What defines a reptile in contrast to other animal classes?

Reptiles are primarily defined by their cold-blooded metabolism, also known as ectothermy. This means that their body temperature is largely determined by the external environment, as they do not have internal mechanisms to regulate body heat like mammals do. This characteristic allows reptiles to conserve energy, particularly in environments where temperatures fluctuate, as they can adjust their activity level based on the warmth of their surroundings.

In addition to their metabolic traits, reptiles exhibit specific anatomical and physiological features distinct from other classes of animals, such as scales that help prevent water loss, laying amniotic eggs, and a three-chambered heart. These adaptations have enabled reptiles to thrive in various habitats, but the defining aspect remains their dependence on external temperatures for metabolic functions, which is a key differentiator from mammals and birds, both of which are warm-blooded.

The other choices represent characteristics of entirely different animal classes; for example, feathers define birds, the ability to live both in water and on land is typical of amphibians, and hairy skin is a characteristic of mammals.