Different animals use their tails for different reasons. A dog uses its tail to keep its nose warm when it sleeps (I have a Siberian Husky who curls up with her tail over her nose.) or to express themselves, like wagging it. A range animal like a horse or cow uses the tail to keep flies from biting them. A monkey uses it to balance itself in trees. A lizard uses it to fool other animals into grabbing the tail when they try to eat them. The tail breaks off and the lizard is able to run away. What other animals can you think of that use their tails?

Most four-legged animals do have tails.They might use them for balance when jumping or running. If you're out in the yard on a summer evening, often you can see rats running along the power lines. Their tails help them balance themselves when they do this. Flying squirrels and kangaroo rats use their tails as rudders. They can change direction in mid-air. Dogs use tails for communication. Some animals have "prehensile" tails, meaning they can grab things with their tail. Many monkeys can do this. (Check out some monkeys with prehensile tails at: http://daphne.palomar.edu/primate/prim_4.htm)
There is also a prehensile tailed skink (a kind of lizard). Cows and horses swat away flies with their tails. Can you think of any other uses?

Some of us are "tail impaired". Bears, and hamsters are some of the animals with very small tails. Frogs, chimps and gorillas are with us in the no-tail category. But if you look at a picture of a human embryo that's under abut 7 weeks old, you will see that we all had tails at one point. We just lose them before we're born. We still have the tiny tailbones but they're not visible from the outside. Why do you think some species don't have tails?