15N signatures do not reflect body condition in Arctic ground squirrels

Studies using stable-isotope analysis documented an enrichment in δ15N values in nutritionally stressed animals. Investigators suggested that changes in δ15N values measured in urine, hair, and blood may be a good indicator of lean-tissue losses. During our investigations into the effects of population density on body condition and reproduction of female Arctic ground squirrels (Spermophilus parryii plesius) near Kluane Lake, Yukon, Canada, we examined the relations between body condition and δ15N values. Data obtained from 20 livetrapped female ground squirrels suggested that reproductive females from a population with moderate density and low food availability experienced a reduction in body condition, as indicated by mass loss and changes in blood urea nitrogen (BUN) and glucose concentrations. In contrast, those from a population that failed to reproduce successfully and had high density and low food availability experienced no nutritional stress. Similarly, those females from a high-density population with high food availability (i.e., supplemented food) that reproduced successfully suffered no noticeable nutritional stress. In contrast to our prediction, δ15N values did not show a decline with increasing body mass, and animals in poor and excellent body condition had similar δ15N values. In addition, female ground squirrels from the same group with access to similar types of food (natural or supplemented) and with similar body masses, BUN, and blood glucose concentrations showed a difference of up to 1.8‰ in δ15N values. Thus, our results suggest that the ecological process (i.e., diet selection) may have obscured the physiological one (i.e., recycling of nitrogen). Therefore, we recommend that field ecologists studying animal diets using stable-isotope analysis use alternative techniques when attempting to evaluate the body condition of their subjects.