Because maintaining an acidic pH environment is costly, acidic stomachs should be present primarily in those cases where it is adaptive (or where it was adaptive in a recent ancestor). The cost of stomach acidity is twofold. The host must invest significant energy for both acid production and protecting the stomach from acid-related damage.
Gastric acid in vertebrates.
Koelz HR. Gastric acid in vertebrates. Scand J Gastroenterol 1992;27 Suppl 193:2-6. Gastric secretion of hydrochloric acid appears to be unique to vertebrates and is almost ubiquitous in all fishes, amphibians, reptiles, birds and mammals. Comparative anatomy and physiology suggest that gastric acid has evolved approximately 350 million years ago. The similarity of the acid-secreting mechanism across all classes of vertebrates implies a major advantage for selection, but the evidence regarding its precise purpose remains inconclusive. Key words: Achlorhydria; Amphibians; birds; evolution; fishes; gastric acid; gastric mucosa; mammals; pepsin; reptiles Hans R. Koelz, M. D.. Dept. of Medicine, Stadtspital Triemli, CH-8063 Zurich, Switzerland