With well over 1000 genera, the class mammalia is a diverse one. Included within it are bears, cats, primates, bats, dogs and even human beings are mammals. Despite this astonishing diversity, ranging from the miniscule field mouse to the picturesque blue whale, all mammals share a few characteristics unique to mammals. The defining feature of a mammal is that the dentary (lower jaw bone) attaches directly to th skull. Mammals are endothermic, which means their body produces its own heat without the need for external aids, such as the sun. Mammals also nurse their young with milk, which is produced by mammary glands, and most mammals have hair on their body.