Antlers are bony extensions growing from a deer’s skull and are one of the most recognizable features in the animal kingdom. The males of most deer species are the only ones to develop these structures. Antlers are composed entirely of bone and are shed and regrown every year. This distinguishes them from horns, which have a bony core covered by a permanent keratin sheath, like those found on cattle. The annual cycle of growth and shedding ties the presence of antlers directly to the male reproductive cycle.
The General Rule Across Deer Species
The majority of the world’s deer, belonging to the family Cervidae, limit antler growth to the male sex. Species such as the white-tailed deer, elk, moose, and red deer follow this pattern. Males are typically referred to as “buck,” “stag,” or “bull,” while females are called “does” or “cows.” This distribution makes antlers a secondary sexual characteristic.
The development of antlers is intrinsically linked to the male hormone testosterone. Antler growth begins when testosterone levels are low after the previous mating season. However, the ultimate hardening and shedding of the antlers are closely timed with fluctuations in this hormone.
The energy required to grow and carry these bony structures means they serve as reliable visual signals of a male’s health and genetic quality. Only the healthiest males with superior access to nutrition can consistently grow the largest, most complex antlers.
The Antler Growth Cycle
The annual process of antler development begins in the spring with the formation of a pedicle, a permanent bony base on the skull. This growth phase is one of the fastest rates of bone growth observed in any mammal. While growing, the antlers are covered in velvet, a soft, fuzzy skin highly vascularized with nerves and blood vessels. The velvet supplies the rapidly growing bone with necessary oxygen and nutrients.
As summer progresses, the antlers begin mineralization, becoming solid bone in preparation for the autumn breeding season. This hardening is triggered by a significant increase in testosterone, which constricts the blood supply to the velvet. The velvet dries and dies, and the male rubs it off against trees, revealing the polished bone beneath. When the breeding season concludes, decreasing day length triggers a drop in testosterone. This leads to the formation of an abscission layer, causing the antlers to shed, or be “cast,” and the cycle begins anew.
Key Exceptions to the Rule
The general rule is broken by the caribou, known as reindeer in Eurasia. Caribou are the only deer species where both males and females grow antlers, though the female’s are typically smaller and less branched. Females retain their antlers through the winter, often longer than males who shed theirs shortly after the rut. This retention allows females to defend patches of winter forage and dig through snow for food when they are pregnant.
Rare, individual exceptions occur in species where females do not normally grow antlers, such as white-tailed deer and red deer. In these isolated cases, a doe may grow small, often malformed antlers due to a hormonal imbalance, typically involving elevated testosterone. This unusual occurrence is often the result of an ovarian issue or a tumor that interferes with normal hormone regulation. Such antlers frequently remain covered in velvet because the female lacks the massive surge of testosterone required to fully mineralize the bone and shed the velvet.
Function and Purpose
The primary role of antlers is success in reproduction, largely through intrasexual competition, rather than defense against predators. During the annual mating season, known as the rut, males use their hardened antlers to fight rivals and establish a dominance hierarchy. These conflicts are ritualized pushing and grappling contests where the antlers interlock, allowing animals to test strength without inflicting severe injury. The winner secures access to the females for breeding.
Antlers also serve as a striking visual display that helps attract mates. The size, symmetry, and complexity of a male’s antlers signal his overall fitness, reflecting his age, nutritional history, and genetic quality. Females use this visual cue to assess the strength and health of a potential mate. Although their main function is mating, antlers can be used secondarily to ward off predators.