Unlike the rest of the body, which takes years to grow to adult size, antlers in even the largest bulls go from nothing to full size in just a few months. Antlers grow faster than any other bone in any animal, and this record speed racks up record energetic costs. Estimates from antlers of a related species, fallow deer, show that while males are growing antlers they more than double their daily energetic needs. In addition, growing antlers suck up so much calcium and phosphorus—minerals that make up the bone—that the males cannot possibly get enough from their food. Instead, they leach these vital minerals out of other bones and shunt them to the antlers, depleting the rest of their skeleton so severely that they experience a seasonal form of osteoporosis. Their bones get weak and brittle at precisely the time of the year—the rut—when they must hurl themselves against eight-hundred-pound rivals in incessant battles for access to females. By the end of the rut, the males will have fought so often and so hard that they've lost a quarter of their body weight, and they emerge from this season battered, starved, and brittle boned. If they cannot replenish their reserves in the few short weeks before winter, they'll starve.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 14