Osmoregulation by Adélie 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 Ad6lie 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 Cl-, 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+. Received 26 July 1996, accepted 1 February 1997. ADPLIE PENGUINS (Pygoscelis adeliae) live in an osmotically challenging marine environment. Adelie Penguins have little or no access to fresh water, and up to 99% of their diet is marine invertebrates, usually krill (mostly Euphausia superba; Volkman et al. 1980). Adelie chicks potentially face the same osmotic challenges as the adults, because the only food they receive is that regurgitated by their parents. Unless adults modify the composition of krill fed to chicks, then the chicks' osmoregulatory physiology must be sufficiently developed to handle potentially high salt loads delivered in the food within hours after hatching (Douglas 1963, 1968). The purpose of this study was to determine the magnitude of salt loads received by chicks, determine if adults alter the salt composition of food given to chicks, and measure the salt concentrations of chick excretions (e.g. urine and salt-gland secretion) to determine salt-output capability. Adelie Penguins are good subjects for physiological studies of osmoregulation for four important reasons. First, their large size (adults weigh 4 to 5 kg) allows collection of relatively large samples of tissues and body fluids, which facilitates ion analyses. Second, Adelie Penguins are found in dense colonies near research 1 E-mail: donjanes@ucracl.ucr.edu stations in Antarctica, and this abundance combined with their relative tameness permits acquisition of large sample sizes. Third, their life history is well known (Ainley et al. 1983). Fourth, all of the material input into chicks is regurgitated by the parents, simplifying measurements of water and salt influx.