Rapid Rehydration and Kidney Function in the Black Bedouin Goat

Abstract

Black goats, tended by Bedouin tribesmen in the extreme deserts of the Middle East, drink only once every 2-4 days and imbibe amounts of water that often exceed 40% of their dehydrated body weight. The water that the goats drink copiously is first retained in the rumen and only gradually flows into the other body fluid compartments. Five hours after the drinking, 81% of the water imbibed was still stored in the spacious rumen of the goat. The kidney of the Bedouin goat responded to the voluminous drinking by a drop in the blood flow that was followed by a compatible drop in both the GFR and the urine flow. The urine flow, even 4 h following the drinking, was below the rate recorded in the dehydrated animal. Urine Na⁺ concentration that amounted to 80.3 mM in the dehydrated animal dropped to only 37.7 mM following the drinking. A drop (from 144 mM to 49 mM) was also simultaneously recorded in Cl⁻ concentration; K⁺ and urea concentration, however, changed only slightly. It is suggested that the rumen in the goat serves as a water reservoir that helps maintain the osmotic stability of the body proper. The kidneys in these animals efficiently conserve the water imbibed as well as the body's Na⁺ and Cl⁻. Studies of sheep that assign similar roles to the rumen and the kidneys indicate that the physiological mechanism described in the present study is generally shared by ruminants tended in a pastoral system.