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Total body water turnover and partitioning of salt excretion in glaucous-winged gulls, larus glaucescens Walter, Anne

Abstract

1. Two aspects of long-term salt and water excretion were measured in Glaucous-winged Gulls, Larus glaucescens. Total body water volume and turnover rate were measured in birds drinking fresh water and drinking sea water by THO disappearance rate. Na+, K+, and CI- excretions over 24 hours were measured by continuous collection of cloacal and salt gland excretions from birds fed fish, fish plus a salt load, or a salt load only. 2. Total body water volume was found to be 79% of body weight on both fresh water and sea water drinking regimes. TBW volume is large compared to other birds. 3. Mean total body water turnover rate was the same for both drinking regimes (0.064 ml/g-day); this value is the same as the predicted rate based on data from other birds. 4. There were no significant differences between the fish and fish salt fed birds in the pattern or amounts of ion excretion. 5. Sodium and chloride were excreted in approximately equal amounts from the salt gland and cloaca. Most potassium was excreted via the cloaca. Thirty-eight percent of the total sodium, 6% of the total potassium and 58% of the total chloride excreted were contained in the salt gland secretion of both the fish and fish + salt groups of gulls. 6. The fluid and solid portions of the cloacal excretion were analyzed for ions. Cations were divided between the two portions. In the fish and fish + salt fed birds, 51.8 + 7.8% of the cloacal Na+ and 61.8 + 4.5% of the cloacal K+ was found in the solid portion of the cloacal excreta. Chloride was detected in the fluid portion only. 7. The two birds given only a salt load had lower rates of evaporative water loss and smaller amounts of cloacal solids compared to the fed birds. 8. The large TBW volume may be advantageous to marine birds as a buffer against excess salt ingestion. The constancy of TBW turnover rate suggests that the gulls are specifically adapted to their environment where salt, but not water, is a stress. 9. The results suggest that the salt gland, cloacal fluids and especially the cloacal solids are important routes for ion excretion for fed birds and that osmoregulatory processes for fed and unfed birds may be different. 10. These data imply that although the salt gland is the primary adaptation of marine birds to salt stress, the entire process of salt and water metabolism also involves other more subtle mechanisms.

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