Nisäkäisen

Nisäkäisen is a Finnish word that translates to "mammal" in English. It refers to any member of the class Mammalia, which is characterized by the presence of mammary glands in females, a neocortex in the brain, fur or hair, and three middle ear bones. Mammals are a diverse group of vertebrates found in nearly all terrestrial and marine habitats, as well as in the air.

The defining characteristic of mammals is the production of milk by mammary glands to nourish their young. This allows for a prolonged period of parental care, which is common among many mammal species. Mammals also possess hair or fur, which helps with insulation, camouflage, and sensory perception. Their skeletal structure includes a unique set of three bones in the middle ear (malleus, incus, and stapes) that are crucial for hearing.

Mammals exhibit a wide range of adaptations to different environments. For example, aquatic mammals like whales and dolphins have streamlined bodies and flippers, while arboreal mammals such as monkeys have prehensile limbs and tails for climbing. Bats are the only mammals capable of sustained flight, possessing wings formed from membranes stretched between their elongated fingers and body.

The class Mammalia is divided into three major subclasses: monotremes (e.g., platypus, echidnas), marsupials (e.g., kangaroos, koalas), and placentals (e.g., humans, dogs, elephants). Monotremes are unique among mammals as they lay eggs. Marsupials give birth to underdeveloped young that complete their development in a pouch. Placentals give birth to more developed young after a longer gestation period, supported by a placenta that transfers nutrients from the mother.