Seasonal changes in the ratio of serum urea to creatinine in feeding and fasting polar bears

• Published in: Canadian Journal of Zoology
• Main Authors: Ramsay, Malcolm A., Nelson, Ralph A., Stirling, Ian
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1991
• Subjects: Ursus maritimus
• Online Access: http://dx.doi.org/10.1139/z91-048; http://www.nrcresearchpress.com/doi/pdf/10.1139/z91-048

When mammals fast, the ratio of serum urea to serum creatinine (U/C) typically rises. Blood samples from 668 free-ranging polar bears (Ursus maritimus) of differing nutritional status were collected for determination of serum concentrations of urea and creatinine. Adult female polar bears in spring that had just emerged with their cubs from dens and had been without food for more than 7 months had markedly low U/C values. This atypical pattern of U/C levels during a fast is similar to that of black bears (Ursus americanus) at the end of their overwinter fast. Whereas black bears only show reduced U/C values during their overwinter fast, however, some polar bears had low serum U/C values in all seasons, suggesting that those bears were in a nutritional state similar to that of a fasting and winter dormant bear. We suggest that when food is unavailable, regardless of the time of year, polar bears can rapidly adopt appropriate physiological and biochemical means to control urea recycling and, consequently, are one of the most proficient terrestrial mammals at undertaking extended fasts.