There are over 5500 species, divided into 1250 genera, over 150 families and almost 30 orders. They are very small, like a mouse, or very large, like a hippopotamus. And everything in between. Mammals are found almost everywhere in the world. They have adapted extremely well to the different conditions found there. For example, some mammals live almost exclusively high up in the trees, while others live underground. Mammals even live in the sea. Think of the blue whale: it is even the largest mammal on earth!