Why do some animals have tails?



Why do some animals have tails?


Depends on what kind of “why” you’re asking.

If you mean “what function does it serve”, it varies from animal to animal, but one of the most common uses is for balance.

If you mean “where does it come from”, it comes down to natural selection: some individuals developed longer tails, that were useful for whatever reason in specific environments, and so those wity shorter or no tails died out or migrated to environments where their tail length was more benefitial, and thus their descendants inherited those traits.

If you mean “how did it come to be in the first place”, it also comes down to evolution, but it’s a more specific awnser this time: it’s simply the result of bilateral symmetry. It was extremely useful for moving in water (compared to biradial simmetry, more suited for static organisms), and thus most if not all mobile animals, descended from whatever specimen first developed it, are symmetric. Tails are just appendixes that develop at the end of the body, and can be useful for many things, including balance, so it’s no wonder so many animals develop it.

But a more “unified” tail is that of vertebrates, which, in many cases (excluding cases like horses, in which “tails” are just strands of hair) are the same: the vertebral column extending past the cervix (hip bones).

Evolution-wise, it’s a remnant from our waterborne history where all water creatures use some sort of tail to move.
It’s a very low cost/movement mechanism.
Then land based animals either kept it because it’s a low cost balance mechanism, or keep it to navigate (birds and so on)