VARIATION IN STABLE CARBON (Δ13C) AND NITROGEN (Δ15N) ISOTOPE COMPOSITIONS ALONG ANTLERS OF QAMANIRJUAQ CARIBOU (RANGIFER TARANDUS GROENLANDICUS)

Male and female Caribou (Rangifer tarandus groenlandicus) from the Qamanirjuaq herd (Nunavut, Canada) grow antlers throughout the summer season, beginning at the start of the spring migration for both male and female and ending before the fall rut for male and after for female. Throughout antler growth, stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) are incorporated into the tissue. As such, yearly antler shedding provides a non-lethal source of data for studying short and long-term changes in caribou ecology. We sampled the antlers of thirteen males and fifteen females culled from the Qamanirjuaq herd in September 1967. We predicted that serial sampling of antlers would reflect the known seasonal dietary change from lichen to a shrub and grass-like plant diet, as found in rumen contents from individuals culled during the same period. Values of δ¹³C and δ¹⁵N were analyzed for food sources as well as for collagen sampled sequentially along each antler’s length. The stable carbon isotope compositions of collagen (δ¹³C_col) varied among individuals and within antlers by ~0.5‰ for males and by ~1‰ for females. Values of δ¹⁵N_col increased within all antlers by 1~3‰ from pedicle to tip and varied by 3‰ among all individuals sampled. Bayesian mixing models produced using these data indicated mixed diets of fungi, horsetail, lichen, and woody plants but did not reveal significant dietary shifts for any individual during antler formation. The measured increases in δ¹⁵N_col of antler tissue may correspond to subtle seasonal dietary changes and/or variations in nutritional stress.