Something I noticed looking at reference images is some animals have legs that are slanted inward relative to the length of their body, like in the two images below. Why have animals evolved this instead of having their legs pointing more straight down like lions or elephants?

enter image description here

  • 2
    $\begingroup$ You picked the two habitual, upright, striding bipeds as examples. $\endgroup$
    – kmm
    Commented Jan 29, 2018 at 0:02
  • $\begingroup$ So? The question remains the same. $\endgroup$
    – John Joe
    Commented Jan 29, 2018 at 0:12
  • 1
    $\begingroup$ I think @kmm is encouraging you to think critically about your question and giving a huge hint about the difference between the examples you came up with that have 'inward slanted' legs versus your counterexamples that do not. $\endgroup$
    – Bryan Krause
    Commented Jan 29, 2018 at 21:31
  • $\begingroup$ No one teaches advanced mechanics until after calculus 3 or maybe 2, and frankly analyzing every possible force at every possible angle of three joints and a body with arbitrary self-locomotion and then guessing why something happened to coincidentally mutate that way is a nightmare for anyone, there's not much to critically think about unless you have a very accurate simulation set up. That's why the sources of answers to questions like these almost always originate from graduate researchers with extensive testing on live animals in motion or expensive robots. $\endgroup$
    – John Joe
    Commented Jan 30, 2018 at 1:06

1 Answer 1


Sorry no references other than illustrations for the moment.

If a running biped has it's feet further away from the line of gravity, then it's got less efficiency in forwards motion and more instability of the line of gravity falling left and right.

When you consider the kinetic downwards weight in kilos that goes into every step, a runner/sprinter who doesn't have aligned steps wastes a lot of energy.

here's are dinosaur footprints: https://australianmuseum.net.au/Uploads/Images/7919/footprint%20comparisons_big.jpg

The hips joints tend to point sideways to give a strong musculature, lots of running muscle, and a wide base of more than 90 degrees, for a wide birth canal and other reasons.

So the tendency for efficiency of running bipeds is to have aligned footsteps and their hips tend to be a strong and wide base.

That's only the resting position, the walking angles actually vary a bit outwards and straight too, the knee goes outwards when it's rising.


here is a comparison of ostrich and human bone angles: http://rsif.royalsocietypublishing.org/content/13/122/20160529

enter image description here


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