Stable Isotopes of Carbon and Nitrogen in Female Qamanirjuaq Caribou (Rangifer tarandus groenlandicus) Antlers in Relation to Diet and Physiology

Populations of Arctic barren-ground caribou (Rangifer tarandus groenlandicus) have fluctuated over the past few decades but are currently in decline. To support conservation efforts of caribou, it is integral to develop both historical and modern datasets; these datasets allow us to understand how caribou have adapted to past climatic shifts and may respond to future change. Caribou are the only extant species in which both males and females grow antlers each year, providing unique sex-specific isotope datasets. Isotopic study of antlers is proving to be a source of annualized data on caribou diet and migration, but uncertainty remains in the magnitude and drivers of isotopic changes across individual antlers. Nitrogen (δ¹⁵N_Col) in male antlers from the Qamanirjuaq herd has been shown to increase along the main antler beam aligned with known shifts in seasonal diet and/or the increased energy requirements. We examined 19 female antlers from the same population to compare δ¹³C_Col and δ¹⁵N_Col between sexes and among varying reproductive statuses. Female antlers were found to be ~0.4‰ lower in δ¹³C_Col at the bases of their antlers compared to males. In both sexes, caribou antler collagen varied in δ¹³C_Col among individuals by ~2‰ (−20.0‰ to −18.1‰) and in δ¹⁵N_col by ~5‰ (+2.3‰ to +7.3‰). We conclude that δ¹³C_Col differences between sexes are the result of differing diets during the onset of antler formation, corresponding to the different seasons that females and males initiate antler growth. Despite dietary differences, both males and females increased in δ¹⁵N_Col along the antler beam by approximately 1‰–1.5‰. Increases in δ¹⁵N_Col along the antler beam coincide with the increased physiological stress and material demands during antler development. Antler tissue remains a promising resource for studying both short- and long-term changes in male and female caribou ecology.