Functional molar macrowear analysis in Pongo pygmaeus and Pongo abelii

Orangutans are found in Southeast Asian tropical rainforest of Borneo (Pongo pygmaeus) and Sumatra (Pongo abelii) and are primarily considered as frugivorous species. However, in seasonal periods when fruit availability is scarce, orangutans tend to eat fibrous and unripe fruits, leaves, nuts, bark, and in rare occasion, wood [1]. The highly seasonal environments, and fruit availability, seem to play an important evolutionary role in shaping the craniodental morphology in Pongo [1]. In fact, while Bornean orangutans, spend more time feeding on fallback foods (those seasonal foods with poor nutritional values and which are more difficult to digest), Sumatran populations consume more ripe fruit pulp. As a consequence, Bornean orangutans are characterised by relatively more robust mandibles, providing greater load resistance abilities to masticatory forces [2].
The aim of this study is to analyse the molar macrowear pattern of Bornean (Pongo pygmaeus) and Sumatran (Pongo abelii) orangutans, and thus investigate if there is any geographic variation in diet between the two species. We employ a well-established method known as Occlusal Fingerprint Analysis [3], that allows the three-dimensional reconstruction of the occlusal movements responsible for the formation of wear facets, those flat and polished enamel areas with well-defined borders. This approach considers the functional aspects of tooth macrowear (buccal phase I, lingual phase I and phase II facets) that occur during the sequential phases of the chewing cycle, or power stroke [4].
The sample consists of second mandibular molars (which provide a good general overview of the development of masticatory functions within a species) of P. pygmaeus (n = 9) and P. abelii (n = 8), characterised by a moderate degree of wear, between stages 2 and 3 [5].