The characteristic of mammals that distinguishes them from reptiles is the presence of mammary glands that produce milk for nourishing their young. Additionally, mammals generally possess hair or fur, and have a higher degree of endothermy, which allows them to regulate their body temperature. While reptiles are mostly ectothermic, relying on external environments for temperature regulation, mammals maintain a constant internal temperature. These features, along with more advanced brain structures, contribute to the success of mammals in diverse environments across the globe.