I'm designing an alien race that in structure to humans. One exception is that they have long muscular tails.

I remembered while I was designing them, the reason why humans have no tails is because for us they were useless appendages.

Which begs the question, "if they're humanoid as well, wouldn't tails be useless for them?"

One option I considered was that their ancestors used to live on top of huge trees and therefore they needed the tail to get a better grip, but in reality would a humanoid race have such an appendage?

Other notes on their design:

  • they have four arms
  • raptor like legs
  • elongated skull sort of like xenomorphs.
  • their planet has a bit lower gravity)
  • 10
    $\begingroup$ Out of interest, you've give them "raptor like legs" and four arms without justification, why do you feel a tail is exceptional in needing an explanation? $\endgroup$ Commented Jul 28, 2018 at 21:37
  • 5
    $\begingroup$ All monkeys save one species have tails. What about their physiology doesn't work for you? What, exactly, are you looking for? Reasonable scientific justification for a humanoid's tail? Will you need that in your story? Because if you don't need it, it's your creature - declare it to be so. (Bear in mind that we don't have tails because they'd be useless, we once had them, we don't have tails today because our evolution deprecated the need.) BTW: the simple answer to your question is "yes." $\endgroup$
    – JBH
    Commented Jul 28, 2018 at 22:16
  • $\begingroup$ It is rare but not terribly unusual for humans to be born with tails. It is roughly comparable in frequency to being born with an extra finger or missing a finger. $\endgroup$
    – ohwilleke
    Commented Jul 28, 2018 at 23:52
  • 1
    $\begingroup$ I gave a perfectly legitimate answer but it was deleted for "not answering the question" and being a "request for clarification". The question, as I interpreted it, is asking if a humanoid alien species living in trees would have reason to have tails. My answer stated that if the creatures had a use for the tail that having one would be reasonable. However to fully understand your concept, a anatomical drawing would be useful. $\endgroup$
    – ZoneWolf
    Commented Jul 29, 2018 at 12:02
  • 2
    $\begingroup$ @ZoneWolf: " it is bipedal and stands upright, therefore, it is humanoid". Bears, ostriches, penguins...? WRT species with functional tails, cats seem to function quite well without tails, or with very short ones, e.g. catsincare.com/tailless-cat-breeds-and-their-features That includes the bobcat & lynx species, as well as breeds of domestic cat. Citing examples of species in which tails serve an important function in no way disproves the claim that in many species there doesn't seem to be an obvious function. $\endgroup$
    – jamesqf
    Commented Jul 31, 2018 at 5:48

1 Answer 1



Tails can serve many uses for creatures.

  • The kangaroo uses its tail as a third leg aiding in stability and balance. Kangaroos kick fight...which is crazy to watch.
  • Monkeys with prehensile tails use them as an extra arm
  • Many creatures use them for balance
  • Many creatures use them as a defense mechanism, they use their tail to appear larger

To this question: if they're humanoid as well, wouldn't tails be useless for them

The short answer is, it depends.

You are designing the world. If you want them to have tails give them a reason to have tails. If you look at the list above there are several reasons they could have tails even if in the modern era of your world they are no longer necessary.

They could

  1. Still actively need them
  2. No longer need them but have needed them more recently in their evolutionary path than humans
  3. They could be useless but a matter of sexual selection, i.e. people like mates with big long, me...anyway.

Their world matters too. The world will in part define evolution, along with predation and natural selection you can come up with a host of factors that drive the evolution of a species wherever you would like.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .