Use of stable-carbon and -nitrogen isotopes to assess weaning and fasting in female polar bears and their cubs

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 (delta C-13) and nitrogen (delta N-15) isotope values for plasma and milk proteins and delta C-13 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 delta C-13 and delta N-15 values. The plasma delta C-13 values for mothers and cubs were similar to milk-protein delta C-13 values and were significantly enriched in C-13 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 N-15 by 1.0 parts per thousand over that of their mothers (delta N-15 = 21.5 +/- 0.8 parts per thousand (mean +/- SD) for cubs and 20.5 +/- 0.5 parts per thousand for mothers) and depleted in C-13 by 0.8 parts per thousand (delta C-13 = -19.6 +/- 0.5 parts per thousand for cubs and -18.8 +/- 0.8 parts per thousand for mothers). For bears who fasted between summer and fall (3-4 months), plasma became depleted in C-13 by 0.5 parts per thousand and in N-15 by 1 parts per thousand. Plasma from females, who had fasted from summer to spring (7-8 months) and given birth to cubs, became enriched in C-13 by 0.7 parts per thousand and in N-15 by 2 parts per thousand. 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 delta C-13 and delta N-15 values. However, we were not able to use stable-isotope analyses to determine the exact time of weaning.