Renal handling of water, urea and electrolytes in wild South America fur seal (<i>Arctocephalus australis</i>)

Abstract

Fur seals alternate foraging at sea and fasting periods ashore. During fasting, energy and water are supplied by fat stores, but little is known about the transition from feeding to fasting metabolism. To evaluate the variation on land of significant parameters of urea and hydromineral metabolism, urine pH, plasma and urine osmolality, creatinine, urea, Na + , K + , Cl - and Ca 2+ were determined in 65 wild South American fur seals ( Arctocephalus australis ) from Lobos Island, Uruguay. Urine/plasma ratio (U:P) and fractional excretion (FE) were calculated for creatinine, urea, Na + , K + , Cl - and Ca 2+ . As no significant differences were found between categories (immature, pregnant and non lactating, pregnant and lactating females, and immature males), pooled data allowed to perform correlation and regression tests. Correlated variables allow to characterize a post-absorptive (high FE of urea, urine urea and osmolality, associated with low U:P quotient for creatinine and urea) as well as fasting condition (low FE of urea, urine urea and osmolality, associated with high U:P quotient for creatinine and urea). The FE of urea versus log creatinine U:P plot suggests active urea secretion at low urine flows. Comparative urine analysis is consistent with a possible event of seawater ingestion in a young male: highest urine Na + (336mmol l -1 ) and Cl - (265.74mmol l -1 ) concentrations associated with low urine urea concentration (657.24mmol l -1 ).