Heating - why it's necessary
Reptiles differ from the so-called ‘warm-blooded’ animals in that they don’t generate enough body heat from their metabolic activity to significantly control their body temperatures. The advantage to this super-low metabolism is that reptiles require far less food than mammals and birds. However this is not to say that reptiles do not prefer to maintain their body temperatures at certain levels – because they do. Reptile activities such as feeding and digestion generally take place within a narrow range of preferred body temperatures. Reptiles generally accomplish their body temperature control through the absorption of radiant heat from the environment – seeking comparatively warmer or cooler positions in order to raise or lower their own core temperatures. Sun-loving species often shuttle between positions exposed to full or partial sun to increase their body temperature, before ducking into shaded areas to avoid over-heating. Some nocturnal or small species shuttle between sun-warmed rocks or other materials during the early hours of the night in order to maintain an optimal body temperature. Other species are able to function with fairly low body temperatures.
As a reptile keeper you will need to recognize and accommodate the individual thermal requirements of whatever species is being kept. Otherwise, if kept at the wrong temperature, reptiles might look healthy and may even feed voraciously, but will not be able to properly digest their food, and will become susceptible to potentially fatal diseases. Many species can withstand extended periods below the preferred 'optimal' temperature range under natural conditions, but under captive conditions where stress may be a factor, extended maintenance of reptiles at suboptimal temperatures is to be avoided since it can result in a breakdown of their defences against disease. The notable exception to this rule is in the provision of a winter 'cooling down' period required to bring some species into breeding condition - which a whole other matter.
It is a failure to provide adequate warmth for their charges that has proven to be the biggest problem confronting novice reptile keepers. There are a number of common traps that first-snake keepers can fall into. An all too common failing has been an inability to recognize that the floor temperature of a cage might be significantly lower than the walls or upper portion of the cage (where a thermometer or thermostatic reading might be taken), especially if the cage is kept on a cold floor and is heated solely from above. Therefore, temperature readings should be taken from various positions within the cage, including in any hide boxes or other 'hiding places' where the reptile spends substantial amounts of time, before judging whether or not suitable heating is being provided. To adequately monitor the various temperature conditions within an enclosure, an invaluable tool is the remote thermometer. These ‘temp guns’ are readily available from on-line reptile equipment suppliers, and no reptile keeper should be without one. In the end, it is the reptile's body temperature, as influenced by the immediate environment, which must be kept within critical limits, not the general cage temperature as interpreted by a standard thermometer mounted on a cage wall.