Mammals

Mammals are a class of animals that are characterized by the presence of mammary glands, which produce milk for feeding their young. They also have a four-chambered heart, hair, and a neocortex, a region of the brain that is responsible for higher-order thinking. Additionally, most mammals give birth to live young, although a few lay eggs.

There are approximately 5,500 species of mammals, which can be divided into two subclasses: the Prototheria, which includes the monotremes, and the Theria, which includes all other mammals. The Theria can further be divided into the Eutheria, or placental mammals, and the Metatheria, or marsupial mammals.

Mammals are found on every continent except Antarctica. They range in size from the tiny bumblebee bat, which weighs less than a gram, to the blue whale, which can weigh up to 150 metric tonnes.

Mammals play important roles in many ecosystems. They are a keystone species in many food webs, and their grazing and foraging habits can have a significant impact on the vegetation of an area. Additionally, their burrowing and nesting habits help to aerate and turn the soil, and their dung provides a valuable source of nutrients for plants.

Mammals are also popularly kept as pets. Dogs and cats are the most popular mammal pets in the world, but many other mammals, such as rabbits, guinea pigs, and hamsters, are also kept as companion animals.

The class Mammalia is divided into two subclasses: the Prototheria, which includes the monotremes, and the Theria, which contains the eutherians and the metatherians. The monotremes are oviparous, laying eggs. The eutherians, or placental mammals, give birth to live young and have a placenta, a complex organ that joins the mother and the fetus and provides nutrients and oxygen to the developing fetus. The metatherians, or marsupials, have a rudimentary placenta and give birth to relatively undeveloped young, which complete their development in a marsupium, or pouch.

The mammalian brain is relatively large and complex, and mammals have well-developed senses of sight, hearing, and smell. Some mammals, such as primates, have specialized features that allow them to grip objects and to make fine movements with their hands and fingers.

Mammals are social creatures, living in groups that may range in size from a pair of mated animals to herds of thousands of individuals. Membership in a group provides protection from predators and assistance in finding food and mates.

Mammals have been studied for centuries and are the subject of intense scientific research. They are important to humans both as companions and as model organisms for the understanding of human physiology and disease.

The study of mammals is known as mammology. Scientists who specialize in this field are known as mammalogists.