1School of Human Evolution and Social Change, Arizona State University, 2Institute of Human Origins, Arizona State University, 3Department of Biological Sciences, Dartmouth College, 4Department of Anthropology, California State University Fullerton
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Geladas (Theropithecus gelada) are regarded as extreme dietary specialists owing to their heavy reliance on grass, a unique adaptation among primates. Consistent with this view, molar microwear analyses have revealed gelada enamel surface features to be highly anisotropic and low in complexity. Further, low variation in complexity, anisotropy, and heterogeneity among individuals has implied dietary homogeneity at the population and species levels. Two weaknesses of these earlier studies are the absence of dietary information for the specimens and their association with heavily degraded habitats. Here we present new molar microwear texture analyses on gelada specimens from an ecologically-intact Afroalpine grassland in Ethiopia, the Guassa Plateau. The dietary ecology of this population is diverse (including grasses, herbs, and invertebrates) and variable across seasons and age-sex classes, with elevated consumption of underground storage organs during low rainfall periods. To test whether this dietary variability is recorded on enamel surfaces, we analyzed dental microwear textures from Facet 9 of M1s from four specimens (three juveniles and one adult). The specimens show lower mean anisotropy, higher mean complexity, and greater variation in heterogeneity compared to previously published results for geladas. These patterns could be linked to (a) the frequent consumption of quartz grains, both from soil-borne grit and heavy seasonal dust accumulation on above-ground vegetation, and/or (b) the seasonal consumption of fallback resources with challenging mechanical properties. These results illustrate a diversity of microwear textures for geladas despite highly specialized anatomical adaptations for graminivory. Implications for interpreting hominin dietary ecology from microwear are discussed.
Funding provided by the Graduate Research Support Program (ASU) and the NSF Graduate Research Fellowship Program.