Antlers, on members of the deer family, are grown as an extension of the animal's skull. They are true bone and are a single structure They are generally found only on males. Antlers are shed and regrown each year.
Horns, found on pronghorn, bighorn sheep, bison, and many other bovine, are two-part structures. An interior of bone (also an extension of the skull) is covered by an exterior sheath grown by specialized hair follicles, as are your fingernails. In fact, your fingernails and the exterior sheath of horns are made of very similar materials. Horns are never shed and continue to grow throughout the animals life. The exception to this rule is the pronghorn which sheds and regrows its horn sheath each year.
Leave them on the Ground
Every year male deer, elk, and moose grow a new set of antlers that is generally larger or more massive than the previous year’s "rack." Deer and moose shed their antlers in the fall and early winter following the end of the breeding season. Elk retain their antlers through most of the winter and shed them in mid-February to late March. Many people like to collect shed antlers to use as ornamentation around their homes or to make things like buttons or chandeliers. However, collecting any natural or cultural object (rocks, flowers, antlers, skulls, old bottles, cans, etc.) is illegal in national parks. This is because national parks are "living museums," and everything in a park is important to the story that is told there or to the natural functioning of the park’s ecosystem.
Whether inside or outside of a park, shed antlers left on the ground also provide an important source of minerals for many small animals. Antlers are bone and are mainly composed of calcium. Humans need calcium to keep their bones and teeth strong and growing normally; so do wild animals. Humans eat a variety of foods, like milk, cheese, ice cream, and leafy green vegetables, to get the calcium they need. For wildlife, calcium is harder to obtain. Small mammals, like mice, voles, chipmunks, and ground squirrels, get calcium by gnawing on shed antlers and animal bones. By leaving antlers on the ground, you are helping these animals to survive.