Mammals are vertebrate animals that have hair on their bodies. Hair can take many forms, including whiskers, spines, fur, and even horns. All mammals have lungs, so even those living in water must come to surface to breath air. Although two mammal species lay eggs, all others give birth to live babies. All female mammals feed their babies with milk from their own bodies and therefore always care for their babies for a period of time after they are born. Like birds, all mammals are warm-blooded, which means that they can make their own body heat and can stay warm even when the sun is not out. Mammals can be predators, herbivores, or omnivores. Different species eat different kinds of food, including other vertebrates (living or dead), invertebrates, green plants, nuts , seeds, or fruit. Most mammals live on the ground, but there are many that live in trees or make burrows under the ground, and some mammals like the dolphins and whales live their entire lives in the oceans.