Urea recycling in muskrats (Ondatra zibethicus): a potential nitrogen-conserving tactic?

The rate of 14C-urea hydrolysis was determined in 32 field-acclimatized muskrats maintained on natural diets during spring, summer, fall, and winter. We hypothesized that urea recycling occurs in muskrats during all seasons and that the conservation of tissue nitrogen via this mechanism is most prevalent in fall and winter, when forage protein levels are lowest. Muskrats exhibited higher rates of urea hydrolysis and a lower serum urea nitrogen-to-creatinine ratio in fall and winter than in spring and summer. Even after correcting for seasonal differences in blood urea pool size, the adjusted rate of urea hydrolysis was 67% higher in fall and winter than in spring and summer. There was no evidence that the maintenance nitrogen requirements of muskrats fed natural vegetation were affected by seasonal changes in the amino acid composition of the diet. We suggest that increased levels of urea recycling, coupled with adaptive mechanisms for reducing nitrogen excretion and possibly conserving carbon skeletons of essential amino acids, may allow muskrats to reduce their nitrogen requirements on fall and winter diets. Our finding that 14C-urea hydrolysis occurred during all four sampling periods suggests that nitrogen derived from this source may also be critical to supporting large hindgut microbe populations that enable this rodent to exploit the appreciable fiber content of its aquatic plant diet throughout the year.