What characteristic is a defining feature of mammals?

A defining feature of mammals is the presence of hair or fur. This characteristic is unique to mammals and plays several important roles in their biology. Hair provides insulation, helping to regulate body temperature, which is crucial for maintaining a stable internal environment. This insulation allows mammals to thrive in a variety of climates, from polar regions to deserts. Additionally, hair serves protective functions and is involved in sensory perception through specialized structures like whiskers. These attributes contribute to the adaptability and success of mammals across different habitats.

In contrast, scaly skin is a characteristic of reptiles, feathered wings are associated with birds, and cold-blooded metabolism is typical of ectotherms like reptiles and amphibians. These features differentiate mammals from other animal groups and highlight the evolutionary adaptations that distinguish them in the animal kingdom.