Adaptive radiation of the ateline primates

Measurements were taken on the upper and lower molars of 37 species of primates and one tupaiid to assess the relative importance of shearing, crushing and grinding features. Significant correlations were found between pairs of allometrically standardized dimensions which measure the same molar function (shearing, crushing, or grinding). Correlations between pairs of dimensions which do not measure the same function are not significant. Second molar adaptations for shearing, crushing, and grinding, as well as the length of the second lower molar, and the total surface of the post-canine dentition are negatively allometric with respect to metabolic rate. Species which take different proportions of fruit, leaves, and insects in their diets have different molar structure. Frugivores have small teeth for their adult body size with poorly developed shearing, crushing, and grinding features on their molars. By contrast, leaf-eating species tend to have large teeth for their adult body size with well developed shearing, crushing, and grinding. The second molars of insectivorous species were found to parallel closely those of leaf-eating species. The two groups are clearly distinguishable from the former on the basis of body size alone: the smallest living primate leaf-eater is on order of magnitude larger than the largest living primate insectivore.

Three categories of dietary adaptation are recognized--faunivory, frugivory, and folivory--according to the distinctive structural and biochemical features of animal matter, fruit, and leaves respectively, and the predominance of only one in the diets of most species. Mammals subsisting mainly on animal matter have a simple stomach and colon and a long small intestine, whereas folivorous species have a complex stomach and/or an enlarged caecum and colon; mammals eating mostly fruit have an intermediate morphology, according to the nature of the fruit and their tendency to supplement this diet with either animal matter or leaves. The frugivorous group are mostly primates: 50 of the 78 mammalian species, and 117 of the 180 individuals included in this analysis are primates. Coefficients of gut differentiation, the ratio of stomach and large intestine to small intestine (by area, weight, and volume), are low in faunivores and high in folivores; the continuous spread of coefficients reflects the different degrees of adaptation to these two dietary extremes. Interspecific comparisons are developed by allowing for allometric factors. In faunivores, in which fermentation is minimal, the volume of stomach and large intestine is related to actual body size, whereas these chambers are more voluminous in larger frugivores and mid-gut fermenting folivores; fore-gut fermenters show a marked decrease in capacity with increasing body size. Surface areas for absorption are related to metabolic body size, directly so in frugivores; area for absorption is relatively less in larger faunivores and more in large folivores, especially those with large stomachs. Indices of gut specialization are derived from these regressions by nonlinear transformation, with references to the main functional features of capacity for fermentation and surface area for absorption. These are directly comparable with the dietary index, derived from quantitative feeding data displayed on a three-dimensional graph, with all species within a crescentic path from 100% faunivory through 55--80% frugivory to 100% folivory, perhaps illustrating, at least for primates, the evolutionary path from primitive insectivorous forms through three major ecological grades.