Mammals are the dominate animals specie of our time. Although they have been on earth for over 220 million years, they have only ruled the last 65 million years. This period has been known as the Age of Mammals. Today, there are over 5,500 species of mammals. Mammals can be found on land and in water. All mammals breath air, so aquatic mammals need to surface every so often to breath. Mammals can be grouped into three groups by diet: carnivores, herbivores, and omnivores. Carnivores (like lions) eat meat, and hunt other animals to keep themselves fed. Herbivores (like zebras) eat plants, and are food for the carnivores. Omnivores (like bears) eat both plants and meat, and most are opportunistic feeders.