干旱区生态环境知识资源中心

Adjustments of renal function can conserve significant amounts of water for terrestrial mammals and birds during periods of water scarcity. By measuring renal function in unanesthetized, unrestrained Gambel’s quail (Callipepla gambelii) while normally hydrated (NH) and after 4-5 d of water deprivation (WD), we tested the hypothesis that these birds respond to water deprivation by decreasing their glomerular filtration rate (GFR), thus lowering urine flow and attendant water loss. Glomerular filtration rate averaged 17.7 mL/h in NH quail and declined to 7.7 mL/h in WD quail. Tubular reabsorption of fluids increased slightly as a result of water deprivation, but the primary renal mechanism for water conservation was reduction in GFR. When deprived of water, Gambel’s quail did not reduce their GFR more, nor concentrate their urine more, than other avian species from more mesic habitats. Urine-to-plasma osmotic ratios averaged 1.6. Final adjustments in water content of the urine are likely made in the lower gastrointestinal tract because NH quail reflux urine from the cloaca into this region. However, as urine osmolality increases because of dehydration, the osmotic gradient between plasma and luminal contents of the lower intestine may eventually prevent solute-linked water flux from the intestine. We suggest that quail no longer reflux urine under such circumstances. We developed an allometric equation relating GFR to body mass for NH birds that predicts a GFR of 40.3 mL/h for a 158.0-g Gambel’s quail, a value 2.3 times higher than measured. We suggest that the evolution of a conservative metabolic rate has permitted a reduction in GFR and attendant water loss by way of urine flow.