The influence of natural diet composition, food intake level, and body size on ingesta passage in primates. Comparative Biochemistry and Physiology-Part A: Molecular and Integrative Physiology, 150(3):274-281. Abstract An important component of digestive physiology involves ingesta mean retention time (MRT), which describes the time available for digestion. At least three different variables have been proposed to influence MRT in herbivorous mammals: body mass, diet type, and food intake (dry matter intake, DMI). To investigate which of these parameters influences MRT in primates, we collated data for 19 species from trials where both MRT and DMI were measured in captivity, and acquired data on the composition of the natural diet from the literature. We ran comparative tests using both raw species values and phylogenetically independent contrasts. MRT was not significantly associated with body mass, but there was a significant correlation between MRT and relative DMI (rDMI, g/kg0.75/d). MRT was also significantly correlated with diet type indices. Thus, both rDMI and diet type were better predictors of MRT than body mass. The rDMI-MRT relationship suggests that primate digestive differentiation occurs along a continuum between an "efficiency" (low intake, long MRT, high fiber digestibility) and an "intake" (high intake, short MRT, low fiber digestibility) strategy. Whereas simple-stomached (hindgut fermenting) species can be found along the whole continuum, foregut fermenters appear limited to the "efficiency" approach. Clauss et al. Primate ingesta retention relationship suggests that primate digestive differentiation occurs along a continuum between an "efficiency" (low intake, long MRT, high fiber digestibility) and an "intake" (high intake, short MRT, low fiber digestibility) strategy. Whereas simple-stomached (hindgut fermenting) species can be found along the whole continuum, foregut fermenters appear limited to the "efficiency" approach. Key words: ingesta passage, mean retention time, digestive anatomy, digestive physiology, feeding ecology, herbivory, foregut fermenter 35 36 Clauss et al. Primate ingesta retention 362, L.C., Wheeler, P., 1995. The expensive tissue hypothesis: The brain and the digestive system in human and primate evolution. gut fill, liquid and particle marker retention in mouflon (Ovis ammon musimon), and a comparison with roe deer (Capreolus capreolus). Acta Theriologica 49, 503-515. Bell, R.H.V., 1971. A grazing ecosystem in the Serengeti. Scientific American 225, 86-93.