Comparative anatomy: shouldn't bipedal animals have tails?

Question

Tails serve several purposes, the most obvious of which is balance. Since balance is more difficult for bipeds than quadrupeds, it seems like a tail would be particularly useful for bipeds.

Though arms are pretty good for balance, humans like to use their arms for tasks like carrying babies, spears or other weapons, making it more difficult to use arms for balance. Is two million years too short a period to evolve tails?

OKIM - Ostriches can run at 40 mph. So did some some dinosaurs. In other words, not all bipeds move slowly. You claim there are no advantages to using a tail but offer no evidence - am I supposed to take your word for this?

Are you sure you should be trying to answer this question?

Dr. Y - Thanks - seems like a fairly thoughtful answer. However, I'm having difficulty 'connecting the dots' on your final paragraph.

It seems like you're saying that an upright posture is required in order to use arms and hands the way humans do. But couldn't there be some hypothetical animal that leans forward, has a heavy tail, and has grasping arms? Imagine a kangaroo with prehensile arms...

Answer 1

Tails have their own drawbacks. A tail necessarily has weight, so it pulls your center of balance backward... exactly the opposite direction one would want to lean for forward motion (I suppose you could have a tail in the front, but that could be awkward for other reasons).

You may notice that animals that use their tails for balance either have huge ones and must lean way forward to move at any speed (kangaroos and many dinosaurs) or have comparatively miniscule tails which can only provide correspondingly slight adjustments and balance aid (cats).

In our case, it would seem the upright posture trumps any kind of tail. Being able to hold and manipulate things in almost any direction - even while running - will probably assure that our successors will never have a genetic predisposition to possess this particular trait. That's my take, anyway, for what it's worth.

Answer 2

Bipedal animals usually move slowly. There is no advance having a tail. All quadrupeds seem to have a tail. Another function of the tail is to keep flies at bay.