Nitrogen metabolism and urea kinetics in the rock hyrax (Procavia habessinica)

Summary Nitrogen metabolism and urea kinetics were studied in rock hyraxes (Procavia habessinica) fed diets of different protein content.The maintenance nitrogen (N) requirement of the rock hyrax (311 mg·kg^–0.75·24 h^–1 of dietary N, or 209 mg·kg^–0.75·24 h^–1 of truly digestible N) is similar to that of several marsupial species, and thus lower than that of other eutherians.Urea recycled to the gut, measured with single injections of¹⁴C-urea, was 63% and 60% of urea entry rate on diets with 14.6% and 8.2% crude protein, respectively. Urea recycling increased to 70% when water intake was restricted, but decreased to 40% on a low (5.3%) protein diet, presumably because of a low energy intake.Urea utilization in the gut, measured with single injections of¹⁵N-urea, was 59% and 53% of urea degradation on the 14.6% and 8.2% protein diets, respectively. Urea utilization increased to 71% on the low protein diet, and increased to 98% with water restriction.The low maintenance nitrogen requirement appears to be the main physiological attribute of the rock hyrax enabling it to survive periods of low dietary protein availability. However, this low requirement can be related to the low basal metabolic rate of the Hyracoidea in general, and thus is not necessarily a primary adaptation to the environment.