Andy M.
Certified Pretend Chef
It's mostly that sex thing. Mine are bigger than yours so I'm more "attractive" for a mate or used to out phase a rival or even to fight one. The Chinese water deer use theirs for aggression - they can move the tusks forward or back.Water deer - Wikipedia, the free encyclopedia
Sometimes they are used for exposing their eats so scraping away a covering to expose food - a lot of the pigs do this.
Ray
Thanks, Ray. If they were on the top of his head, they'd be horns or antlers.