Reptiles separated from their aquatic ancestors to live on land, but they retain some of their water-related adaptations. They conserve water through scaly skin, for instance, and have efficient excretory systems that concentrate waste.
크레스티드게코They’re ectotherms, too, and can regulate their body temperature with behavioral adaptations such as basking or seeking out shady spots. They also can divert blood flow from their lungs to other parts of the body, conserving energy.
Dry Skin
Reptiles’ dry scaly skin helps them conserve water by not allowing excess moisture to escape. It also protects against predators and other hazards.
Scales are composed of a tough keratin layer that prevents water loss. They are especially thick on the head, neck and tail, where they form plates and shields to protect these areas from abrasion. [2]
These structures can be modified to provide additional support or protection, such as the enlarged scales that make up the turtle shell. Some scales are widened ventrally to form gastropeges that help with locomotion. In crocodiles and some lizards, the outer horny cell layer of the scales can fuse to become a bony plate called an osteoderm. [1]
A reptile’s kidneys are also specially adapted to conserve water. Reptiles do not need to drink water on a regular basis because they can obtain most of the liquid they need from the food they eat. Their bodies also waste less water because they concentrate the body’s waste products into semisolid bundles of uric acid that require very little fluid to be flushed away from the body.
Reptiles separated from their water-dwelling ancestors and climbed onto land during the Paleozoic era, more than 280 million years ago. They dominated Earth’s terrestrial habitats for more than 250 million years until the extinction of the dinosaurs and the rise of mammals, birds, fish, and invertebrates. Today, reptiles include snakes, lizards, and crocodiles along with the turtles, tortoises, and the extinct tuatara of New Zealand.
Water-Resistant Scales
Reptile scales help to keep the body hydrated and prevent water loss. They also offer protection from the sun’s UV rays. Scales are thicker than the skin of mammals and are less susceptible to thermal burns.
The keratin that makes up a reptile’s skin is highly water resistant, and the scales themselves can withstand the pressure of deep diving. They are also capable of abrasion resistance.
Because they are not able to absorb oxygen through their skin like amphibians, reptiles have adapted to breathe in other ways. For example, snakes use muscles of the chest wall to move air into and out of their lungs. Crocodiles, alligators, and other tetrapods have a more complex method for moving air in and out of their lungs: they have specialized dermal bones called osteoderms that are located under their epidermal scales.
Unlike the dermal scales of fish, which are easily scraped off, snake scales can be shed and replaced. They are also arranged in different patterns that can serve to identify the species of snake. Epidermal scales can also be modified into crests, spines, and horn-like processes; these are referred to as scutes.
The squamate reptiles—lizards, snakes, and worm-lizards—are a diverse group that diverged from amphibians approximately 340 million years ago. These are the only tetrapods that lay amniotic eggs, and they have a number of adaptations to help them survive on land:
Water-Conserving Kidneys
Reptiles evolved to live on land with very little water, so they needed to rethink the way their kidneys worked. They had to find a way to conserve body water by optimally reabsorbting the water they filtered through their kidney nephrons. This allowed them to excrete urine with concentrations of solutes largely above that of their blood plasma.
This was possible because reptiles were able to convert their waste nitrogen compounds into uric acid, which is soluble in water but forms a salty paste when it’s excreted from the kidneys into the cloaca. This process used very little of the kidneys’ filtration capacity, which conserved a lot of the water that would otherwise have been wasted during the urea cycle.
To re-concentrate the urine in the cloaca, the kidneys used a very clever technique: They made the ascending limb of the Loop of Henle initially impermeable to sodium chloride (NaCl) and then permeable to water. This created a high salt concentration in the medulla, so it would be easier to concentrate urine in the cloaca by osmosis later on.
The first step of this process occurred during embryonic development. The embryos of vertebrates—including reptiles, birds, and mammals—develop three sets of kidneys. The first set, called the pronephroi, are vestigial organs that soon degenerate and don’t contribute to the final functional kidneys. The nephrons in the second set, known as the mesonephroi, are functional kidneys.
Eggs That Can Survive on Land
The development of a semi-permeable shell in amniote eggs was one of the key evolutionary innovations that allowed reptiles to leave the water and live on land. This important development freed them from the need to return to the water to breed and spawn. It also permitted them to explore new environments, since even aquatic animals must surface to breathe.
Today, crocodilians, turtles, snakes and some lizards lay amniotic eggs that can survive on land. In addition, many reptiles are ovoviviparous, meaning their eggs remain in the mother’s body until they are ready to hatch. A few, such as the platypus and echidna, are viviparous, giving birth to their young alive.
All reptiles have lungs, which allows them to travel farther from their aquatic environment than amphibians. Lungs also permit them to stay hydrated on dry land, since they cannot absorb oxygen through their skin like amphibians.
The first tetrapods to leave the water, reptiles have a number of other important adaptations. For example, their scales, which overlap and protect each other, help prevent them from injury, as shown in this photo of a tree viper snake. Additionally, they can maintain their internal temperature by absorbing heat from rocks or plants that lie near them (thigmotry). In addition, most reptiles do not have a larval stage. Rather, their babies are miniature adults. This reduces their risk of predation and ensures that they can begin eating as soon as they hatch.