Absorption by the cecum of wild birds: is there interspecific variation

The major functions proposed for the avian cecum--energy balance and osmoregulation--involve absorption of material from the cecum, yet little is known of these absorptive processes in wild birds. The magnitude of transport of compounds from the ceca depends on the concentrations of compounds in the ceca, the presence of transport pathways for those compounds, and the size of the reabsorptive surface. Interspecific variation in size of avian ceca is well documented, but the other two determinants of absorption are much less well studied. All ceca examined contained acetic, propionic, and butyric acid; water content varies from less than 70 to greater than 90%. Most other constituents have been quantified in just one or two species. A model for cecal transport, derived from studies of mammalian ceca but consistent with observations on birds, is described. A luminal (mucosal) Na+/H+ exchanger couples the absorption of Na+ to that of volatile fatty acids, which are absorbed in their uncharged (protonated) form; these two compounds are thereby mutually stimulatory in their absorption. The cecum also absorbs water, sodium, carbon dioxide, and probably some nitrogenous compounds (e.g., ammonia). Too few data exist to analyze interspecific variation in cecal transport in birds. However, variations in the following characteristics do exist: cecal contents, cecal absorptive surface area, transport characteristics of other regions of avian intestine (e.g., rectum), cecal transport among mammalian species, and histology of avian ceca. Studies of the physiological consequences of these variation should yield insight into the function and contributions of the avian cecum.