Why do deer shed their antlers?

Antlers are made of bone and are grown by nearly all male deer and, in reindeer, by females too. They grow each spring and summer covered in a soft, blood-rich skin called velvet. Rising testosterone before the autumn rut hardens the antlers and the velvet is rubbed off. After mating season, testosterone drops, cells dissolve the bone where the antler attaches, and the antler falls off. A new, often larger set then regrows, making antlers among the fastest-growing tissues in the animal kingdom.