How mangabey molar form differs under routine vs. fallback hard-object feeding regimes.

Background: Components of diet known as fallback foods are argued to be critical in shaping primate dental anatomy. Such foods of low(er) nutritional quality are often non-preferred, mechanically challenging resources that species resort to during ecological crunch periods. An oft-cited example of the importance of dietary fallbacks in shaping primate anatomy is the grey-cheeked mangabey . This species relies upon hard seeds only when softer, preferred resources are not available, a fact which has been linked to its thick dental enamel. Another mangabey species with thick enamel, the sooty mangabey , processes a mechanically challenging food year-round. That the two mangabey species are both thickly-enameled suggests that both fallback and routine consumption of hard foods are associated with the same anatomical feature, complicating interpretations of thick enamel in the fossil record. We anticipated that aspects of enamel other than its thickness might differ between and We hypothesized that to function adequately under a dietary regime of routine hard-object feeding, the molars of would be more fracture and wear resistant than those of .

Methods: Here we investigated critical fracture loads, nanomechanical properties of enamel, and enamel decussation in and Molars of , a genus not associated with hard-object feeding, were included for comparison. Critical loads were estimated using measurements from 2D µCT slices of upper and lower molars. Nanomechanical properties (by nanoindentation) and decussation of enamel prisms (by SEM-imaging) in trigon basins of one upper second molar per taxon were compared.

Results: Protocone and protoconid critical fracture loads were significantly greater in than and greater in both than in . Elastic modulus, hardness, and elasticity index in most regions of the crown were greater in than in the other two taxa, with the greatest difference in the outer enamel. All taxa had decussated enamel, but that of uniquely exhibited a bundle of transversely oriented prisms cervical to the radial enamel. Quantitative comparison of in-plane and out-of-plane prism angles suggests that decussation in trigon basin enamel is more complex in than it is in either or . These findings suggest that molars are more fracture and wear resistant than those of and . Recognition of these differences between and molars sharpens our understanding of associations between hard-object feeding and dental anatomy under conditions of routine vs. fallback hard-object feeding and provides a basis for dietary inference in fossil primates, including hominins.