title: Phylogenetic constraints on digesta separation: Variation in fluid throughput in the digestive tract in mammalian herbivores.
abstract: 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.
authors.affiliation: Clinic for Zoo Animals, Exotic Pets and Wildlife, Vetsuisse Faculty, University of Zurich, Winterthurerstr. 260, 8057 Zurich, Switzerland. dmueller@vetclinics.uzh.ch
authors.forename: Dennis W H
authors.initials: DW
authors.lastname: Müller
authors.forename: Judith
authors.initials: J
authors.lastname: Caton
authors.forename: Daryl
authors.initials: D
authors.lastname: Codron
authors.forename: Angela
authors.initials: A
authors.lastname: Schwarm
authors.forename: Roger
authors.initials: R
authors.lastname: Lentle
authors.forename: W Jürgen
authors.initials: WJ
authors.lastname: Streich
authors.forename: Jürgen
authors.initials: J
authors.lastname: Hummel
authors.forename: Marcus
authors.initials: M
authors.lastname: Clauss
mesh_terms: D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D004032:Diet; D004064:Digestive System; D005772:Gastrointestinal Transit; D006801:Humans; D008322:Mammals / Q000145:classification / Q000502:physiology*; D010802:Phylogeny