Female caribou possess antlers, and their biology makes them unique among all deer species. Caribou, known as reindeer in Eurasia (Rangifer tarandus), are the only member of the Cervidae family where both sexes typically grow headgear annually. The structures are classified as antlers because they are composed of bone and are shed and regrown each year, unlike true horns, which are permanent bone cores covered by a keratin sheath. This unusual trait in the female is a specialized adaptation to the harsh Arctic and subarctic environments they inhabit.
The Biological Distinction of Antlers
Antlers, found exclusively in the deer family, are intricate structures made entirely of bone that grow from bony supports on the skull called pedicles. The growth process is remarkably fast, representing some of the fastest-growing tissue in the animal kingdom. While growing, the antlers are covered in velvet, a soft, fuzzy skin rich with blood vessels and nerves that supply the necessary calcium and nutrients for rapid bone formation.
Once the antlers are fully grown, the blood supply to the velvet cuts off, and the caribou rub this dead tissue off against vegetation. Unlike the permanent, unbranched horns found on animals like bison or goats, antlers are typically branched and are shed completely at a predetermined point in the annual cycle. The shedding occurs when a layer of specialized bone cells at the pedicle weakens the connection, causing the heavy bone structure to drop off.
The Female Antler Cycle and Timing
Male Antler Cycle
The timing of antler growth and shedding is governed by seasonal hormonal shifts, particularly testosterone, which creates a staggered cycle between the sexes. Male caribou begin growing their antlers in the spring, shed their velvet by mid-September, and use their hardened racks during the autumn rut for fighting and dominance displays. Older, dominant males typically shed their massive antlers quickly after the rut, often by early November, to conserve energy during the winter.
Female Antler Cycle
Female caribou maintain a different schedule, beginning their antler growth slightly later, between June and September. Their cycle is uniquely linked to their reproductive status, allowing them to retain their antlers throughout the entire winter season. Non-pregnant females, sometimes referred to as barren cows, will shed their antlers earlier in the winter, typically around December or January.
Pregnant females retain their antlers for the longest period, often carrying them until just a few days after they give birth in the late spring, around April or May. This extended retention is thought to be triggered by the sharp drop in progesterone and other hormones associated with calving. This biological timing difference means that for much of the winter, the pregnant female is the only member of the herd, besides some young males, still bearing antlers.
Why Female Caribou Need Antlers
The unusual retention of antlers by female caribou is an adaptation that provides a significant survival advantage during the coldest and most difficult months. The primary function is not for defense against predators, but for successful competition for limited food resources. During the Arctic winter, caribou must access lichen and other vegetation hidden beneath deep, compacted snow, a process called cratering.
Having antlers allows pregnant females to assert dominance and displace other caribou, including the now-antlerless males, from the best foraging pits. Securing the most accessible food sources is crucial for maintaining the energy reserves needed to sustain a pregnancy through the winter and to successfully deliver a calf in the spring. Studies suggest that in populations with deeper snow cover, the proportion of females with antlers is higher, supporting the hypothesis that the headgear is a tool for interference competition for winter food. This ability to dominate feeding areas ensures the pregnant female has the best chance of survival and reproductive success in their challenging environment.