Energy is used by organisms to maintain living processes, power locomotion and other activities, and fuel the synthesis of tissue in growth and reproduction. All vertebrates derive their usable energy from the oxidation of carbohydrates, lipids, and proteins in the food they eat. Reptiles do not expend large amounts of energy to maintain high levels of body temperature as do birds and mammals, which are en-dothermic. The latter maintain high body temperature with internal heat production coupled with insulation—fur or feathers—that minimizes losses of heat to the environment. Reptiles have lower rates of heat production and are not well insulated from thermal exchanges with the environment. They are ectothermic, meaning they depend on external sources of heat in the environment for maintenance of body temperature.
During periods of activity, many reptiles regulate their body temperature within narrow limits by adjusting both position and posture in relation to thermal complexity in the environment. The simplest example of such behavioral thermoregulation is that of many lizards that shuttle between sunlit and shaded microenvironments, maintaining high body temperature, sometimes with remarkable precision. Body temperatures are maintained at levels between 95°F and 108°F (35°C and 42 °C) during daylight hours in most diurnal species. Thus ectothermic reptiles can be as warm as many mammals. However, during nighttime hours or colder seasons in the temperate latitudes or high altitude, reptiles usually retreat to locations where temperatures are moderated, such as underground, in rock crevices, or in bodies of water. Unlike mammals, reptiles cannot maintain activity during colder conditions.
The time reptiles spend exposed to solar radiation for ther-moregulatory purposes is inversely related to body size and ambient temperature. Thus altitude greatly affects the behaviors required for thermoregulation. Decreasing air temperatures and increasing cloud cover generally accompany increases in altitude. Lizards that have been studied tend to compensate for these conditions by adjusting their exposure to sunlight. Thus at increasingly higher altitudes there is a shift from occupancy of shaded habitats, as at low altitudes, to occupancy of open habitats. Above approximately 9,800 ft (3,000 m), high body temperatures can be attained only by exposure to direct sunlight, a circumstance that also increases exposure of basking lizards to predators.
Because reptiles do not depend on internal metabolism for body heat, their metabolic requirements are sevenfold to tenfold lower than those of most endotherms. Moreover, when the environment cools at night, the decreasing body temperature of reptiles reduces energy expenditure even further, whereas mammals must produce additional heat to maintain warmer bodies. Thus reptiles use far less energy during a day or a season's activity than do birds or mammals of comparable size.
Metabolic rates vary with body size, but the relation usually is not linear; energy expenditure scales typically to the 0.6-0.8 power of body mass. The rate of metabolic energy expenditure per gram of body mass therefore increases at smaller body sizes. Although this relation holds for almost all animals, the rates of energy expenditure in birds and mammals are many-fold higher than those of reptiles of comparable size. All factors that contribute to energy differences considered, a lizard in nature uses only approximately 3% of the energy used by a mammal of similar body size during the course of a day.
Being ectothermic has both advantages and disadvantages for reptiles. The lower metabolic energy requirements of reptiles decrease demand for food resources and increase the efficiency at which energy from food is transformed into production of body tissue. Thus reptiles are better able than endotherms to live in environments of low biological productivity and to withstand periods of food scarcity related to changes in weather or other factors. Ectotherms, however, cannot maintain optimal body temperature in all environmental conditions. The activities of reptiles are limited in time and space by environmental conditions, which also limit geographic distribution and reproductive output. Numerous body functions and processes operate most effectively at higher temperatures, and reptiles are not always in conditions that allow optimal performance, as is the case for endother-mic mammals. Exposure to sun or other heat sources also may render reptiles more vulnerable to predation than would be the case if shuttling between environments were not required for thermoregulation.
There are two notable exceptions to the general pattern of ectothermy among reptiles. Females of several species of pythons generate metabolic heat from spasmodic contractions of skeletal muscles during brooding when they coil around their eggs. Thus brooding female pythons are endothermic and transfer body heat to the incubating eggs, which are maintained at approximately 86°F (30°C). Metabolic heat also may accumulate in other reptiles of large body mass. The result is periodic or facultative endothermy. In comparatively cold seawater, body temperatures of leatherback seaturtles (Dermochelys coriacea) have been shown to be as much as 32°F (18°C) greater than the surrounding water temperature.
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