Deer grow bony extensions from their skulls called Antlers.
Antlers are grown and shed in one year by the male deer, known as a stag. Once shed, a new pair of antlers grows.
While antlers are being grown they are fed by the deer's body through blood vessels in the soft, furry skin-covering called velvet.
When fully grown, the antlers no longer need the nourishing blood from the velvet and it is vigorously rubbed off, exposing the hard, bony weapons.
Stags use their antlers against other male deer in fights over females during the breeding seasons.
Both the male and female antelope grow a pair of horns. Antelope horns continue to grow throughout their lifetime. Horns are made up of two parts - the inner, bony core growing from the skull, and the outer covering, which is not like the velvet of the deer.
The bony core is covered by a tough growth of keratin, which is very similar to human fingernails.