What feature distinguishes mammals from other animal groups?

Mammals are uniquely characterized by the presence of mammary glands, which are specialized organs that produce milk. This feature is critical for nurturing their young, providing essential nutrients and antibodies during the early stages of development. The evolution of mammary glands is a defining trait that sets mammals apart from other vertebrate groups, enabling a higher level of parental investment in offspring care, which is vital for survival and successful maturation. This distinguishes mammals as warm-blooded animals that can maintain a stable internal body temperature and provide a unique form of nourishment to their young, in addition to other traits associated with mammals such as hair or fur.