The biggest creature that has ever lived, the blue whale, is a mammal. So too are tiny pygmy shrews, giant pandas and human beings. Mammals are an amazingly varied group but they are all warm-blooded vertebrates. They have a bony skeleton and hair on their bodies at some stage of their lives. All mammals feed their young on milk from their own bodies and adult females have mammary glands for producing this milk. Mammals are generally agreed to be the most intelligent of all animals and they have the most highly developed nervous systems. Most mammals have four limbs, but in some these have become adapted into flippers for swimming power (seals and whales) or even wings (bats). There are about 4600 species of mammal. Most live on land, but whales, dolphins, seals and sea lions are among those that have become adapted to life in water. Bats wings are made of skin stretched over a flexible bone structure like the human hand, so they can control their direction very easily. This makes them expert fliers and they spend much of the time in the air. Mammals are usually divided into three groups—monotremes (egg-laying mammals), marsupials (pouched mammals) and placental mammals. The placental group is by far the largest.