Defining Characteristics:
* Hair or Fur: All mammals have hair or fur at some point in their lives, even if it's only present during a larval stage. This helps with insulation, protection, and sensory functions.
* Mammary Glands: Mammals are named for their mammary glands, which produce milk to nourish their young. This is a defining characteristic and sets them apart from other animal groups.
* Three Middle Ear Bones: Mammals have three bones in their middle ear (malleus, incus, and stapes) that help transmit sound vibrations. These bones evolved from the jaw bones of their reptile ancestors.
* Live Birth: Most mammals give birth to live young, although a few lay eggs (monotremes).
* Endothermy: Mammals are warm-blooded, meaning they can regulate their own body temperature internally. This allows them to live in a wider range of environments.
* Four-Chambered Heart: Mammals have a four-chambered heart, which is more efficient at circulating oxygenated blood throughout the body.
* Diaphragm: Mammals have a diaphragm, a muscular sheet that helps with breathing.
Other Common Characteristics:
* Teeth: Most mammals have teeth that are specialized for different functions, such as cutting, tearing, grinding, or chewing.
* Advanced Nervous System: Mammals have a relatively large brain compared to their body size, which allows for complex behaviors and learning.
* Sweat Glands: Many mammals have sweat glands, which help regulate body temperature.
* Social Behavior: Many mammals exhibit complex social behaviors, including forming groups, communicating, and cooperating.
Exceptions:
It's important to note that there are always exceptions to these rules. For example, some mammals lack teeth (like anteaters) or have fur that is very reduced (like whales). However, these exceptions don't change the fundamental characteristics that define mammals.