The world has nearly 4,500 kinds of mammals – all of which are unique, from how they reproduce to their various body sizes.
Most female mammals carry their young, nurse them when they are born, and nurture them for a time—much like humans do with their young. These mammals are called “placental mammals.” But a few mammals, called monotremes lay egg. These mammals include platypi, echidnas and anteaders.
Another way in which mammals are unique is their varying body sizes. Some mammals can be big while others are small. The blue whale, the largest sea mammal, is humongous, weighing about 210 tons! The elephant is the largest land mammal. The largest elephant the African Savanna can weigh about eight tons. The smallest mammal in the world, the tiny hog-nosed bat, is the size of a human eye ball! Another tiny mammal, the mouse deer, is about the size of a rabbit and lives in Africa.
Even though there are thousands of mammals all over the world, each one is special in its own way. How they care for their young and sizes are just two factors that make mammals different than other types of animals.
[Source: 100 Things You Should Know About Mammals]