Only male moose have antlers. The antlers are broad and flat, and shed at the end of every mating season.
Female moose choose mates based on horn size. Moose in their prime, 5 to 12 years old, have the largest antlers. Regrowing them each year reflects the health and condition of the animal as a mate.
Antler growth is tied into testosterone production. They grow to full size in the spring and summer months, while testosterone levels are building for the fall mating season. Calcium for antler-building comes from food and from the moose's own bones, especially the rib cage.
When mating season is over, testosterone levels fall, and the bone at the base of the antlers begins to dissolve. The antlers become loose and eventually fall off, usually in November to mid-January. Some of the antler calcium is resorbed into the moose's body before shedding. Moose conserve energy in winter by not having to carry around the heavy antlers.