Optimal digestive strategies for arboreal herbivorous mammals in contrasting forest types. Why Koalas and Colobines are different

The eucalypt forests of temperate eastern Australia contrast with many forests in tropical Australia, Asia, Africa and Central America in terms of degrees of sclerophylly, concentrations of nutrients in foliage, types and amounts of chemical challenges posed for herbivores, and the range of alternatives to foliage as food. Comparisons between digestive strategies employed by arboreal herbivorous marsupials and primates inhabiting these forests reveal several trends: (i) foregut-fermenters occur only in tropical and sub-tropical forests, whereas caecum and caecum-colon fermenters occur in both temperate eucalypt and the tropical forests; (ii) the arboreal marsupials and primates (koalas, greater gliders, ringtail possums and sportive lemurs) that are the most folivorous are caecum or caecum-colon fermenters with colonic specializations for selective retention of small particles of digesta; (iii) no caecum or caecum- colon fermenters lacking colonic separation mechanisms achieve consistently high degrees of folivory; (iv) even the most folivorous foregut-fermenting arboreal marsupials (tree kangaroos) of primates (colobus monkeys) include a substantial proportion of high-quality components (fruits or seeds) in their diet. Caecum or caecum-colon fermentation with colonic separation is postulated to be essential for a high degree of folivory in small mammals because it offsets potential limits on intake of high-fibre food and minimizes losses of faecal nitrogen that otherwise would be prohibitive. This digestive strategy is common among folivores in eucalypt forests because the low abundance of large seeds and fleshy fruits limits folivory-frugivory and folivory-granivory. Foregut fermentation employing a sacciform/tubiform forestomach (as in tree kangaroos and colobus monkeys) is postulated to be optimal for mixed leaf/fruit and leaf/seed diets that are available in many tropical forests, assuming it allows high-fibre meals to be retained for microbial fermentation while permitting low-fibre meals to pass rapidly to the hindstomach for acid/enzymic digestion. However, foregut fermentation appears innappropriate for either primary frugivory or exclusive folivory due to the inefficiency of microbial digestion of simple sugars and constraints on microbial fermentation as a primary energy source for small mammals.