Final answer:
Reptiles thrive in arid environments due to their scaly skin with keratin that prevents dehydration, their ability to conserve water through waste management, and their adaptations for ectothermy that make them energy-efficient.
Step-by-step explanation:
Reptiles have developed numerous adaptations to thrive in arid environments, one of the most significant being their scaly skin containing the protein keratin. The scales with keratin significantly reduce water loss, helping to prevent dehydration. These animals are also capable of excreting nitrogen as a uric acid paste, which conserves water in their bodies. Additionally, reptile eggs are protected by a shell and their embryos are encased within an amniotic membrane, adaptations that enable them to reproduce away from water sources.
Their lifestyle as ectotherms allows them to regulate their body temperature using external heat sources, conserving energy that would otherwise be spent on metabolic heat production. With these adaptations, reptiles are less dependent on frequent access to water, and they can survive on fewer calories compared to endotherms of similar size. Therefore, the adaptations of reptiles' skin and their ability to conserve water are key aspects of their success in dry environments.