Plasma electrolyte concentrations in captive and free-ranging African penguins (Spheniscus demersus) maintained with and without dietary salt supplements

African penguins (Spheniscus demersus) are common display animals in North American zoos and aquariums. At present, 43 American Zoo and Aquarium Association (AZA)-accredited institutions maintain over 700 of these birds. A survey of facilities found that only seven institutions maintain their birds in "saltwater habitats" (natural, brine, or artificial seawater systems), while the remaining institutions provide freshwater habitats. Penguins maintained in freshwater habitats commonly receive dietary salt supplements, though evidence is lacking as to the value of this practice. This study was designed to evaluate the necessity of salt supplementation in African penguins. We report the results of a randomized, 10-month comparison of plasma electrolytes (Na, Cl, and K) between groups of 10 salt-supplemented and nonsupplemented birds living in a freshwater environment. The results showed no significant differences between the two groups. An interfacility comparison at the same time points revealed temporal pattern differences in analyte values between the facilities; however, the absolute concentrations did not exceed clinical ranges found in healthy birds. Furthermore, single time-point comparisons between wild and captive African penguins indicated similar sodium concentrations, while potassium and chloride concentrations varied between groups. Finally, the plasma electrolyte levels in 20 birds remained statistically unchanged after a 60-day withdrawal from salt supplementation. This study provides experimental evidence that African penguins maintained in freshwater exhibits on a herring-, capelin-, and squid-based diet do not require salt supplementation. In addition to the practical implications regarding the need for salt supplementation for captive birds, the results also have theoretical significance. They provide evidence that penguins do not need to ingest high salt concentrations to remain in electrolyte balance. In this regard, the ion regulatory mechanisms in penguins are similar to those in terrestrial mammals and birds. (C) 2004 Wiley-Liss, Inc.