Reptiles occupy a variety of thermally heterogeneous environments. They must be able to select from a large range of operative body temperatures.
Ecological critical maximum and minimum body temperatures set fundamental limits on behavioural repertoires, thermal set points and distribution patterns. They must also avoid environmental conditions which are lethal.
Reptiles use a variety of behaviors to warm and cool themselves, trying to maintain body temperatures within an optimal range where internal chemical processes are optimized. Often, this means finding or creating heat sources in their environment.
Because reptiles are ectotherms, they cannot generate their own internal body heat through the enzymatic breakdown of fats or other metabolic reactions. Consequently, they must spend a large portion of their time seeking out the external heat they need to survive. When a reptile’s body temperature drops too low, it seeks out warmth by burrowing in the sand or hiding in crevices, and when its temperature rises too high, it basks in the sun to warm itself.
Thermoregulatory behaviors also vary depending on the habitat in which a reptile lives. For example, desert-dwelling reptiles such as the sand viper Cerastes will often bury themselves completely in sand with only their heads poking out to conserve body heat and maintain a stable environment.
When a reptile finds the source of the heat it needs, it must be able to recognize it in its surroundings. For this reason, the pit organs of many snake species can detect small changes in air and ground temperatures that are invisible to human eyes. When a snake encounters a thermal gradient, the pit organ signals its brain to take action.
Heat Sensitive Receptors
Physiological thermoregulation is a process by which an organism matches internal conditions to external variation through a series of automatic, involuntary responses that produce or dissipate heat(Seebacher and Franklin,2005). In mammals this involves sensing the environment (via afferent neural pathways) and signaling the brain to initiate the response. This brain signaling involves a complex network of neurons in the hypothalamus (shown here with red and green coloring in the crocodile brain).
Upon receiving a temperature change from the outside world, thermal receptors (heat sensors) located on the skin and elsewhere respond. Then, the signals are relayed to a group of nerve cells called the preoptic nucleus in the hypothalamus. These neurons are responsible for the thermosensory information that is sent to the rest of the body and to the cellular metabolic reactions that regulate the body’s thermal state(Seebacher and Franklin,2005).
Thermosensitive neurons in the skin (red) transmit information about the ambient temperature via afferent pathways to the hypothalamus (shown here in red and green coloring in the crocodile brain). In turn, the POA neurons trigger efferent responses (blue) such as cardiovascular adjustments and modulation of cellular metabolism. These responses can dissipate heat by constricting blood vessels and increasing cardiac output(see the animation), promote heat production in muscle by triggering tremor mechanisms (see the animation) and promoting metabolic acclimation to changing environments through transcriptional controllers such as peroxisome proliferator activated receptor gamma coactivator 1a(See the animation). Blocking TRPM8 and TRPV1 pharmacologically causes hyperthermia in rats, suggesting that these ion channels are critical for sensing environmental temperatures and triggering a thermosensory response in the hypothalamus.
Reptiles use a variety of methods to warm and cool themselves in order to keep their body temperatures within an optimum range. They do this by using a combination of behaviors such as moving to cooler or warmer places, changing their skin color or behavior, and altering their respiration rate.
Some reptiles, such as snakes, have evolved to control the amount of heat they lose by regulating blood flow to their extremities through vasodilation (widening of blood vessels) and vasoconstriction (narrowing of blood vessels). This allows them to conserve heat when they are cool or elicit heat when they are warming up.
Other reptiles, such as terrestrial lizards and tortoises, may be able to regulate their body temperature by utilizing thermal gradients in their environment. For example, the surface of land can be much hotter than its surrounding vegetation and these reptiles will move to the hottest areas in order to warm themselves.
Other reptiles, such as burrowing forms or aquatic species, may have a more difficult time maintaining their optimum body temperature. For instance, water cooled by sun’s rays is likely to have a low thermal gradient and these reptiles will need to spend longer periods of time in the cool water to reach their optimum temperatures.
Reptiles can use a variety of heat-absorbing and radiative surfaces to reduce their body temperature. They may flatten their bodies to maximize surface area for absorbing radiant energy or raise themselves off the ground to avoid direct contact with hot surfaces. Many species become more active during warmer times of day or during specific seasons and others enter a state of torpor or hibernation at cooler temperatures.
While endotherms can increase their internal body heat production to maintain a stable internal temperature, ectotherms like iguanas, geckos, lizards and most reptiles depend on external sources of thermal energy to regulate their body temperature. As the temperature of their environment rises, ectotherms’ internal body temperature will rise and fall in response.
When lizards are stimulated to run around a track, their body temperature (Tb) is recorded and used to determine the highest value of locomotor performance (Topt). Topt was determined by finding the average of the Tb measurements taken during the trial.
The results showed that families generally classified as thermoregulators had higher mean values for all thermal traits than those classified as thermoconformers, although mean values of Tb, VTmax and CTmin were similar in both groups. The family of Teiidae (forest-dwelling lizards) had the lowest Tb and the family of Gekkonidae (dry forest-dwelling lizards) the lowest VTmin, but all of these differences were small.