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.

Nasalization by Nasalis larvatus: Larger noses audiovisually advertise conspecifics in proboscis monkeys.

Male proboscis monkeys have uniquely enlarged noses that are prominent adornments, which may have evolved through their sexually competitive harem group social system. Nevertheless, the ecological roles of the signals encoded by enlarged noses remain unclear. We found significant correlations among nose, body, and testis sizes and a clear link between nose size and number of harem females. Therefore, there is evidence supporting both male-male competition and female choice as causal factors in the evolution of enlarged male noses. We also observed that nasal enlargement systematically modifies the resonance properties of male vocalizations, which probably encode male quality. Our results indicate that the audiovisual contributions of enlarged male noses serve as advertisements to females in their mate selection. This is the first primate research to evaluate the evolutionary processes involved in linking morphology, acoustics, and socioecology with unique masculine characteristics.

Fecal Nutrients Suggest Diets of Higher Fiber Levels in Free-Ranging than in Captive Proboscis Monkeys (Nasalis larvatus).

Understanding the natural diet of species may provide useful information that can contribute to successful captive maintenance. A common problem experienced with captive foregut-fermenting primate (colobine) diets is that they are deficient in fiber and therefore highly digestible. This may contribute to gastrointestinal disorders often observed in zoos. An approach to obtain information relevant for the improvement of diets is to compare the nutrient composition of feces from free-ranging and captive individuals. In theory, fecal material can be considered a proxy for diet intake integrated over a certain period of time. We collected fecal samples from eight free-ranging proboscis monkey (Nasalis larvatus, a highly endangered colobine species) groups from a secondary forest along the Kinabatangan River and four from a mixed mangrove-riverine forest along the Garama River, Sabah, Borneo, Malaysia. We also collected fecal samples from 12 individual captive adult/sub-adult proboscis monkeys from three different zoos. We confirmed that feces from free-ranging monkeys contained more fiber and less metabolic fecal nitrogen than those from captive specimens, indicating a less digestible diet in the wild. Modifying the diets of captive colobines to include more fiber, comparable to those of free-ranging ones, may contribute to their health and survival.