Defining Physical Features
Reptiles are vertebrates characterized by distinct physical attributes. Their outer covering, often consisting of scales, provides protection and helps prevent water loss. These scales are formed from keratin, a tough protein also found in human nails and hair, and differ from fish scales. Their dry skin, lacking glands or moisture, helps them thrive in diverse, often arid, environments.
Reptiles regularly shed their outer layer of skin, a process known as ecdysis, to accommodate growth and repair damage. Structurally, most reptiles possess a backbone and typically exhibit a four-limbed body plan. Even snakes, which are limbless, evolved from ancestors that had four limbs.
Many reptiles display a sprawling posture where their limbs extend outwards from their body. Unlike many mammals and birds, all reptiles breathe air throughout their lives using lungs. This respiratory method distinguishes them from amphibians, which often rely on gills or skin respiration during at least part of their life cycle.
Temperature Regulation
Reptiles manage their body temperature primarily through external sources, a characteristic known as ectothermy. This means they rely on the surrounding environment to gain or lose heat, rather than generating significant internal heat like mammals or birds. While often called “cold-blooded,” this term can be misleading, as a reptile’s blood can be quite warm when basking in the sun.
To regulate their temperature, reptiles engage in various behavioral strategies. They may bask in direct sunlight to absorb heat, or seek shade and burrows to cool down when temperatures become too high. Some species flatten their bodies to increase sun exposure, maximizing heat absorption. They also press against cool surfaces to dissipate excess heat.
The ectothermic nature of reptiles results in a lower metabolic rate compared to endothermic animals. This means reptiles require less food and energy. Their activity levels are directly influenced by ambient temperatures, leading to reduced activity or dormancy during extreme heat or cold.
Reproduction and Development
Reptiles reproduce through internal fertilization, where the male’s sperm fertilizes the female’s eggs inside her body. A significant evolutionary adaptation that allowed reptiles to colonize terrestrial environments is the amniotic egg. This specialized egg provides a self-contained aquatic environment for the developing embryo, freeing reptiles from the need to return to water bodies for reproduction.
The amniotic egg contains several key membranes that protect and nourish the embryo:
- The amnion encloses the embryo in a fluid-filled sac, preventing desiccation and providing cushioning.
- The chorion facilitates gas exchange with the external environment.
- The allantois stores metabolic wastes and assists in respiration.
- The yolk sac provides a rich source of nutrients for the developing embryo.
Most reptiles are oviparous, meaning they lay eggs that hatch outside the mother’s body. However, some species, particularly certain snakes and lizards, are viviparous, giving birth to live young.
Even in viviparous species, the developing embryos retain the amniotic characteristics and are nourished within the mother’s body, rather than through a direct placental connection like in most mammals. Parental care among reptiles is generally limited; most species lay their eggs and offer little to no further involvement, though notable exceptions exist, such as crocodiles, which actively guard their nests and young.