Enamel thickness in Bornean and Sumatran orangutan dentitions

Abstract

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.

Dental tissue proportions in fossil orangutans from mainland Asia and Indonesia

“…Virtual 2D section planes were generated from three-dimensional models with VG Studio MAX 2.0 software (Volume Graphics, Inc.) according to published protocols. 29,35,41,45 Several variables were quantified on 2D section planes following Martin: 41,45 enamel cap area (c), enamel-dentine junction length (e), and coronal dentine area enclosed by the enamel cap (b) ( Figure 1). Average enamel thickness (AET) is calculated as [c/e], yielding the average straight-line distance (mm units), or thickness, from the enamel-dentine junction to the outer enamel surface.…”

“…It was not possible to assess temporal variation due to the uncertainty of dates for the Chinese apothecary and Sumatran cave material (see Supplementary Information), which constitute the majority of the fossil sample. Variables were also compared between pooled fossil samples and a comparative extant sample of 193 Bornean and Sumatran postcanine teeth 28,29 using the Mann-Whitney U test. …”

“…16 Thick enamel is often interpreted an adaptation to resist tooth damage and/or abrasion while feeding on hard, brittle, or abrasive objects. 33,34 The lower AET found in extant orangutans relative to fossil populations may indicate a change in dietary behavior towards less mechanically demanding or abrasive food, although orangutans have a broad and variable diet (reviewed in 29,30 ), as is true of recent human populations.…”

“…[25][26][27] Comparisons of dental enamel thickness between the two species also reveal highly overlapping ranges and statistically indistinguishable means. 28,29 This is somewhat surprising given differences in jaw morphology and the material properties of dietary items between the two species. 30, 31 Gantt 32 and Ho et al 4 noted that fossil orangutans show thick enamel, although this was not quantified in either study, nor were comparisons made among fossil groups.…”

“…5,24,36,37 Finally, these results are considered in light of recent studies of enamel thickness within fossil and extant Homo sapiens, 35,[38][39][40] which are known to show a similar pattern of dental reduction over the same period. Given the significance of enamel thickness in assessments of hominoid systematics 28,29,32,35,41 and dental functional morphology, 33,34,42,43 characterization of enamel thickness within a geographically and temporally diverse hominoid genus will also permit more refined comparisons of limited samples of other fossil apes and humans.…”