Nitrogen metabolism and kidney function in the nubian ibex (Capra ibex nubiana)

Nitrogen metabolism and kidney function of the dik-dik antelope (Rhynchotragus kirkii) were studied under a range of controlled, experimental conditions and diets. Dik-dik antelope remained in nitrogen balance even when fed a diet low in protein and high in fibre. When fed a diet high in protein (20%) and water ad-libitum, 55.3% of the urea filtered by the kidney was reabsorbed. Limiting water intake increased urea reabsorption to 77.2%. The U/P urea concentrations were maintained at similar ratios on all diets, as well as during dehydration and solute loading. Minimum endogenous nitrogen excreted was 74 mg/kg^0.75/day. Dehydration (water deprivation) and solute loading (intraruminal infusion of 0.25 M NaCl) had varying effects on nitrogen metabolism. It is concluded that the metabolic nitrogen economy of the dik-dik antelope is qualitatively similar to that of other domestic and wild ruminants.

Goat living in harsh environments represents a climax in the capacity of domestic ruminants to adjust to such areas. This ability is multifactorial: low body mass, and low metabolic requirements of goats can be regarded as an important asset to them for it minimise their maintenance and water requirements, in areas where water sources are widely distributed and food sources are limited by their quantity and quality. An ability to reduce metabolism allows goats to survive even after prolonged periods of severe limited food availability. A skillful grazing behaviour and efficient digestive system enable goats to attain maximal food intake and maximal food utilisation in a given condition. There is a positive interaction between the better recycling rate of urea and a better digestion of such food in desert goats. The rumen plays an important role in the evolved adaptations by serving as a huge fermentation vat and water reservoir. The water stored in the rumen is utilised during dehydration, and the rumen serves as a container, which accommodates the ingested water upon rehydration. The rumen, the salivary glands and the kidney coordinately function in the regulation of water intake and water distribution following acute dehydration and rapid rehydration. Goats in the tropics, when possible, eat a diet composed of tree-leaves and shrubs (browse), which ensure a reliable and steady supply of food all year around, albeit, from a low to medium quality food. Some of the physiological features of ruminants defined as intermediate feeders like large salivary gland, the large absorptive area of their rumen epithelium, and the capacity to change rapidly the volume of the foregut in response to environmental changes are most likely responsible for the goat’s superior digestion capacity.

Variations in nitrogen isotope ratios in terrestrial foodwebs are described, and alternative models for variation in the enrichment between trophic levels are evaluated. Nitrogen isotope ratios in bone collagen have been used to determine trophic levels and differentiate marine from terrestrial resource consumption among prehistoric humans. However, recent research in terrestrial ecosystems has revealed significant variation in nitrogen isotope ratios between habitats, and within trophic levels in the same environment. Foodwebs in hot, arid environments tend to have higher nitrogen isotope ratios than cool, wet ones. Within ecosystems, the stepwise enrichment between trophic levels is often greater in hot, arid environments. Within ecosystems, herbivore species with physiological adaptations to water conservation have higher nitrogen isotope ratios than water-dependent species. The nitrogen isotope ratios of human bones may be affected by climate and physiology and thus cannot be directly compared between different types of ecosystems without first determining the isotopic composition of the local foodweb and the stepwise enrichment between trophic levels.