What main characteristic defines a mammal?

The main characteristic that defines a mammal is their warm-blooded nature. Mammals are part of a class of animals known as Mammalia, and one of their key traits is the ability to regulate their body temperature internally, regardless of the surrounding environment. This warm-blooded quality allows mammals to maintain a stable internal temperature, which is crucial for their metabolic processes and overall survival in various habitats, from hot deserts to cold polar regions.

Mammals also typically have other distinguishing features such as fur or hair and the presence of mammary glands that produce milk to nourish their young. However, the aspect of being warm-blooded is essential as it sets them apart from cold-blooded animals, which rely on external environmental temperatures to regulate their body heat.

The other options do not accurately represent mammalian characteristics. For example, the ability to fly is not exclusive to mammals, as seen in birds and some insects. The presence of feathers is a defining feature of birds, while the ability to live underwater is common in many different animal groups, including fish and some mammals like whales, but does not alone define the entire class of mammals.