Dental enamel thickness has received considerable attention in ecological models of the adaptive significance of primate morphology. Several authors have theorized that the degree of enamel thickness may reflect selective pressures related to the consumption of fallback foods (dietary items that may require complex processing and/or have low nutritional value) during times of preferred food scarcity. Others have speculated that enamel thickness reflects selection during mastication of foods with particular material properties (i.e., toughness and hardness). Orangutans prefer ripe fruit when available, but show interspecific and sex differences in the consumption of fallback foods (bark, leaves, and figs) and other preferred foods (certain seeds). Bornean orangutans (Pongo pygmaeus) have also been reported to masticate more mechanically demanding foods than Sumatran orangutans (Pongo abelii). To test these ecological models, we assessed two-dimensional enamel thickness in orangutan full dentitions using established histological and virtual quantification methods. No significant differences in average enamel thickness (AET) were found between species. We found significant differences in the components of enamel thickness indices between sexes, with males showing greater enamel-dentine junction lengths and dentine core areas, and thus relatively thinner enamel than females. Comparisons of individuals of known sex and species revealed a dentition-wide trend for Bornean females to show greater AET than Sumatran females. Differences between small samples of males were less evident. These data provide only limited support for ecological explanations of enamel thickness patterns within great ape genera. Future studies of dietary ecology and enamel thickness should consider sex differences more systematically. Am J Phys Anthropol, 2012. © 2012 Wiley Periodicals, Inc.