Energy, Nitrogen, and Electrolyte Flux and Sea Water Drinking in the Sea Otter Enhydra Lutris

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Abstract
An elevated metabolism requires the sea otter, Enhydra lutris, to include large quantities of invertebrate prey in its diet. This necessitates processing large quantities of energy, nitrogen, electrolytes, and water. Feeding studies were conducted to document these high material and energy fluxes and the dilution of tritiated water through time was used to measure total water flux. Total energy influx on a diet of clam and squid was 234 ± 19 (X ± SE) kcal/kg-day with an assimilation efficiency of 82% and an estimated metabolic efficiency of 72%. Material influx from food was 5.22 ± 0.41 (X ± SE) g/kg-day nitrogen and 166 meq/kg-day electrolytes (Cl, Na, K, Mg). The presence of large lobulate kidneys may allow for the production of large volumes of urine at elevated concentrations. Urine samples from wild and captive animals had a mean osmotic pressure of 1,627 ± 91 mOsm/kg (range, 910-2,130), which was composed of 647 meq/L Na, K, Cl and 698 ± 67 mmol/L urea. Total water flux was 269 ± 25 mL/kg-day. Mean seawater ingested was estimated to be 62 mL/kg-day (range, 0-124), and it may aid in the urea elimination by increasing the urinary osmotic space.