Reptiles use their behaviour to regulate their body temperature, adjusting their position to the sun or into the shade and retreating to burrows to avoid heat. This is an essential aspect of their survival.
Behavioural thermal ecology is a major research topic in ectotherms and many of the results have important practical implications. The following sections will highlight some of the main lines of investigation.
Reptiles are very sensitive to altered environmental temperatures as a result of their ectothermy. They rely on ambient environmental temperatures for critical physiological processes such as metabolism, sex determination and embryonic development. As a result they are more vulnerable than most vertebrates to climate change.
Thermoregulation is defined as the ability to elevate body temperatures above environmental temperatures. However, the reptiles’ behavioural responses to elevated environmental temperatures vary considerably. Thermosensitivity in reptiles is set by their environment and not by their internal body temperature, as demonstrated by the beer can experiment of Heath (1965).
In many species a reptile’s operative temperature range (which varies from one habitat to another) is restricted by the availability of thermal energy and the habitat’s ability to provide it. In addition, the ability to attain optimum physiological body temperatures in natural habitats is often limited by lifestyle and other factors such as competition with other reptiles or inter-specific predators.
Oxygen capacity of reptile blood is temperature sensitive and peaks at temperatures close to the species’ activity-temperature range, as shown by experiments on lizards and snakes. In fact, the blood oxygen capacity of reptiles in vitro correlates closely with their eccritic temperatures (Cowles and Bogert, 1944; Saint Girons and Spellerberg, 1974).
In many cold regions the temperature of the ground surface can be as low as a reptile’s minimum critical temperature. In such a situation a reptile’s ability to move about is severely restricted and it may dig itself into the soil in an attempt to lower its body temperature.
Reptiles are able to operate within their ideal thermal environment in which they can attain and maintain their physiological optimum body temperatures for extended periods. This is a highly desirable condition since they do not have to expend as much energy heating their bodies to perform typical activities such as feeding, digestion and fighting off disease or infection.
The thermoregulatory mechanisms in reptiles are highly sophisticated and adapted to the specialized habitats they inhabit. For example the pit organ of a snake is an unusually dense, innervated structure with a very narrow membrane that separates it from the bottom of the pit. The membrane is lined with treelike structures of bare (unmyelinated) nerve fibres that are highly sensitive to radiant heat. When a reptile detects heat, it is sensed by the receptors and causes a dynamic increase in heart rate with an accompanying change in blood flow to warm or cool the body. This elicits the characteristic heart rate hysteresis that is so well known and is an effective thermal regulation mechanism that maximises the time spent at an optimal temperature.
Other reptilian thermoregulatory behaviours are based on exploiting the thermal gradients of their habitats. For instance sand-living reptiles such as the Saharan viper Crotalus cerastes will bury itself in the sand with only its head protruding through to utilise the thermal energy of the underlying soil. Similarly desert iguanas (Dipsosaurus dorsalis) will adjust their posture in relation to solar radiation both inside and outside their burrows to achieve the desired body temperatures.
Thermoregulation in the Tropics
Reptiles occupy a variety of habitats and are subject to very different climates. Some of them (for example, turtles, tortoises and a few lizards) are ectothermic and rely mainly on body surface area and conductive heat exchange to raise body temperatures above environmental temperatures. Others, such as the sand-living Sahara viper Cerastes, can maintain a preferred body temperature by burrowing into the soil and avoiding direct contact with the air or other surfaces. These behaviours are highly sophisticated and probably have considerable adaptive value as they allow the animal to keep their metabolic rate high and their blood temperatures within their preferred ranges for much of the day.
In intertropical forests and desert regions thermal regulation poses far less of a problem as the environment is relatively stable and the preferred body temperature can be achieved with very little variation. Thus a great many squamate species, such as the tuatara (Tuatara novaezelandiae), are heliothermic, basking regularly and often allowing their operative body temperature to rise above ambient temperatures. However, these animals experience a thermal lag and do not attain their operative body temperatures until later in the day than do males.
In contrast, burrowing thigmothermic reptiles are able to use the temperature gradients of their microhabitats to maintain preferred body temperatures, such as the sand-dwelling Anguis fragilis, above environmental temperatures throughout the day. They also exhibit a rapid response to changes in air temperature and are able to rapidly increase the frequency of nerve impulses discharged at their pit organs.
Thermoregulation in Water
Reptiles are ectothermic, meaning they cannot produce metabolic heat on their own and must use the thermal environment to regulate their body temperature. Because of this, they require a narrow range of temperatures in which they can perform typical functions such as locomotion, digestion, and reproduction. When temperatures fall outside of this range, these functions can become impossible. This is why it is so important for reptiles to be able to thermoregulate in order to survive.
In general, reptiles use both behavioral and physiological methods to thermoregulate. Typical behavioral thermoregulatory strategies include emergence, retreat, selection of temperatures, basking, orientation changes, and postural adjustments. Physiological mechanisms include the use of pit organs to detect temperatures (and their rate of change) in the environment, and vasomotor responses affecting rates of heating and cooling.
A classic test of thermoregulatory capability involves measuring the relationship between a reptile’s body temperature and its environmental temperature. The test involves plotting the slope m and the intercept b on the regression equation y = mx + b. A reptile that has a perfect thermoconformer will have a slope m of 1 which indicates that its body temperature is exactly equal to its environment’s temperature.
Unfortunately, this method of testing does not always give a true picture of thermoregulatory ability. The famous “beer can” experiment performed by Heath (1964) shows that even empty beer cans have temperatures that are above their environment’s temperature, so this method of determining thermoregulatory ability is flawed.