Antlers, the bony headgear of the deer family (Cervidae), are overwhelmingly associated with males. These structures typically signal sexual dimorphism, used for competition and display during the breeding season. However, female animals that regularly grow and use antlers are a rare biological exception. This deviation highlights unique evolutionary pressures acting on a single species that requires both sexes to possess this specialized cranial appendage.
Understanding Antlers Versus Horns
Antlers and horns are fundamentally different in structure and composition, though often mistaken for one another. Antlers are true bone, growing as extensions of the skull from bony outgrowths called pedicles. They are shed and regrown entirely each year, making them one of the fastest-growing tissues in the animal kingdom.
During their annual growth cycle, antlers are covered in a layer of skin and blood vessels known as velvet, which supplies nutrients for rapid growth. Once fully grown, the velvet is shed, leaving the hardened, branched bone structure that is eventually cast off, usually after the breeding season.
Horns, by contrast, are permanent structures found on species in the Bovidae family, such as bison, sheep, and goats. They consist of a bony core covered by a tough outer sheath made of keratin. Horns are not shed yearly and often grow continuously throughout an animal’s life. In many horned species, both males and females possess them, though the female’s horns are often smaller.
The Unique Case of Female Reindeer
The only species in the Cervidae family where females predictably and routinely grow antlers is the Reindeer, known in North America as the Caribou (Rangifer tarandus). This makes the female Reindeer an outlier, as most female deer (does) are completely devoid of cranial appendages. While both sexes grow antlers, the timing of their shedding differs significantly, which relates to the female’s unique needs.
Male Reindeer typically shed their massive antlers in late autumn or early winter, following the mating season (rut). Pregnant female Reindeer retain their smaller antlers much longer, often keeping them throughout the harsh winter months. They do not cast their antlers until late spring, usually just before or shortly after giving birth to their calves in May.
This difference ensures pregnant females are equipped with headgear during the most energetically challenging season. Retaining antlers throughout the winter provides a clear advantage in securing resources and defending against rivals within the herd.
Evolutionary Reasons for Antlers in Females
The evolutionary reason for female Reindeer retaining antlers is linked to the environmental challenges of their Arctic and sub-Arctic habitat. During winter, food is buried beneath deep snow, requiring Reindeer to use their hooves to dig feeding craters for lichens and mosses. This process is highly competitive, especially for pregnant females who require maximum nutrition to sustain their developing fetus.
The retained antlers serve as weapons for interference competition, allowing females to assert dominance and defend the feeding craters they have dug. Females with larger antlers secure better access to food resources, linking the headgear directly to enhanced survival and reproductive success. The retention of the hard antler state is regulated by elevated levels of gonadal hormones, such as oestradiol and testosterone, which are linked to the pregnancy cycle. This hormonal synchronization ensures the female possesses antlers when the need for resource defense is highest.