Reptiles separated from their water-dwelling ancestors and climbed onto land over 280 million years ago. They developed several adaptive features to help them survive and thrive on land.
These include scaly skin that reduces water loss; lungs; internal fertilization of eggs; and more.
Marine reptiles also have adapted to saltwater by developing mechanisms for osmoregulation and lung capacity to allow for oxygen storage.
Reptiles are ectothermic, meaning they have an internal body temperature that is dependent on external temperatures. In contrast, mammals, including dogs and people, are homiothermic animals that have an internal thermostat that maintains their bodies at a constant level, regardless of environmental conditions. The ability of reptiles to regulate their body temperatures is critical to their survival in the wild.
Temperature regulation is achieved by a combination of behavioral strategies. In the simplest case, a reptile moves to different areas of its habitat to gain access to warm or cool sources of energy. This enables it to achieve its optimum body temperature, which is the range within which typical metabolic activities can be carried out efficiently.
For example, if a reptile moves to a cooler environment, it must adjust its body temperature by spending more time basking to increase the warmth of its skin and maintain a higher thermal set point. However, the increased time spent basking could lead to a reduced capacity to hunt or perform other ecological activities.
As an early definition of thermoregulatory behaviour, the elevation of a reptile’s body temperature above environmental temperatures was accepted. However, in a famous experiment (Heath I 964) filled beer cans also exhibited this behaviour, and it was later realised that the true test of thermoregulatory behaviour should be measured against an artificial set point.
Water Resistant Skin
Reptiles’ dry and scaly skin may seem like an adaptation for desert habitats, but it is also very useful in protecting them from water-borne germs. Millions of households have reptiles, including turtles, snakes and lizards, but many people do not understand that these animals often carry germs that make them sick or can be passed to humans and other mammals.
Reptilian skin is incredibly waterproof because of an arrangement of fatty bilayer lipid molecules in the outer layer known as the stratum corneum. This arrangement creates an impermeable force that prevents water from passing in or out of the skin in either direction.
As a result, the skin is almost impervious to water and allows the reptile to stay in deep pools for longer periods of time. A new study shows that gecko skin has an additional feature to allow lizards to “slither” across surfaces using their scales, which grab onto tiny surface imperfections and generate frictional forces to move the animal forward.
During a process called ecdysis, or molting, the outer layers of reptile skin change. These changes include a separation and loss of the old superficial layer known as the stratum basale. Then, the deeper layer of granulosum and corneum duplicates itself, pushing up through the stratum intermedium (a temporary layer between the old and new skin). White blood cells invade this area and promote this change.
Reptiles have a number of special urinary adaptations. Their scaly skin, which contains the protein keratin and waxy lipids, reduces water loss from the body. They also do not breathe through their skin, like amphibians; they have lungs for respiration.
During the long periods of inactivity (estivation and hibernation) that desert reptiles spend underground, their metabolic processes slow considerably. At this time, water, urea and other wastes accumulate to potentially toxic levels. To prevent this, desert tortoises have enlarged urinary bladders that can store over 40 percent of their body weight in water, urea and nitrogenous wastes.
In addition, a complex medullary structure has evolved in these animals. This is likely related to the environmental aridity of their habitats. The kidneys are connected to the stomach via a pylorus, with the spleen and pancreas located adjacent to the stomach, and the gall bladder nearby.
Marine reptiles use specialized flipper-like limbs to swim. They may have a flattened tail that aids in propulsion. They often have salt glands that enable them to regulate the salt content of their bodies, allowing them to survive in marine habitats. This ability to excrete excess salt is important to surviving in marine environments where the depletion of coral reefs, seagrass beds and nesting beaches can directly affect the breeding success of these reptiles.
Reptiles are essentially dependent on the external environment for heat or cooling. Consequently, they have developed a great number of means for maintaining their body temperature within a preferred range.
The problem is particularly acute for reptiles which are ectothermic and thus lack the internal metabolic mechanisms to counteract changes in body temperature – unlike endothermic mammals for example.
Those reptiles which live in warm environments such as intertropical humid forests have at their disposal a large thermal gradient – their bodies can easily be raised to the ambient air temperature by exposure to sunlight. In contrast, reptiles which inhabit cold regions must contend with very small environmental gradients. This imposes the requirement on them to be able to use the sun’s rays and other natural sources of heat for accelerating the rate of heating or to reduce the rate of cooling by abrasion or immersion in icy water.
The sand-living lizard Sceloporus magister, for example, is able to maintain its body temperature within the optimum range by immersing itself in dew, which evaporates very rapidly, during the day. In the same way, the burrowing thigmotherms, such as the Sahara viper Cerastes, are able to bury themselves in sand and only expose their heads for breathing. These species are able to do so by using the natural thermal gradients of their habitats – namely the fact that the surface soil is warmer than the surrounding air.