The moose (Alces alces) is the largest member of the deer family, inhabiting the boreal and mixed forests of the Northern Hemisphere. These massive animals are instantly recognizable by the imposing, broad structures that often crown their heads. Antlers serve a purpose in the life cycle of the species, acting as a dynamic signal of health and strength. Their presence is often the first characteristic used to distinguish between the sexes, leading to confusion when nature deviates from this pattern.
The Standard Rule: Antlers Belong to Males
In almost all species within the deer family (Cervidae), including moose, antler growth is reserved exclusively for the male sex. Male moose, known as bulls, develop and carry these structures to display dominance and secure mating rights during the autumn breeding season, or “rut.” The size and symmetry of a bull’s antlers are direct indicators of his physical condition and genetic quality, allowing female moose, or cows, to select the most vigorous mates. While the cow moose is a formidable animal, she does not typically invest the massive energy required to grow antlers.
The antlers of a mature bull can span up to six feet across and weigh over 40 pounds, a significant burden carried only when necessary. These structures are used primarily for ritualistic display to intimidate rivals, often preventing a physical fight. When two bulls of similar stature spar, the antlers are locked together in contests of strength to establish a clear hierarchy for breeding.
The Biology of Antler Growth and Shedding
Antlers are unique among animal structures because they are not horns, which are permanent keratin sheaths grown over a bony core. Instead, moose antlers are pure bone, forming the fastest-growing tissue known in the animal kingdom. The process of growth and shedding is governed by the interplay between the length of daylight, known as photoperiod, and the male hormone testosterone. As the days lengthen in spring and summer, a bull’s testosterone levels begin to rise, initiating rapid growth from a base on the skull called the pedicle.
During this growth phase, the antlers are covered in a fuzzy, highly vascular skin called “velvet,” which supplies oxygen and nutrients to the developing bone tissue. The antlers can grow at an astonishing rate, sometimes adding up to an inch per day. By late summer, the antlers are fully mineralized and hardened, and the spike in testosterone causes the blood flow to the velvet to cease. This dried skin is then rubbed off by the bull on trees and shrubs, leaving the hard, polished bone ready for the rut.
Once the breeding season concludes and the days become shorter, the bull’s testosterone production declines dramatically. This hormonal drop triggers the formation of an abscission layer at the base of the antler, causing the structure to loosen and eventually fall off, a process known as casting or shedding. Shedding typically occurs between late November and early March, allowing the bull to conserve energy and move more easily through deep winter snows. The cycle then begins anew in the spring, driven by the recurring hormonal surge.
When Female Moose Grow Antlers
The rule that only male moose grow antlers is occasionally broken by a rare biological anomaly. Female moose, or cows, can develop antlers when their bodies experience a significant hormonal imbalance, most commonly an overproduction of androgens like testosterone. This abnormal hormonal profile is often caused by a tumor or cyst on the ovaries or adrenal glands, which regulate hormones. These antlered cows represent a tiny fraction of the overall moose population.
The antlers grown by these females are typically smaller, irregular, and often malformed compared to the large, palmated racks of healthy bulls. Because the hormonal cycle required for proper hardening and shedding is profoundly disrupted, the cow’s antlers may remain permanently covered in velvet. The constant presence of the vascular tissue suggests that testosterone levels are not cycling to trigger final mineralization and casting. Furthermore, an antlered cow is often sterile or reproductively compromised, as the hormonal disruption that causes antler growth interferes with the normal reproductive cycle.