Final answer:
Mammals have three main characteristics: hair, mammary glands, and a complex placenta.
Step-by-step explanation:
Mammals have three main characteristics:
- Hair: Mammals have hair or fur, which can serve various functions such as insulation, sensory perception, and social signaling.
- Mammary glands: Female mammals have mammary glands that produce milk to nourish their young. This distinguishes mammals as the only group of animals that nurse their offspring with milk.
- The presence of a complex placenta: Mammals have a specialized organ called the placenta that provides oxygen and nutrients to the developing fetus.
These three characteristics collectively set mammals apart from other animal groups and are fundamental to their biology and ecology.