Grown as an extension of their skull generally by only male deer and antelope, antlers are used to attract females and also for fighting other males. Reindeer are the exception where both the male and the female of the species grow antlers (they use them to clear snow to get access to the vegetation beneath so it is practical for both sexes to have them).
Amazingly given the size they can reach, many of the species that have antlers in temperate and arctic zones shed and regrow them each year! When you see them covered in velvet you know they are growing new antlers. The velvet provides oxygen and the required nutrients to grow the necessary bone to form the antlers.
It is fascinating to see these big antlers but as always it is so much better to see them on the magnificent creatures that grow them. Luckily Denali National Park offers plenty of opportunities to see big bull Reindeers and Moose.