Why are chimps/apes/monkeys bums inside out?!?

Question

I mean they kinda look wrong and painful! Any ideas?

Answer 1

They seem to like it that way. Their sexual preference express themselves in the survival of traits like this.

Answer 2

Their called ischial callosites and they are visual signals to tell other's where they are in their reproduction cycle.  At ovulation, they are the most swollen ready for copulation.

Answer 3

Cause other chimps think it's s**y.

Same reason human female mammary glands are so large.

Answer 4

The female genitalia get swollen when they are receptive.  As far as their bum being inside out, I am not sure what you have been seeing.