Retention marker excretion suggests incomplete digesta mixing across the order primates

The digestive tract of animals, and the patterns how passage markers are excreted from them, have been fruitfully compared to chemical reactor models from engineering science. An important characteristic of idealized reactor models is the smoothness of the curves plotting marker concentrations in outflow (i.e., faeces) over time, which is the result of the assumed complete mixing of the marker with the reactor contents. Published excretion patterns from passage experiments in non-primate mammals appear to indicate a high degree of digesta mixing. In order to assess whether marker excretion graphs from primates differ from ideal outflow graphs, we performed passage experiments in eight individuals of three foregut-fermenting species (Pygathrix nemaeus, Trachypithecus auratus and Semnopithecus vetulus), and added them to available marker excretion curves from the literature. In the resulting collection, 23 out of a total of 25 patterns in foregut fermenters (21 individuals of 10 species from 7 studies), and 13 out of 15 in hindgut fermenters (9 individuals of 2 species from 2 studies), showed an irregular, 'spiky' pattern. We consider this proportion to be too high to be explained by experimental errors, and suggest that this may indicate a taxon-wide characteristic of particularly incomplete digesta mixing, acknowledging that further data from less related primate species are required for corroboration. Our hypothesis is in accordance with previous findings of a comparatively low degree of 'digesta washing' (differential retention of particulate and fluid digesta) in primates. Together with literature findings that suggest a low chewing efficiency in primates compared to other mammals, these observations indicate that in contrast to other herbivores, the success of the primate order is not derived from particularly elaborate adaptations of their ingestive and digestive physiology.