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The hominoid digestive strategy

Judith M. Caton

     The diets of extant hominoids, which are plant-based, contain appreciable amounts of non-starch polysaccharides that require microbial fermentation before they can be used as an energy source. Hominoids use the digestive strategy of colon fermentation for processing the non-starch polysaccharide component of their diets.
     The diagnosis of digestive strategy is based on three groups of data* wild diet, gastro-intestinal morphology and digesta passage experiments. Hominoid diets consist of ripe fruit, young leaves, and other plant parts, supplemented with a range of animal foods. Human hunter-gatherers eat similar diets. The morphology of the gastro-intestinal tract has a common pattern in all hominoids: a unipartite stomach, a long, narrow small intestine, a small caecum ending in a vermiform appendix, and a colon divided into proximal and distal segments. The musculature of the hindgut is complex as it is adapted for mixing and retaining digesta for microbial fermentation.
     Microbial fermentation is a much slower process than enzymatic digestion, thus digesta passage through the guts of plant-eaters is much slower than for mammals eating diets requiring no fermentation. A fluid and a particle marker are used to measure the rate of digesta passage from mouth to anus. The results of these studies with hominoids demonstrated that there was prolonged retention of digesta both markers in all species, with the particle marker being retained for longer than the fluid marker. This pattern of marker elimination from the guts of apes is consistent with that recorded from other plant-eaters using the digestive strategy of colon fermentation.
     The three-pronged approach to the study of extant primate digestive strategies incorporating ecological, morphological and physiological data is a means of providing new and valuable insights into the digestive strategies of closely related species, e.g. Homo erectus and Homo sapiens. Walker and Shipman, (1996) discussed the conclusion, based a skeleton exhibiting symptoms of hypervitaminosis A, tooth microwear and the Expensive Tissue Hypothesis* of Aiello and Wheeler (1993) that H. erectus was a carnivore. Similarities in gut morphology and the common digestive strategy of the hominoids suggest that this was unlikely, and that H. erectus was an opportunistic scavenger, supplementing its diet with animal foods when available.