Reptiles are often called cold-blooded creatures, an assertion that mistakenly leads many people to believe they have cold core temperatures. In reality, reptiles are ectothermic animals, which means their metabolism does not generate enough heat byproducts to maintain body temperature above air or surface temperatures. Unlike warm-blooded mammals (including humans), reptiles regulate their body temperatures by behavior, such as basking in the sun or moving between warm and cold spots.
There are advantages and disadvantages to ectothermy: Reptiles need less energy than mammals. This allows reptiles to survive long fasts and to save energy better as compared to mammals.
However, this means a reptile's level of activity depends upon the ambient temperature - it cannot survive extreme climate changes. They need to maintain their core body temperature well above ambient temperatures for at least part of the day.
Some reptiles (i.e., tropical and desert species) need to be able to maintain temperatures cooler than the surrounding air. Reptiles can minimize body temperature fluctuations behaviorally. Reptiles becoming too cool seek an elevated area to bask. They lie perpendicular to the sun's rays, maximize their surface area by expanding their rib cage, and darken the pigment of their skin to maximize heat absorption. If no sunlight is available, some reptiles burrow into warm soil or lie on or under a large object (such as a rock) that acts as a heat sink when the sun is shining.
Overheated reptiles place themselves parallel to the sun's rays, seek shade, pant, lighten skin color, and burrow into cool soil. Some pythons use a series of muscle contractions to elevate their body temperatures a few degrees above ambient while incubating eggs.
In general, reptiles will increase their body temperature before bouts of activity. This is because nerves and muscle cells function better at warmer temperatures. A warm reptile is able to hunt more efficiently and is better able to escape predators. Reptiles will also increase their body temperatures when they are pregnant, digesting food, or sometimes for no apparent reason.
In contrast, reptiles will sometimes choose cool body temperatures, especially when there is a food shortage. By cooling their body temperatures, they can wait out droughts or low food sources. This is one reason why reptiles are often more abundant than warm- blooded animals in deserts and habitats with seasonal rains.
Sometimes, it's difficult to predict whether a reptile will choose warmer or cooler body temperatures. Snakes have been known to choose both cool and warm body temperatures during shedding. Although the immune system functions better when reptiles are warm, occasionally reptiles given experimental infections choose cooler body temperatures. It is thought that the cooler temperatures may help slow the growth rate of bacteria until a time when the reptile has enough energy stores to fight off the infection or be able to attain a very warm body temperature.
Overall reptiles have a limited ability to control their core body temperature. If unable to cool themselves, thyroid gland dysfunction and/or death from hypothermia (chilling) ensues. Chilled reptiles fall into torpor (a slowed metabolic state). Under natural conditions some reptiles hibernate. In captivity, reptiles are unable to prepare for hibernation on their own. When the body temperature falls below a critical point enzymes are unable to function. This means that digestion ceases or is incomplete, immunity is impaired, and reproductive function declines. Chilled animals are at high risk of disease.
The ideal temperature range for a reptile is referred to its preferred optimum temperature range (POTR). This range includes all the temperatures that a reptile needs to maintain optimal body function. Reptiles should be supplied as much of their preferred optimum temperature range as possible so that the animals have an opportunity to self regulate much as they would in the wild.
Suggested guidelines for temperature depends on natural habitats and the species of reptile. In general, tropical reptiles need temperatures ranging from 80 to 100 Fahrenheit; desert species require temperatures over 100 degrees in the daytime and between 60 and 80 degrees at night; temperate species usually need temperatures between 70 F and 90 degrees. However, certain individual species may be outside these ranges. For temperature ranges recommended for a particular type of reptile check the appropriate husbandry pages.
Circadian and Annual Rhythms
Tropical animals are exposed to almost constant temperatures and cannot handle large fluctuations. This is in contrast to desert animals that experience high daytime temperatures with an evening cool down period. Temperate reptiles can be very tolerant of temperature extremes (within reason) if they are free of disease. Many temperate reptile species experience seasons in the wild and require seasonal changes in light cycle, temperature, water or food availability to stimulate normal behaviors such as hibernation and breeding. Most healthy North American snakes and lizards benefit from a winter cool down period.
Hibernation is a period of dormancy marked by a decrease in metabolic rate. In a natural setting, most reptiles have completed metabolically taxing activities (reproduction) and have built up energy reserves. This period is then followed by a period of reduced food sources, which results in an empty gastrointestinal tract before the animal actually hibernates. Cooling occurs to the point that metabolism slows.
In captivity, it is important to duplicate the energy loading to provide fat stores and the following fast to empty the intestinal tract to prevent putrefaction (spoilage) of undigested food. The most common mistake made by pet owners attempting to hibernate their reptile is to cool the animal into torpor but not cool it enough to truly slow metabolism. In this state, the reptile slowly starves or develops systemic infection because its metabolic processes are too slow to fight off disease, but not so cold that microorganisms are slowed also. Females cannot hibernate successfully if they are in the process of producing eggs or offspring.
Humans are not good at determining the exact temperature that a reptile requires for any given period of time. Therefore, it is best to supply as much of the temperature range as possible.