1Department of Anthropology, California State University Fullerton, 2Department of Biology, Penn State University, 3Department of Biological Sciences, Dartmouth College
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Geladas (Theropithecus gelada), graminivorous monkeys endemic to the alpine grasslands of Ethiopia, and their extinct theropithecine relatives have long been used as models for understanding the diets of extinct hominins. To date, however, no published account of gelada feeding ecology over an annual cycle exists, and previous investigations of gelada ecology have been limited to populations occupying heavily degraded habitats. During a recent 15-month period, we found that a ~220-member gelada band inhabiting an unusually ecologically-intact grassland at Guassa, Ethiopia consumed a more diverse diet than extremely graminivorous populations studied at more disturbed sites elsewhere. At Guassa, geladas consumed >50 species of plants and >12 species of invertebrates and exhibited wide variability in their monthly diet, with monthly percentage consumption of grasses, herbs, and invertebrates ranging from 35%-74%, 19%-61%, 0%-8%, respectively. Overall, the geladas’ annual diet consisted of 55% grasses (42.0% tall grass leaves; 8.8% short grass leaves; 2.4% other short grass parts; 2.2% tall grass seed heads), 37% herbs (28.8% herb leaves; 7.5% herb roots; 0.6% herb flowers), 3% invertebrates, and 5% other items (1.9% unidentified underground items; 1.6% underground storage organs; 1.3% pith). While geladas at Guassa can be classified as graminivores, their diet is considerably more varied than assumed by most modelers of extinct hominin evolution. Given that the ecosystem at Guassa is probably similar to the one in which geladas evolved, a diet dominated by a variety of grasses and herbs, with occasional bouts of invertebrate consumption, may be the ‘natural’ diet of geladas.
This study was funded by Pittsburgh Zoo, Margot Marsh Biodiversity Foundation, Primate Conservation Inc., and Cleveland Metroparks Zoo.