Osmoregulation by Adelie Penguin chicks on the Antarctic Peninsula

Abstract

Physiologists have long contended that consumption of a diet of marine invertebrates imposes a high salt load on animals. I measured water and salt contents in the food, excreta, and body fluids of Adelie Penguin (Pygoscelis adeliae) chicks to determine patterns of osmoregulation in a bird that eats almost exclusively marine invertebrates. Adelie Penguins eat krill (Euphausia spp.) and feed the same to their chicks by regurgitation. Adelie chicks sometimes receive food that is significantly saltier than their body fluids, but on average the food has salt concentrations similar to those in their plasma. Adelie chicks excrete excess solutes via the salt glands, which are fully functional at hatching, and via the kidneys, which produce urine that is more concentrated relative to urine of other bird species. Larger chicks receive food that is drier than that eaten by smaller chicks, and larger chicks may compensate by excreting more of a given salt load via the salt glands, thus conserving water. Regurgitated food has significantly less Na+ than CI, but the salt glands secrete equal amounts of these two ions. Consequently, in larger chicks Na+ may be shunted from the urine to be excreted via the salt glands. The loss of this cation in the urine is compensated for by addition of NH4+.