Bull moose have antlers like other members of the deer family. Cows select mates based on antler size. Bull moose use dominant displays of antlers to discourage competition and will spar or fight rivals. The size and growth rate of antlers is determined by diet and age; symmetry reflects health.
The male’s antlers grow as cylindrical beams projecting on each side of the head at right angles to the midline of the skull, and then fork. The lower prong of this fork may be either simple, or divided into two or three tines, with some flattening. Moose antlers are broad and palmate (flat) with tines (points) along the outer edge. The antlers of mature Alaskan adult bull moose (5 to 12 years old) have a normal maximum spread greater than 200 centimetres (79 in). By the age of 13, moose antlers decline in size and symmetry. The widest spread recorded was 210 centimetres (83 in) across. (An Alaskan moose also holds the record for the heaviest weight at 36 kilograms (79 lb)).
Antler beam diameter, not the number of tines, indicates age. In North America moose (A. a. americanus) antlers are usually larger than those of Eurasian moose and have two lobes on each side, like a butterfly. Eurasian moose antlers resemble a seashell, with a single lobe on each side. In the North Siberian moose (A. a. bedfordiae), the posterior division of the main fork divides into three tines, with no distinct flattening. In the common moose (A. a. alces) this branch usually expands into a broad palmation, with one large tine at the base and a number of smaller snags on the free border. There is, however, a Scandinavian breed of the common moose in which the antlers are simpler and recall those of the East Siberian animals. The palmation appears to be more marked in North American moose than in the typical Scandinavian moose.
After the mating season males drop their antlers to conserve energy for the winter. A new set of antlers will then regrow in the spring. Antlers take three to five months to fully develop, making them one of the fastest growing animal organs. Antler growth is “nourished by an extensive system of blood vessels in the skin covering, which contains numerous hair follicles that give it a ‘velvet’ texture.” This requires intense grazing on a highly-nutritious diet. By September the velvet is removed by rubbing and thrashing which changes the colour of the antlers. Immature bulls may not shed their antlers for the winter, but retain them until the following spring. Birds, carnivores and rodents eat dropped antlers as they are full of protein and moose themselves will eat antler velvet for the nutrients.
If a bull moose is castrated, either by accidental or chemical means, he will quickly shed his current set of antlers and then immediately begin to grow a new set of misshapen and deformed antlers that he will wear the rest of his life without ever shedding again. The distinctive looking appendages (often referred to as “devil’s antlers”) are the source of several myths and legends among many groups of Inuit as well as several other tribes of indigenous peoples of North America.
In extremely rare circumstances, a cow moose may grow antlers. This is usually attributed to a hormone imbalance.