On a sunny summer day in Southcentral Alaska, a young bull moose steps into a clearing sporting a growing set of antlers. Across Alaska, male deer, moose, and caribou are growing antlers. Antler is the fastest growing bone known, and the velvet antler of a big healthy moose or elk can grow three-quarters of an inch per day in summer. It's a tremendous expenditure of energy, comparable to a pregnant female gestating a calf. And it's an annual occurrence - all those antlers will be shed in the winter.
In spring and summer, the growing antler has a complex network of veins and arteries in the marrow of the developing bone. The skin covering the growing antler is called velvet, and has nerves and hair and oil glands. The growing antler feels warm to the touch, and the deer would feel you touching it.
A young male deer, a fawn born in the spring, gets a surge of testosterone in his first summer that causes the growth of two bony bumps called pedicles, which will become the platform for future antlers. The first fall, small buttons develop, but it's not until the next summer that the first recognizable antlers form, small spikes. Antler size depends on genetics, nutrition and the health of the deer, and its age. A young buck, two or three years old, is still growing its skeleton, but once it's full grown, nutrients that went to bone growth are transferred to the antlers.
Antlers are not horns. Deer, moose, elk and caribou have antlers, sheep and goats have horns. Horns have a bony core covered with keratin, the same protein that hooves, claws and fingernails are made of. Horns are not shed, and if they're cut off, they don't grown back.