According to National Geographic.
Male, or bull, moose grow their antlers each year through the spring and summer. Female moose, called cows, don’t grow antlers. But males’ impressive headgear is made of bone that is an extension of the skull. When they first form, antlers are covered with a layer of skin called velvet, which nourishes the bone as it grows. When finished, the velvet sheds off, a process that the moose often accelerate by rubbing on trees.