1. Hair or fur: Mammals are the only animals that have hair or fur covering their bodies. This serves various purposes, including insulation, camouflage, and sensory functions.
2. Mammary glands: Mammals are named after these glands, which produce milk to nourish their young. This is a unique feature that distinguishes mammals from other animal groups.
3. Live birth: Most mammals give birth to live young, rather than laying eggs. This allows for a longer period of parental care and nurturing of the offspring.
4. Three middle ear bones: Mammals possess three bones in their middle ear (malleus, incus, and stapes) which are essential for hearing. These bones evolved from bones that were part of the jaw in their reptile ancestors.
While these four traits are the most defining characteristics, there are other features common to most mammals:
* Endothermic (warm-blooded): Mammals can regulate their own body temperature, unlike reptiles and amphibians.
* Diaphragm: A muscular sheet that helps with breathing.
* Four-chambered heart: This allows for more efficient blood circulation and oxygen delivery.
* Complex nervous system: Mammals have highly developed brains and complex social behaviors.
Remember, these characteristics are general trends, and there are exceptions within the vast diversity of mammals.