There are approximately 5,400 known mammal species on Earth. All mammals have hair or fur, are warm blooded, give live birth and have mammary glands for nursing their young. Of course, in the animal world nothing is absolute. There is an unusual group of egg-laying mammals known
as monotremes. The smallest mammal is the Kitti’s hog-nosed bat, which only weighs 0.05 ounces and can sit on top of your finger. The largest is the blue whale, which can be 100 feet long and weigh 150 tons.