The Handling of Constant Volumes of Various Concentrations of Seawater by the Jackass Penguin Spheniscus Demersus
This paper reports on the effects of varying the concentration of sea water dosed at a rate of 10% of body mass on the handling of fluid and solutes by jackass penguins (Spheniscus demersus). The salt gland only secreted when the seawater dosed was hypertonic to the plasma and caused a rise in plasma osmotic concentration of at least 7,5%. The gland reacts to different marginal osmotic loads (equivalent to 40 and 50% seawater) by varying the flow rate and total concentration to match the degree of osmotic stress. High osmotic loads resulted in maximum flow rates and concentration from the onset of secretion. The relative composition of the ions was not affected by flow rate. Cloacal excretion decreased with a decreasing osmotic load, especially at those seawater concentrations that failed to stimulate the salt gland. This cloacal control is interpreted as a graded mechanism to dispose of osmotic loads. The cloacal system is stimulated to excrete by the same stimuli as the salt gland. Apparent selective Na* absorption by the cloaca was also noted.