The males of this species have a penis that is almost 8 inches long. They can fully extend it in 1/3 of a second! This means that females have a very short amount of time to escape if they don't want to mate with a particular male.
In order to not get raped, the females evolved a vaginal canal that twists and turns, blocking the penis from going completely in. The male can only impregnate the female if she relaxes all these muscles inside her! Biologists think this is the first evidence they've seen of evolution driving a competition in genitals.