviour) and gasping (in extreme cases when death is near). In a well planned terrarium, where a variety of temperatures are available. it is impossible for the lizards to overheat, because they simply move to a cooler area when desired. To establish a good thermal gradient several maximum-minimum thermometers should be placed around the enclosure and the position of fumishings and heat sources altered until an acceptable variety of temperatures is achieved.

Primary heat suurces
The purpose of the primary heater is to provide a source of constant warmth both day and night that prevents the lizards from getting too cold, and so it must be powerful enough to keep the enclosure suffidently warm during even the coldest weather. The simplest way to heat small and medium sized enclosures is with one or more ceramic heaters, controlled with an electronic thermostat to give precise control over the temperature. These are easily obtained from reptile dealers. Methods of heating such as mats and cables, which are designed to fit at the bottom of the terrarium are unsuitable for large monitors unless they can be positioned so that the animals cannot dig them up. If many terraria are kept together it is usually more economical to heat the entire room. Enclosures for large monitor lizards require more powerful heaters which can be powered by gas or electricity. A local heating specialist should be able to suggest an efficient and economic system which must be highly controllable and free from all fumes or smell. The minimum temperature should be a year round 25°C for tropical species and 12-20°C for temperate species depending on the time of year (see below).

Secundary heat sources
The purpose of the secondary heat source is to provide the lizards with "hot spots" or basking areas during the day, so that they can regulate their own temperature as they do in the wild. There must be a basking spot available for each animal in the enclosure, or the weaker monitors will be deprived and unable to thermoregulate. The basking areas below the heat source should be planned with care. Some species like to bask on a platform above the ground such as a branch or a large flat stone, whilst others prefer to lie flat on the ground in an area cleared of debris. Depending on the species to be kept, either stone, wood or bare substrate can be used for the basking platform. Be sure to position all heat sources where the lizards cannot burn themselves, even if they try.

Reflector-type parabolic spotlights are the usual choice for the secondary heat source. The harsh glare of spotlights can be reduced by positioning them at the front of the enclosure and directing them towards the back. Bigger enclosures may benefit from the use of an infra-red bulb that generates more heat without producing visible light However monitor lizards often associate heat with light. and invisible infrared heaters are responsible for most of the serious burns that are incurred by captive specimens. Therefore particular care must be taken to ensure that it is impossible for the lizards to injure themselves when heat sources of this type are utilised.

The wattage needed will depend on the distance between the heat source and the basking area and heat absorbing properties of the basking surface. Pick one strong enough to keep the hottest basking surfaces between 40-48°C. Some individual lizards may find this too hot (others will happily bask at even higher temperatures for short periods). but in a well planned terrarium they will be able to select cooler basking sites. A thermostat should be attached to the heat source to prevent the enclosure from overheating in warm weather.

Daily Variations in Temperature
For tropical species the night-time temperature should not be more than a degree or two lower than the ambient temperature maintained during the day. For many temperate dwelling monitors. especially desert species. a more substantial drop in temperature at night is beneficial.

Seasonal Variations in Temperature
When naturally seasonal lizards from temperate climates are kept at temperatures that permit year-round activity they may suffer in a number of ways. They may become obese or conversely may waste away and die. Without a period of inactivity it may be impossible for their reproductive organs to mature. Some temperate species mate almost immediately after their period of hibernation. During the previous months the reproductive organs have become active and the large fat deposits accumulated by females are turned into eggs whilst the males have expended much less energy on the production of sperm. A period of a few months inactivity therefore seems to be desirable. However exposing captive monitor lizards to periods of hibernation is a risky business at best. Very little is known about the body temperatures maintained by wild monitor lizards during cold weather and at present providing temperatures at which the lizards should be kept during artificial hibernation would be purely speculative. Many animals that enter hibernation apparently healthy emerge from it very poorly and sometimes they may fail to emerge from it at all. Maintaining the lizards at too warm a temperature would result in them slowly wasting away rather than entering a state of true hibernation whilst the use of a temperature that is too cold would kill them outright. Because so little is known about this subject I recommend that you do not consider letting the animals hibernate unless you have the guidance of somebody with experience of the practice - try the herpetological society. However an annual period during which temperatures are reduced and the lizards become less active may be beneficial for many species. In some cases maintaining the animals at temperatures cooler than normal for a period of just a few days may trigger reproductive activity. Only well nourished. perfectly healthy animals that are known to have originated in a temperate climate should be treated in this way. The animals should be fed well for several months prior to cooling and the basking periods reduced a month or so before turning off all of the lights and allowing the animals to rest. Minimum ambient temperature can be allowed to drop as low as 12°C at night and need be no higher than 17°C during most of the day. The lizards will become immobile or sluggish but providing they are in good health to begin with and the period of cooling does not exceed 10-12 weeks no problems should result (be warned however that cooling the lizards will inevitably subject them to some stress and that this can lead to an impairment in their ability to ward off disease).

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