Reptiles are ectothermic amniote vertebrates traditionally grouped within the class Reptilia and commonly characterised by keratinised skin, lungs, internal fertilisation and reproduction adapted to terrestrial environments. The traditional category of Reptiles includes turtles, crocodilians, lizards, snakes and several extinct groups, but modern evolutionary classification recognises birds as descendants of reptiles and therefore treats a non-bird Reptilia as a potentially incomplete evolutionary grouping depending on the classification system. Reptilian skin generally reduces water loss through scales, scutes or other keratinised structures. Most reptiles rely heavily on environmental heat sources to regulate body temperature, although they can maintain controlled thermal states through behavioural and physiological mechanisms. They occupy deserts, forests, grasslands, wetlands, oceans and many other environments.
The evolutionary history of Reptiles extends into the Carboniferous Period, when early amniotes diverged into lineages that eventually produced reptiles, birds and mammals. The evolution of the amniotic egg was a major transition because it allowed reproduction to become less dependent on open water. Amniotic development involves specialised membranes that support gas exchange, waste storage and nourishment of the embryo.
Reptile ancestors subsequently diversified into numerous terrestrial and aquatic forms, including dinosaurs, pterosaurs, marine reptiles and the lineages leading to modern reptiles and birds. Major evolutionary radiations occurred during the Mesozoic Era, when reptiles occupied many dominant terrestrial and aquatic ecological roles. The fossil record demonstrates that modern reptiles represent only a fraction of historical reptilian diversity.
Reptile anatomy shows strong adaptations for conserving water and functioning on land. Their skin is covered by keratinised scales or scutes that limit evaporative water loss, although the exact structure varies among groups. Most reptiles breathe using lungs rather than relying on skin respiration, and rib movements contribute to ventilation in many species.
Turtles possess highly modified shells composed of bony structures covered by keratinous material, while snakes have elongated bodies and highly specialised skulls that permit ingestion of relatively large prey. Crocodilians have powerful jaws and specialised sensory structures associated with aquatic predation. Lizards vary enormously in body form, from tiny climbing species to large terrestrial predators. These differences illustrate how a common amniote heritage has produced diverse anatomical solutions.
Reptile thermoregulation is strongly influenced by ectothermy. Rather than relying primarily on internally generated metabolic heat, many reptiles regulate body temperature through behaviours such as basking, seeking shade, changing posture, moving between microhabitats and altering activity times. Ectothermy can reduce energetic requirements compared with maintaining a high constant metabolic temperature, but it can also make activity and physiological processes sensitive to environmental temperature.
Some reptiles have physiological mechanisms that allow substantial tolerance of thermal variation, while others require relatively narrow temperature ranges. Climate affects digestion, growth, reproduction, locomotion and immune function. Consequently, changes in temperature regimes can alter reptile distributions, breeding seasons and survival, although responses differ according to species, habitat and behavioural flexibility.
Reptile reproduction is diverse. Many species lay eggs containing protective membranes and shells, while some give birth to live young through different developmental mechanisms. Sex determination varies, with some species using genetic mechanisms and others showing temperature-dependent sex determination. In species with temperature-dependent sex determination, incubation conditions can influence the proportion of males and females produced, making climate and nest microhabitats biologically significant. Courtship can involve visual displays, chemical signals, body movements, biting or vocalisations. Parental care ranges from almost none to prolonged nest guarding in some crocodilians and other species. Reproductive rates and maturation times vary substantially, with some reptiles producing many offspring but experiencing high juvenile mortality, while others invest heavily in fewer young.
Reptiles occupy important ecological roles as predators, prey, herbivores and ecosystem modifiers. Snakes regulate populations of rodents and other prey, lizards consume insects and vegetation, turtles can influence aquatic food webs and crocodilians can affect wetland communities. Herbivorous reptiles may contribute to seed dispersal, while aquatic reptiles can transfer nutrients between water and land. Large predators can influence prey behaviour and abundance, producing indirect ecological effects. Reptiles also serve as prey for birds, mammals and other reptiles. Because many species have relatively specialised habitat and thermal requirements, their populations can respond to changes in vegetation, water availability and temperature. Ecological studies therefore examine reptiles not merely as isolated species but as components of food webs and energy flows.
Reptiles have substantial cultural, economic and scientific importance. Some are kept as companion animals, while others support ecotourism or traditional industries. Certain species provide meat, eggs, skins or other products, although harvesting is regulated or prohibited in many jurisdictions where conservation concerns exist. Reptiles have contributed to scientific research on physiology, venom, regeneration, reproduction, behaviour and evolution. Human impacts include habitat destruction, road mortality, illegal collection, persecution, pollution, invasive species and climate change.
Conservation measures include protected habitats, nesting-site protection, captive breeding, regulated trade and population monitoring. Sea turtles face additional pressures associated with marine pollution, fisheries interactions and changes to nesting beaches. Successful conservation frequently requires protecting both breeding sites and the broader landscapes or marine environments used during feeding and migration.
A common misconception is that Reptiles are primitive or biologically inferior to mammals and birds. Reptiles possess highly specialised sensory, physiological and behavioural adaptations that reflect their evolutionary histories. Another misconception is that all reptiles lay eggs, whereas live-bearing reproduction occurs in multiple reptile lineages. It is also inaccurate to treat snakes, turtles, crocodilians and lizards as biologically interchangeable because their anatomy, ecology and evolutionary relationships differ substantially.
Modern evolutionary classification further complicates the everyday meaning of Reptiles because birds descend from reptilian ancestors and are nested within the broader reptile evolutionary tree. Understanding Reptiles therefore requires distinguishing traditional ecological usage from modern phylogenetic classification and recognising that the group encompasses extensive diversity in physiology, reproduction, behaviour and habitat use.
