Reptiles are a diverse group of vertebrates with ancient origins. Their lineage dates back to the Carboniferous period, 315 million years ago, when early proto-reptiles evolved from tetrapods that adapted to life on dry land. Roughly 12,000 species of reptiles inhabit various environments, including turtles, crocodiles, lizards, and snakes.
Defining Physical Characteristics
Reptiles possess scaly skin, a primary identifier. These scales are composed of keratin, a tough protein that provides a protective barrier against physical damage and significantly reduces water loss. Unlike the permeable skin of amphibians, reptilian skin is designed for water retention, often featuring overlapping scales and sometimes lipid layers that act as a waxy, water-repellent coating.
Their body plan involves four limbs or descent from four-limbed ancestors, though some species like snakes have lost their limbs over evolutionary time. The skeletal system of reptiles follows a general vertebrate pattern, including a bony skull, a long vertebral column enclosing the spinal cord, and ribs that form a protective cage around internal organs. Specific adaptations exist across different reptile groups; for example, turtles have a unique shell that integrates with their ribs and vertebral column, while snakes have a highly flexible skull with mobile jaw bones to accommodate large prey.
Thermoregulation and Metabolism
Reptiles are ectothermic, meaning they rely on external sources to regulate their body temperature, rather than generating internal heat like mammals or birds. This reliance on environmental heat leads to a metabolic rate that fluctuates with the surrounding temperature; a cooler environment results in a decreased metabolic rate, while warmer temperatures increase it. This is why reptiles are often seen basking in sunny spots, absorbing heat to raise their body temperature and enhance metabolic activity for functions like digestion and hunting.
To avoid overheating, reptiles also engage in behavioral thermoregulation by seeking shade, burrowing underground, or retreating to cooler water. This ability to adjust their behavior allows them to maintain a preferred body temperature range that supports their physiological processes. Compared to endotherms, ectothermic reptiles have lower resting metabolic rates, 10-20% of that of similarly sized mammals and birds. This lower energy demand allows many reptiles to survive for extended periods without food, making them highly efficient in converting consumed energy into biomass, with production efficiencies ranging from 10-40%.
Reproduction and Sensory Adaptations
Reptiles exhibit varied reproductive strategies, but a common feature is the amniotic egg, a major adaptation for terrestrial life. This egg contains specialized membranes, such as the amnion, chorion, yolk sac, and allantois, which protect and nourish the developing embryo in a dry environment. Most reptiles are oviparous, meaning they lay eggs, which have leathery shells and are buried for incubation. Some snake and lizard species are viviparous, giving birth to live young, with embryos developing inside the mother and sometimes receiving nourishment through a placenta-like structure. Another strategy, ovoviviparity, involves eggs hatching internally, with live young emerging from the mother’s body.
Beyond reproduction, reptiles possess specialized sensory adaptations for navigating their surroundings and locating prey. Many lizards and snakes use a unique chemosensory organ called Jacobson’s organ, located in the roof of their mouth. They flick their tongues to collect scent particles, transferring these to Jacobson’s organ for interpretation, allowing them to “taste” their environment and detect prey or mates.
Certain snakes, such as pit vipers, boas, and pythons, have heat-sensing pits on their heads that detect infrared radiation emitted by warm-blooded animals. These pits are highly sensitive, capable of detecting temperature changes as small as 0.001°C, which enables these snakes to accurately strike prey even in complete darkness. While vision varies among species, some nocturnal reptiles have slit-shaped pupils for low-light conditions, and some arboreal snakes have horizontal pupils that enhance depth perception.