Caribou (known in Eurasia as reindeer) are unique among the deer family (Cervidae) because females, or cows, routinely develop antlers. While most female caribou grow these bony appendages, a small percentage (3 to 5%) may not grow antlers in a given year. The presence of antlers on both sexes makes Rangifer tarandus a biological anomaly, setting it apart from all other cervid species.
The Antler Anomaly
Antler growth in nearly all other deer species is driven by elevated testosterone levels and is restricted to males. The existence of antlers on female caribou is a biological difference from the rest of the deer family, where females typically lack head ornamentation. Caribou and reindeer are the same species; caribou generally refers to wild North American populations, and reindeer to domesticated or Eurasian populations.
Female antlers are generally smaller and less complex than those of males, but they are still significant structures. Female antlers can reach lengths of up to 20 inches, while a large male’s rack can exceed 50 inches and weigh as much as 20 pounds. This shared characteristic suggests an evolutionary advantage for females to invest the high metabolic cost required to grow and maintain these structures.
Antler Cycles and Seasonal Differences
The distinguishing feature of caribou antlers is the difference in shedding cycles between the sexes, which is tied directly to their reproductive status. Male caribou, or bulls, shed their antlers shortly after the breeding season (rut), typically in late fall (November or December). This shedding is triggered by the decline in testosterone levels following mating competition.
In contrast, female caribou maintain their antlers throughout the winter months. Pregnant females retain their antlers until calving occurs in the spring (late May or early June), shedding them only days after giving birth. Non-pregnant females may shed theirs earlier in the spring, but still later than the males. This delayed shedding means that during winter, female caribou are often the only adult members of the herd that possess antlers.
This difference in timing is hormonally regulated, with the female’s reproductive state maintaining the connection between the antler and the skull. The retention of antlers through winter and late spring by pregnant cows supports a survival strategy. By the time males regrow their antlers in the summer, females have already shed theirs, reversing the social dynamics of the herd.
Biological Purpose of Female Antlers
The explanation for this unique trait lies in the competition for resources during the winter, which is severe in their Arctic and subarctic habitats. Female caribou use their antlers to establish a dominance hierarchy within the herd, granting them preferential access to the limited winter food supply. This dominance helps a pregnant female secure the necessary nutrition for herself and her developing fetus.
The antlers are physically used as tools to defend foraging patches on the ground. They allow the female to dig through deep, compacted snow to reach the winter food source, primarily lichen. Studies show that the percentage of antlered females correlates positively with annual snowfall and snow depth, supporting the idea that antlers provide an advantage in competition for food.
Retaining antlers also protects the pregnant cow’s feeding craters from other caribou, including larger, antlerless males. By keeping their antlers until calving, females ensure they maintain body condition during the final, demanding stage of pregnancy, which dictates the health and survival of the newborn calf. The antlers are also a source of calcium and phosphorus; once shed, nursing females are often observed gnawing on the discarded antlers to supplement mineral intake during lactation.