Male fiddler crabs each have a single super-sized claw that they use as a weapon to threaten and fight other males and as beautiful adornment to attract females. Here, see a video of two male dancing fiddler crabs (Uca terpsichores) battle for territory. The male on the left has no burrow of his own—and no mate—while the male on the right tries to defend his own. The crabs with two small claws that look like they're twiddling their thumbs are females picking microscopic food from the sand.
And what happens at the end? The battle-losing burrowless male follows a female into her burrow—not to mate, but to try to oust her and take over her home.
Read more about the evolution of fiddler crabs' beautiful and powerful claws at the blog post "The Design of a Beautiful Weapon."