The fastest-growing bone there is
Why Do Deer Grow New Antlers Every Year?
A deer throws away a pair of antlers every year and grows a whole new set, faster than any other bone grows anywhere.
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Step 1 of 5
Antlers are not horns.
A horn is a permanent bony core in a sleeve of the same stuff as your fingernails, and it grows all its owner's life. An antler is bare bone, and it is temporary.
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Step 2 of 5
They grow under living skin.
A growing antler is soft, warm and wrapped in furred skin full of blood vessels. Under that cover the bone builds outward at a speed nothing else in the body matches.
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Step 3 of 5
Then the blood is cut off.
When the antler is finished the blood supply shuts down. The skin dies and is rubbed away on trees, and what is left is dead, hard bone — a weapon and a display.
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Step 4 of 5
A big set is an honest advertisement.
Antlers cost an enormous amount of food and mineral to build. A deer in poor condition simply cannot grow a large pair, so the size cannot be faked.
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Step 5 of 5
Starting again fixes every mistake.
A broken horn stays broken for life. A broken antler is dropped with the rest at the end of the season, and next year's pair grows back perfectly.
Shed antlers rarely lie around long. Mice, squirrels and even deer themselves gnaw them for the minerals.
The short version
Antlers are bone rebuilt every year, so their size honestly shows a deer's condition and any damage is undone next season.
Try it yourself
Press a fingernail and then your shin. Horn is the first kind of material; antler is the second.