The Avian Cecum: a Review

-The ceca, intestinal outpocketings of the gut, are described, classified by types, and their occurrence surveyed across the Order Aves. Correlation between cecal size and systematic position is weak except among closely related species. With many exceptions, herbivores and omnivores tend to have large ceca, insectivores and carnivores are variable, and piscivores and graminivores have small ceca. Although important progress has been made in recent years, especially through the use of wild birds under natural (or quasi-natural) conditions rather than studying domestic species in captivity, much remains to be learned about cecal functioning. Research on periodic changes in galliform and anseriform cecal size in response to dietary alterations is discussed. Studies demonstrating cellulose digestion and fermentation in ceca, and their utilization and absorption of water, nitrogenous compounds, and other nutrients are reviewed. We also note disease-causing organisms that may be found in ceca. The avian cecum is a multi-purpose organ, with the potential to act in many different ways-and depending on the species involved, its cecal morphology, and ecological conditions, cecal functioning can be efficient and vitally important to a bird’ s physiology, especially during periods of stress. Received 14 Feb. 1994, accepted 2 June 1994. The digestive tract of most birds contains a pair of outpocketings that project from the proximal colon at its junction with the small intestine (Fig. 1). These ceca are usually fingerlike in shape, looking much like simple lateral extensions of the intestine, but some are complex in structure. Within the Class Aves, ceca range in size from very long to very short, or they may be entirely absent (Table 1). Unlike the case in almost all mammals, most avian ceca are paired and of approximately equal length, with separate lateral or ventrolateral openings into the colon (rectum). In some species the openings are dorsal or ventral, and a few paired ceca share a common orifice, but the great majority open into the lateral colon opposite one another (McLelland 1989). There has been confusion over how the passage of material through cecal openings is controlled, but it is now believed that the interdigitating meshwork of villi at the cecal entrance act as a filter, excluding large particles and allowing only fluid and fine particles to be separated and pushed from the colonic contents into the ceca by colonic antiperistalsis (Duke 1986a). This villous meshwork exists even in species, such as the House Sparrow (Passer domesticus), with very small, possibly nonfunctional, ceca (Klem et al. 1983). In the few species with larger ceca that have been studied histologically in detail (domestic chickens, G&us, other galliforms, and ducks, Anus; Calhoun 1954, Clarke 1978, Fenna and Boag 1974b, Mahdi and ’ Div. of Gastroenterology, Dept. of Internal Medicine, The Univ. of Texas Medical Branch, Galveston, Texas 71555-0764.