Colobine monkeys are known for the anatomical complexity of their stomachs, making them distinct within the primate order. Amongst foregut fermenters, they appear peculiar because of the occurrence of two different stomach types, having either three ('tripartite') or four ('quadripartite', adding the praesaccus) chambers. The functional differences between tri and quadripartite stomachs largely remain to be explained. In this study, we aim to compare the apparent digestibility (aD) in tripartite and quadripartite colobines. Hence, we measured the aD in two colobine species, Nasalis larvatus (quadripartite) and Trachypithecus cristatus (tripartite), in two zoos. We also included existing colobine literature data on the aD and analysed whether the aD of fibre components is different between the stomach types to test the hypothesis of whether quadripartite colobines show higher aD of fibre components than tripartite colobines did. Our captive N. larvatus specimen had a more distinctively varying nutrient intake across seasons with a larger seasonal variation in aD than that of a pair of T. cristatus, which mostly consumed commercial foods with a lower proportion of browse and less seasonal variation. We observed higher aD of dry matter (DM), neutral detergent fibre (NDF) and acid detergent fibre (ADF) in the N. larvatus specimen, suggesting a higher gut capacity of N. larvatus provided by the additional praesaccus forestomach chamber. Based on the analysis of literature data for aD, we also found that quadripartite species achieved higher fibre digestibility at similar dietary fibre levels compared with tripartite species, supporting the hypothesis that the additional gut capacity offered by the praesaccus facilitates a longer retention and hence more thorough microbial fermentation of plant fibre.
In animal husbandry, diets should help in maintaining a healthy body condition, support reproduction, and promote species-specific longevity. It is recommended to feed folivorous primates kept in zoos a high-fiber diet, i.e., leaves, although satisfying such a requirement is challenging in temperate regions because it is difficult to obtain fresh leaves, especially in autumn and winter. As equally important for their appropriate treatment, it is valuable to provide details of clinical reports of medical problems and pathological findings, although such clinical reports are rather limited. Therefore, in foregut-fermenting proboscis monkeys (Nasalis larvatus), we (1) described the individual clinical reports of renal disease and weight loss at the Yokohama Zoological Gardens in Japan, (2) determined the nutritional profile of the diets supplied to these animals because other potential triggers for their renal disease and weight loss could be excluded, (3) modified the diet regimen to minimize weight loss and the development of hypercalcemia and hypophosphatemia, and (4) assessed the effects of such dietary modification by comparing the body weight and the Ca and P concentrations and the Ca/P ratios in the blood before and after diet modification with a comparison of these measurements between zoo and free-ranging individuals. Based on the nutritional profile of the diets, we concluded that the reported cases of renal failure might be caused by consumption of leaves with a Ca/P ratio far above the appropriate level in autumn and winter. Additionally, the dietary modification of minerals and metabolizable energy achieved certain beneficial effects on zoo-kept proboscis monkeys.