nisäkäs

Nisäkäs is the Finnish word for mammal. Mammals are a class of vertebrate animals distinguished by the presence of mammary glands which in females produce milk for feeding their young, a neocortex (a region of the brain), hair or fur, and three middle ear bones. They are endothermic, meaning they generate their own body heat, and are thus also known as warm-blooded animals.

The class Mammalia is remarkably diverse, encompassing over 6,400 species that inhabit every major habitat on Earth. From the smallest shrew to the largest whale, mammals exhibit a wide range of sizes, shapes, and adaptations. They are found in oceans, deserts, forests, grasslands, and even polar regions.

Key characteristics that define mammals include their vertebral column, a backbone that protects the spinal cord. Most mammals give birth to live young, although a small group called monotremes, like the platypus and echidna, lay eggs. Mammals breathe air using lungs, and typically have four limbs, though these can be modified for various functions such as wings for flight in bats or flippers for swimming in marine mammals. Their skeletal structure is also a defining feature, with a single bone in the lower jaw and the aforementioned three middle ear bones: the malleus, incus, and stapes.

The evolutionary history of mammals dates back to the Mesozoic Era, the age of dinosaurs, with early mammals being small, shrew-like creatures. Over millions of years, they diversified and eventually came to dominate terrestrial ecosystems after the extinction of the dinosaurs. Humans (Homo sapiens) are also mammals, belonging to the order Primates.