Mammals are characterized by a number of physical and behavioral traits. They have a four-chambered heart and a backbone made up of vertebrae, which gives them a high level of mobility and agility. Most mammals give birth to live young, although a few species, such as marsupials and monotremes, have a more primitive reproductive system in which the young are born in an underdeveloped state and complete their development in a pouch or external structure.

Mammals are also distinguished by their ability to regulate their body temperature. This allows them to live in a wide range of environments, from cold polar regions to hot deserts. They have a number of physiological adaptations that help them maintain a consistent body temperature, such as insulation provided by their fur or blubber and the ability to shiver to generate heat.

Mammals are classified into a number of different orders, including primates (e.g., humans, apes, and monkeys), carnivores (e.g., cats, dogs, and bears), and rodents (e.g., rats, mice, and squirrels). Within each order, there are many different species with a wide variety of physical and behavioral characteristics.

They play important roles in the ecosystems in which they live. Many species are important predators, helping to control populations of other animals and maintain a balance in their environment. Some mammals, such as bees and bats, are important pollinators, helping to spread pollen and seeds and support the growth of plants. Others, such as cows and goats, are used for milk production and meat, and their grazing can help maintain the diversity of grasslands and other habitats.

In addition to their ecological roles, mammals have a long history of interaction with humans. Many species, such as dogs and cats, have been domesticated and are kept as pets. Others, such as cows, sheep, and goats, are important sources of food, clothing, and other products. Humans have also hunted and exploited many species of mammals for their fur, ivory, and other body parts, leading to declines in some populations and the extinction of others.

Overall, mammals are a diverse and important group of animals that play a variety of roles in the world's ecosystems and have a long history of interaction with humans.