Mammals

Mammals are a group of vertebrate animals constituting the class Mammalia (/məˈmeɪliə/), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, with which they share a common ancestry. The class is divided into two subclasses: the egg-laying monotremes and the therians, which include the marsupials and placentals. As of 2021, there are approximately 6,400 recognized species of mammals, distributed in about 29 orders.

Characteristics

Mammals are endothermic amniotes, distinguishing them from reptiles and amphibians, which are ectothermic. The majority of mammals possess a neocortex, a brain region unique to them. This part of the brain is responsible for higher order brain functions such as spatial reasoning, conscious thought, and language. Mammals have a heart with four chambers and a closed circulatory system, which separates the oxygenated from deoxygenated blood, allowing for a higher metabolism and activity level.

Reproduction and Development

Mammalian reproduction is notably diverse. Most mammals are viviparous, giving birth to live young. However, the monotremes lay eggs, and the young hatch from these eggs. Marsupials, on the other hand, give birth to undeveloped young that complete their development in the mother's pouch. Mammals have various gestation periods, ranging from about 12-13 days in some rodents to 22 months in elephants.

Classification

The class Mammalia is divided into two subclasses: the Monotremata, which includes only five species of egg-laying mammals such as the platypus and echidnas; and the Theria, which includes all other mammals. Theria is further divided into two infraclasses: Marsupialia (marsupials), which includes kangaroos, koalas, and opossums; and Eutheria (placentals), which encompasses the vast majority of mammals, including humans, whales, and elephants.

Evolution

Mammals are believed to have evolved from synapsid ancestors during the late Carboniferous period, with the first true mammals appearing in the Jurassic period. The early mammals were small and likely nocturnal to avoid predation by dinosaurs. After the Cretaceous-Paleogene extinction event 66 million years ago, mammals began to diversify into the myriad forms seen today.

Habitat and Distribution

Mammals inhabit a wide range of environments, from the deepest oceans to the highest mountains, and from the Arctic tundra to the tropical rainforests. They have adapted to life in the air, on land, in fresh water, and in the sea. The distribution of mammals is global, with Antarctica being the only continent without native mammal species, although marine mammals do visit its waters.

Conservation

Many mammal species are threatened by human activities such as habitat destruction, hunting, and pollution. Conservation efforts are underway globally to protect endangered mammals, including legal protection, habitat restoration, and the establishment of wildlife reserves.