Mammalian biology studies the characteristics and functions of mammals, a diverse class of animals known for their ability to produce milk through mammary glands. Mammals typically have hair or fur, warm-blooded metabolism, and three middle ear bones, which aid in hearing. They are found in various habitats, from oceans to forests, and exhibit a wide range of sizes and behaviors. Mammals are divided into three main groups: monotremes, which lay eggs; marsupials, which carry their young in pouches; and placental mammals, which nourish their young through a placenta during gestation. This classification highlights the evolutionary adaptations that enable mammals to thrive in different environments.