Reindeer do shed their antlers, but the cycle is a complex biological process that varies significantly between the sexes. Reindeer (known as caribou in North America) are unique among the deer family because both males and females regularly grow antlers. Unlike horns, which are permanent structures made of keratin and bone, antlers are true bone structures that are shed and regrown entirely each year. The annual renewal of these bony growths is tightly regulated by seasonal changes and hormonal shifts, making the timing of their loss a defining feature of the species.
The Annual Antler Growth and Shedding Cycle
Antler growth is one of the fastest bone growth processes in the animal kingdom, fueled by a dense network of blood vessels and nerves. While growing, the antlers are encased in a soft, fuzzy skin known as “velvet,” which supplies the necessary oxygen and minerals for the rapidly forming bone tissue. Once the antler has reached its maximum size, typically by late summer, the blood supply to the velvet restricts and the velvet dries out. The reindeer then rubs this dead skin off against trees and shrubs to reveal the hard, mineralized bone underneath.
The shedding process, or casting, is triggered by a drop in testosterone levels following the mating season. This hormonal change causes specialized cells called osteoclasts to dissolve the bony connection at the pedicle (where the antler meets the skull), weakening the structure until it detaches. Shedding usually happens rapidly, often within a day or two of the connection weakening, preparing the animal for the next cycle of growth.
Timing Differences Between Males and Females
The precise timing of antler shedding is governed by sex and reproductive status, creating a distinct annual rhythm for the herd. Mature male reindeer shed their antlers immediately following the rutting season in the late fall, between November and mid-December. Shedding the heavy antlers after breeding allows males to conserve energy needed for winter survival. They remain antlerless until the following spring when they begin to regrow a new set.
In contrast, female reindeer retain their antlers throughout the entire winter, which is a unique adaptation among deer species. Pregnant females keep their antlers until just after they give birth, typically shedding them in late spring, around May or June. Non-pregnant females usually shed slightly earlier than their pregnant counterparts, often between late March and early May. This extended retention is directly linked to the hierarchy and survival needs during the most food-scarce period of the year.
The antler cycle of juveniles and castrated males further illustrates the role of hormones in this process. Young males, who have lower testosterone levels, often retain their antlers longer than mature bulls, sometimes keeping them through the winter. Castrated males, lacking typical hormone fluctuations, can maintain their antlers for an extended period, sometimes shedding them in late spring like the females.
Antlers and Winter Survival
The timing of antler retention is directly tied to the reindeer’s strategy for winter survival and resource competition. During the cold months, food becomes scarce and is often buried beneath compacted snow and ice. Females who retain their antlers gain an advantage over the males, who have already shed theirs. They use their headgear to establish dominance over the antlerless males, securing access to the best feeding craters.
This dominance allows pregnant females to preferentially access the lichen and other vegetation they dig up from under the snow. This access sustains the high energy demands of pregnancy and ensures the health of the developing calf. Keeping her antlers through the winter is a biological strategy that maximizes her chances of survival and successful reproduction.