The magnificent pronged antlers growing from the skull of a male red deer are billboards. Their size reveals his strength and fighting ability to other males, and his health and quality as a mate to females. Similar horns, antlers and crests adorn the skulls of other animals, including giraffes, antelope, goats, cows, sheep, and dinosaurs like Triceratops. They’re all highly visible, as befitting their role in signalling. What use, after all, is a billboard that you can’t see?

But what if that wasn’t necessary? What if an animal had a way of “seeing” inside a rival’s body? Would antlers need to be visible at all?

Gol’din thinks that these structures are internal versions of a red deer’s antlers. They serve a similar