Reptiles are a diverse group of vertebrates belonging to the Class Reptilia. This lineage emerged approximately 320 million years ago, evolving from ancestral amphibians and successfully colonizing terrestrial environments. The defining characteristics of reptiles allowed them to overcome the constraints of water dependence. These adaptations center on a unique physiological strategy, a protective body covering, a specialized reproductive method, and distinct internal anatomy.
Temperature Regulation: Ectothermy
Reptiles are characterized by ectothermy, deriving the majority of their body heat from external sources rather than generating it internally through metabolism. The term “cold-blooded” is misleading because reptiles actively regulate their temperature, often maintaining a preferred range that can be quite warm. This reliance means reptiles employ complex behavioral strategies to control their thermal state.
They frequently bask in direct sunlight or on warm surfaces to raise their body temperature, optimizing physiological processes like digestion and movement. This warming is important for increasing their metabolic rate.
When temperatures become too high, reptiles seek shade, burrow, or retreat into water to prevent overheating and dehydration. Adjusting their body posture, such as flattening the body or raising it off a hot surface, further aids in precise temperature control. This behavioral thermoregulation is highly efficient, allowing reptiles to operate on a significantly lower energy budget compared to endotherms.
The Integumentary System: Scales and Skin
A defining feature of reptiles is their tough, dry integument, covered in scales or scutes. The skin is highly keratinized, containing layers of the structural protein keratin that form a robust, protective outer layer. This specialized covering acts as a waterproof seal, effectively minimizing water loss and preventing desiccation in terrestrial habitats.
The scales are formed from the epidermis and are composed of alpha-keratins for flexibility and beta-keratins for rigidity and hardness. In many species, such as lizards and snakes, the outer layer of skin is periodically shed in a process known as ecdysis, or molting. This shedding allows for growth and removes worn-out skin and external parasites.
In crocodilians and turtles, the scales—often called scutes—may be reinforced with underlying bony plates known as osteoderms, providing additional armor. This durable, glandless skin offers substantial protection from physical abrasion and injury.
Reproductive Strategy: The Amniotic Egg
The amniotic egg freed reptiles from the necessity of returning to water for reproduction. This self-contained unit allows the embryo to develop in a protected, aquatic environment even on dry land. Fertilization is internal, and the egg contains several specialized extra-embryonic membranes.
Extra-Embryonic Membranes
The innermost membrane, the amnion, surrounds the embryo and is filled with amniotic fluid, providing a cushion and a stable, moist environment. The yolk sac supplies the developing embryo with nutrient-rich food. The allantois functions as a repository for nitrogenous waste products and assists in gas exchange.
The chorion is the outermost membrane, enclosing the others and facilitating gas exchange with the outside air. It is protected by a porous shell, which is either leathery or calcified, allowing gas exchange while retaining moisture. While many reptiles are oviparous (egg-laying), some lizards and snakes exhibit viviparity or ovoviviparity, where the young are born alive.
Unique Internal Anatomy and Movement
Reptiles rely exclusively on pulmonary respiration, breathing solely through lungs, since their tough skin prevents the cutaneous gas exchange seen in amphibians. Their lungs are structurally complex, featuring numerous internal partitions that increase the surface area for efficient gas exchange. Lizards and snakes typically use their intercostal muscles to expand and contract the rib cage for breathing.
Crocodilians have evolved a unique mechanism where the movement of their liver acts as a piston to ventilate the lungs, similar in function to the mammalian diaphragm. The reptilian heart is typically three-chambered, with two atria and one ventricle partially divided by a septum. Crocodilians are the exception, possessing a fully four-chambered heart.
Most reptiles possess a tetrapod body plan and move with a sprawling gait where the limbs project sideways from the body. This posture results in a lateral undulation of the spine during locomotion. Snakes, which are limbless, utilize specialized movements of their ribs and scales for propulsion.