Thermoregulation in reptiles involves a range of very different strategies. In ideal conditions where reptiles have access to a broad thermal energy resource they are expected to maintain physiologically optimum body temperatures for long periods of time each day.
크레스티드게코Historically this has been measured by observing whether body temperatures are higher than environmental temperatures. However this method may give misleading results especially for larger animals.
Thermoregulation in Tropical Forests
The ectotherms that live in trop 크레스티드게코 ical forests have to deal with a wide range of temperatures. For example, a Tuatara that is kept at its preferred temperature in the laboratory dies within two or three months, which shows that this species requires constant temperature variation to survive. Temperatures in tropical forests also vary throughout the day. This makes it hard for lizards such as A. gundlachi to find a spot to warm themselves.
Most of these lizards spend their lives below the tree canopy inside deeply shaded forests where the fields Tb are close to air temperatures (Inger 1959; Ruibal 1961; Porter & James 1979). They rarely bask, and if they do, they seek out shaded shelters where the surface soil and leaves may be warmer than the ambient temperature.
They have very small thermal safety margins – a low Tb, a high To and a high CTmax (see Table 2g in the electronic supplementary material). Thermal safety margins are not significantly correlated with latitude or basking behaviour but do show a weak phylogenetic signal.
As climate warming causes the forest canopy to open up, these lizards may not be able to keep 크레스티드게코 their body temperatures up without having to reduce activity or go into hibernation. This could put them at risk of extinction in future. This contrasts with the situation of reptiles that live in temperate climates, such as polar bears, which must cope with extreme temperature fluctuations but have relatively large thermal safety margins.
Thermoregulation in Deserts
Unlike mammals and birds reptiles do not produce much metabolic heat, so their body temperature depends entirely on ambient temperatures. Thermoregulation is particularly challenging for desert dwelling reptiles, where a small change in ambient temperatures can lead to overheating or chilling. To cope with this problem, lizards in the Sonoran Desert regulate their body temperatures by daily cyclical movements between microhabitats. They roost in bushes during the day and re-enter the open ground at dusk, displaying four thermoregulatory postures: flanking, crouching, stilting and stem-shading (Burmeister, 1967).
In addition to these behaviours, lizards also utilise a range of thermosensory inputs, including cutaneous pit organs. These are relatively insensitive to steady temperature changes but are highly sensitive to variations in radiant energy: a rise in infrared radiation elicits a dynamic increase in impulse discharge from the pit organ, and cooling results in an inhibition of the resting discharge. As a result of this response, the pit organs of these reptiles provide a clear signal to the animals when it is time to move to a new microhabitat.
A key constraint on thermoregulation in desert lizards is the existence of ecological critical body temperatures which restrict the occurrence of essential activities such as digestion and spermiogenesis. These threshold temperatures are typically found at temperatures close to operative field body temperature, for example ecological maximum temperatures in chelonian species occur around 41-43degC (Hutchison, 1979). A consequence of the presence of such temperatures is that these reptiles must spend the greater part of their lives basking in the sun to reach their preferred body temperature. This increases the cost of thermoregulation and reduces the amount of time available for other ecological functions.
Thermoregulation in Water
Thermoregulation by reptiles in aquatic environments is quite complicated. The most obvious problem is that water is a poor conductor of heat, but in cold conditions it can also be very dense and slow to warm. This can create an intractable situation because the body has to expend much energy simply trying to keep up with the cold temperatures of the environment.
Reptiles are able to maintain their preferred body temperature by the use of a variety of behaviors. They may shiver to generate metabolic energy or move to a more sheltered area where it is easier to warm up. They can also take advantage of evaporative cooling by allowing their skins to absorb the vapor from the surrounding air.
Many researchers have attempted to study the thermoregulatory options of reptiles by testing their behavior under laboratory conditions. By constructing a thermal gradient chamber and placing reptiles in it they can see how they choose different combinations of behaviors to meet their environmental needs.
Reptiles are generally regarded as not being as efficient at generating and storing heat as endothermic mammals or birds. Until recently this led to the view that reptiles were effectively failed endotherms, but research has shown that this is not true. Instead reptile ectothermic physiology is a low budget, energy cost system that drains environmental resources at a much lower rate than mammals and birds, thereby enabling these animals to colonise vast areas of the world.
Thermoregulation in Cold Environments
In cold environments reptiles may use a combination of behaviors to warm themselves. They can also cool themselves by evaporative cooling. For example, a fish can remain functional when its water temperature falls below freezing by using natural antifreeze proteins in its tissues (Kosh & Smith 1992).
A common method of testing the extent to which a reptile is capable of thermoregulating is to compare body temperatures of a reptile with those of its environment. This is called the thermoconformity test (Huey 1982). An animal is considered a thermoregulator when its body temperature agrees with the environment. However, this measure does not take into account the costs of achieving these body temperatures.
For example, a lizard buried in sand to conserve energy is unable to achieve its preferred activity temperature. It would have to spend a great deal of energy warming itself and this is likely to cause a reduction in the amount of food it can eat.
This is a problem for a reptile whose survival depends on consuming a lot of calories. In order to avoid these costs, reptiles are often able to adjust their operative field body temperature closer to the optimum. In addition, many reptiles have evolved to be able to maintain their internal temperatures at levels that are below the physiological critical minimum and maximum body temperature thresholds that ultimately set the limits on their survival.