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In some species, stable-isotope techniques can provide insights into dietary regimens where there are temporal shifts in trophic level or feeding frequency. We determined stable carbon (δ 13 C) and nitrogen (δ 15 N) isotope values for plasma and milk proteins and δ 13 C values for milk lipids from female polar bears (Ursus maritimus) and cubs to (i) ascertain whether cubs are at a higher trophic level than their mothers as a result of nursing and whether we can determine when weaning occurs, and (ii) determine the impact of seasonal fasting on δ 13 C and δ 15 N values. The plasma δ 13 C values for mothers and cubs were similar to milk-protein δ 13 C values and were significantly enriched in 13 C compared with those for milk lipid. Plasma from cubs of the year (COYs) in spring, when milk was their only diet, was isotopically enriched in 15 N by 1.0 over that of their mothers (δ 15 N = 21.5 ± 0.8 (mean ± SD) for cubs and 20.5 ± 0.5 for mothers) and depleted in 13 C by 0.8 (δ 13 C = 19.6 ± 0.5 for cubs and 18.8 ± 0.8 for mothers). For bears who fasted between summer and fall (34 months), plasma became depleted in 13 C by 0.5 and in 15 N by 1. Plasma from females, who had fasted from summer to spring (78 months) and given birth to cubs, became enriched in 13 C by 0.7 and in 15 N by 2. By using stable-isotope analyses we were able to show that (i) young cubs were at a higher trophic level than their mother when milk was their only food source, and (ii) seasonal fasting influenced δ 13 C and δ 15 N values. However, we were not able to use stable-isotope analyses to determine the exact time of weaning.
Polischuk et al. (Thu,) studied this question.