While not exhaustive, I have not been able to find another ape species wherein some of the males have retractable penises and others don't. Why do humans have this trait alone among apes? Is it a trait only found in certain ethnic groups among humans, or is it species-wide?
I've been thinking about Bison and cows of late, and how they are able to breed and produce fertile offspring even though they are different species. They are able to do this because they share the same number of chromosomes as I understand it.
So if that's the case - and I know this is a stupid question - but since Humans and Platypuses both have 46 chromosomes, could we breed with platypuses? If not, why not? Why don't the same rules apply?