Effects of fasting and lactation on blood chemistry and urine composition in the grey seal (Halichoerus grypus).

1. The effect of fasting and lactation on osmolality, electrolytes, zinc, protein, urea, uric acid, alkaline phosphatase and vitamin C (ascorbic acid) in plasma and urine was studied in grey seals (Halichoerus grypus) during the breeding season. 2. Plasma osmolality, electrolytes and proteins were not affected by lactation in females or by suckling and post-weaning fast in pups. Higher levels of calcium, potassium and zinc were observed in fetal compared to maternal plasma. Low urea levels in plasma and urine indicate the low catabolism of proteins during fasting. 3. Higher levels of electrolytes in the urine of juveniles and adult males compared to females point to the ingestion of sea water. The increased urine concentration in lactating females compared to that in pregnant ones might be due to additional drainage of body water with the milk. 4. Metabolic water derived from the oxidation of lipids offsets the losses through milk, urine and evaporation, thus guaranteeing water and electrolyte homeostasis even under the combination of fasting and lactation.