Shed Female Antlers as a Mineral Resource for Caribou on Spring Calving Grounds

Caribou (Rangifer tarandus) is the only extant deer species in which females produce antlers, but what are the roles and biological benefits of this feature? Female antler shedding is roughly synchronous with calving and, within populations, occurs at approximately the same time and place every year. Interestingly, spring and summer forage on caribou ranges can be insufficient in key minerals (calcium and phosphorus) that support nursing, and strategies for offsetting these deficits are poorly understood. We test the hypothesis that female caribou antlers, rich in calcium and phosphorus, provide mineral supplements for nursing females. Using antler and bone materials collected from the Coastal Plain calving grounds of the Porcupine Caribou Herd (Arctic National Wildlife Refuge, Alaska), we assessed the consumption of bone-derived nutrients by ungulates (Rangifer), carnivorans (Ursus, Canis, Vulpes), and rodents (Urocitellus, Microtus). Taphonomic surveys yielded over 1,300 antlers and hundreds of skeletal bones. We visually inspected each element for modifications and compared observed features to those generated by candidate modifiers. We identified 20 modification classes within the collection and attributed 10 to caribou. We found caribou gnawing on ~90% of shed antlers, but <10% of skeletal material, indicating a clear focus on antler resources. Co-occurring mammals rarely targeted antlers, with rodents gnawing 30%). On the calving grounds, caribou monopolize antler nutrients, likely driven by nursing females. Antlers can form dense accumulations on caribou calving grounds (>1,000 antlers/km2) and persist for centuries or longer, potentially serving as an important mineral-rich attractor. The annual return to calving grounds, during which females consume available antler resources and contribute new ones, may function as an unrecognized feedback mechanism for maintaining calving ground fidelity.