Animals have tails because tails can be useful in a remarkable number of ways. There is no single answer that fits every species. A cheetah uses its tail differently from a monkey. A deer uses its tail differently from a rattlesnake. A kangaroo uses its tail differently from all of them.
That is the important idea: evolution does not hand every animal the same tool for the same job. A body part can be modified over generations to solve very different problems.
Tails can help animals balance
For many animals, a tail acts as a counterbalance. Watch a squirrel race along a narrow branch and you can see the tail shifting as the animal turns and adjusts its weight.
Cats also use their tails while climbing, jumping, and changing direction. The tail can help the body make small corrections without requiring the animal to stop moving.
Fast-running animals can benefit too. A cheetah’s long tail helps with stability during rapid turns while chasing prey. When the front of the body changes direction, the tail can help control rotation and keep the animal from simply tumbling sideways.
Some tails help with movement
Fish use their tails as major sources of propulsion. Side-to-side movement of the tail pushes against water and helps drive the body forward.
Whales and dolphins move their tail flukes up and down instead. Their tails are not just steering equipment. They are powerful engines.
In trees, some monkeys have prehensile tails that can wrap around branches. In certain species, the tail can support part or even all of the animal’s weight for short periods. That effectively gives the animal another gripping limb.
Tails can be communication systems
Dogs are the obvious example. A dog’s tail position and movement can provide information about excitement, uncertainty, fear, confidence, and social intent. The tail is only one part of canine body language, but it is an important part.
White-tailed deer raise the bright underside of the tail when alarmed. The flash can warn nearby deer and may help keep a group together while escaping.
Rattlesnakes use a specialized structure at the end of the tail to produce a warning sound. The message is wonderfully direct: something large is too close.
Some tails help animals escape
Many lizards can deliberately detach part of the tail when grabbed by a predator. The detached tail may continue wriggling, distracting the attacker while the lizard escapes.
The animal can often grow back part of the tail, although the replacement is not always identical to the original.
That sounds like an extreme feature until you compare the alternatives. Losing a tail is inconvenient. Being eaten is worse.
Tails can store energy or provide warmth
Some animals store fat in their tails. Fat-tailed geckos and several other species can use that stored energy when food is scarce.
A thick furry tail can also work like insulation. Foxes and other animals may curl up and cover parts of the face or body with the tail in cold weather.
Kangaroos use their tails in an especially impressive way
A kangaroo’s heavy muscular tail helps with balance during hopping, but it can do even more. During slow movement, the tail can press against the ground and contribute force along with the legs.
Calling it “just a tail” undersells the equipment.
Why do humans not have tails?
Human embryos briefly develop a tail-like structure, but most of it is absorbed during development. Adult humans retain the coccyx, or tailbone, at the base of the spine.
Our primate ancestors lost an external tail millions of years ago. Other changes in body shape and movement made an external balancing tail less important.
So there is no universal reason animals have tails. Tails are evolutionary raw material. Balance, swimming, climbing, signaling, defense, warmth, fat storage, and escape are all possible jobs.
The better question is often not “What is a tail for?”
It is “What has this particular animal turned its tail into?”
A related animal question is Why do dogs wag their tails?, which shows another way biology shapes familiar behavior or anatomy.
And that’s the short explanation.
Related explanations: Why do dogs wag their tails?, Why do rabbits thump their feet?, and Why do lizards do push-ups?