mammals

Mammals are a diverse class of vertebrates within the phylum Chordata. They are characterized by mammary glands that produce milk for nursing young, hair or fur, and three middle ear bones—the malleus, incus, and stapes. Mammals are endothermic amniotes with a relatively high metabolic rate and, in many species, a well-developed brain capable of complex behavior. Most mammals give birth to live young and nurse them with milk, but the monotremes (platypus and echidnas) lay eggs.

Anatomically, mammals have lungs with extensive alveolar surfaces, a muscular diaphragm, and a four-chambered heart. The lower jaw typically consists of a single dentary bone with a varied dentition adapted to different diets. Hair provides insulation and sensory functions, and many species use vocalization, scent marking, and other behaviors to communicate. Mammals also possess a wide range of sensory adaptations, including acute hearing and, in some groups, specialized navigation such as echolocation.

There are about 6,400 species, ranging from the tiny bumblebee bat to the blue whale, occupying nearly all ecosystems from oceans to deserts. They play diverse ecological roles as predators, herbivores, pollinators, and seed dispersers. Many mammals exhibit complex social structures and parental care, with behaviors shaped by ecological pressures.

Mammals originated in the Mesozoic era from cynodonts and first appeared in the late Triassic, later diversifying after the extinction of non-avian dinosaurs. Modern mammals are divided into three major groups: monotremes, marsupials, and placental (eutherian) mammals, each with distinct reproductive strategies and life histories.