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The relevance of the mean retention time (MRT) of particles through the gastrointestinal tract (GIT) is well understood and MRT(particle)GIT is an important parameter in digestion models. Solute markers have been used to estimate MRT(solute)GIT (or 'fluid passage') in animals, but the relevance of this measure is less evident and is usually sought in its relation to MRT(particle)GIT. The ratio between the two measures indicates the degree of 'digesta washing', with little washing occurring at ratios of 1, aborad washing at ratios >1 (where the solute marker travels faster than the particle marker), and orad (retrograde) washing at ratios <1 (where the solute marker travels slower than the particle marker). We analysed digesta washing in a dataset of 98 mammalian species including man of different digestion types (caecum, colon and nonruminant foregut fermenters, and ruminants), controlling for phylogeny; a subset of 72 species allowed testing for the influence of food intake level. The results indicate that MRT(solute)GIT and the degree of digesta washing are related to digestion type, whereas variation in MRT(particle)GIT is influenced mainly by effects of body mass and food intake. Thus, fluid throughput and digesta washing emerge as important correlates of digestive anatomy. Most importantly, primates appear constrained to little digesta washing compared to non-primate mammalian herbivores, regardless of their digestion type. These results may help explain the absence of primates from certain herbivore niches and represent a drastic example of a physiologic limitation in a phylogenetic group. More experimental research is required to illuminate relative benefits and costs of digesta washing.

Behavioral observations and small fecal particles compared to other primates indicate that freeranging proboscis monkeys (Nasalis larvatus) have a strategy of facultative merycism (rumination). In functional ruminants (ruminant and camelids), rumination is facilitated by a particle sorting mechanism in the forestomach that selectively retains larger particles and subjects them to repeated mastication. Using a set of a solute and three particle markers of different sizes (b 2, 5 and 8 mm), we displayed digesta passage kinetics and measured mean retention times (MRTs) in four captive proboscis monkeys (6-18 kg) and compared the marker excretion patterns to those in domestic cattle. In addition, we evaluated various methods of calculating and displaying passage characteristics. The mean ± SD dry matter intake was 98 ± 22 g kg−0.75 d−1, 68 ± 7% of which was browse. Accounting for sampling intervals in MRT calculation yielded results that were not affected by the sampling frequency. Displaying marker excretion patterns using fecal marker concentrations (rather than amounts) facilitated com-parisons with reactor theory outputs and indicated that both proboscis and cattle digestive tracts represent a se-ries of very few tank reactors. However, the separation of the solute and particle marker and the different-sized particle markers, evident in cattle, did not occur in proboscis monkeys, in which all markers moved together, at MRTs of approximately 40 h. The results indicate that the digestive physiology of proboscis monkeys does not show typical characteristics of ruminants, which may explain why merycism is only a facultative strategy in this species. Abstract 24Behavioral observations and small fecal particles compared to other primates indicate that 25 free-ranging proboscis monkeys (Nasalis larvatus) have a strategy of facultative merycism 26 (rumination). In functional ruminants (ruminant and camelids), rumination is facilitated by a 27 particle sorting mechanism in the forestomach that selectively retains larger particles and 28 subjects them to repeated mastication. Using a set of a solute and three particle markers of 29 different sizes (<2, 5 and 8 mm), we displayed digesta passage kinetics and measured mean 30 retention times (MRTs) in four captive proboscis monkeys (6-18 kg) and compared the 31 marker excretion patterns to those in domestic cattle. In addition, we evaluated various 32 methods of calculating and displaying passage characteristics. The mean ± SD dry matter 33 intake was 98 ± 22 g kg −0.75 d −1 , 68 ± 7% of which was browse. Accounting for sampling 34 intervals in MRT calculation yielded results that were not affected by the sampling frequency. 35Displaying marker excretion patterns using fecal marker concentrations (rather than amounts) 36 facilitated comparisons with reactor theory outputs and indicated that both proboscis and 37cattle digestive tracts represent a series of very few tank reactors. However, the separation of 38 the solute and particle marker and the different-sized ...

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