The Ecophysiological Role of the Avian Lower Gastrointestinal Tract

Abstract

In birds, lower intestinal Na-absorptive capacity is quantitatively important (compared to Na-intake rates or to body Na pools) in ostriches ( Struthio camelus), emus ( Dromaius novaehollandiae), chukars ( Alectoris chukar), and desert quails ( Callipepla gambeli), all species lacking nasal salt glands and having an ecophysiological need for Na retention; it is also important for ducks ( Anas platyrhynchos) on low-Na intake. However, Schmidt-Nielsen's hypothesis (that marine birds may conserve urinary water loss via Na-linked water absorption in the lower intestine and desalinate the absorbate via the salt glands) is not supported by studies on ducks and glaucous-winged gulls ( Larus glaucescens) maintained under hyperosmotic conditions. Evidence for regulation of lower intestinal Na transport in response to Na status is confined to chukars amongst these species, but regulated suppression of urinary reflux is inferred to occur in chukars, sand partridges ( Ammoperdix heyi), desert quails, house sparrows ( Passer domesticus), ducks, and glaucous-winged gulls when urine osmolality exceeds that from which lower intestinal transport can make net water gains against serosal-mucosal osmotic losses. Consequently, lower intestinal water absorption seems important in hydrated terrestrial birds, in which its principal role may be net absorption of water (particularly from intestinal fluid) after the small intestine has recovered water secreted into-intestine more anteriorly.