UREA RECYCLING IN MUSKRATS (ONDATRA-ZIBETHICUS) - A POTENTIAL NITROGEN-CONSERVING TACTIC

The rate of C-14-urea hydrolysis nas determined in 32 field-acclimatiz ed muskrats maintained on natural diets during spring, summer, fall, a nd winter. We hypothesized that urea recycling occurs in muskrats duri ng all seasons and that the conservation of tissue nitrogen via this m echanism is most prevalent in fall and winter, when forage protein lev els are lowest. Muskrats exhibited higher rates of urea hydrolysis and a lower serum urea nitrogen-to-creatinine ratio in fall and winter th an in spring and summer. Even after correcting for seasonal difference s 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 ev idence that the maintenance nitrogen requirements of muskrats fed natu ral vegetation were affected by seasonal changes in the amino acid com position of the diet. We suggest that increased levels of urea recycli ng, coupled with adaptive mechanisms for reducing nitrogen excretion a nd possibly conserving carbon skeletons of essential amino acids, may allow muskrats to reduce their nitrogen requirements on fall and winte r diets. Our finding that C-14-urea hydrolysis occurred during all fou r 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 aq uatic plant diet throughout the year.