1Interdepartmental Program in Ecology, Pennsylvania State University, 2Department of Biological Sciences, Dartmouth College, 3Department of Anthropology, California State University
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Grazing theropithecines were once widespread and abundant in the Plio-Pleistocene; today, however, only gelada baboons (Theropithecus gelada) practice manual graminivory. Given the diversity of ancient grazing primates, possibly including hominins (i.e., Paranthropus boisei), it is surprising that the mechanisms of foraging in T. gelada have not been explored in more detail. Here we use mixed-effects models to preliminarily evaluate the determinants of feeding rate (plucks/second) in 40 adult and juvenile gelada individuals from the Guassa Plateau population in northern Ethiopia over a two-month period. We found that pluck rate decreased with plucking bout duration only for certain graminoids and herbs, indicating bout termination was not consistently associated with patch depletion. Adult sex interacted with food category to influence pluck-rate. Males plucked herbs 13% faster than they did graminoids, whereas females plucked graminoids 12% faster than they did herbs. Males plucked herbs 21% faster than females, but there was no difference between sexes for graminoid pluck rates. When subterranean components (e.g. bottoms or roots) were included in a feeding bout, pluck rates were significantly lower (p<.0001), suggesting plant physical properties, including stem and blade toughness, constrain harvesting rates. The maximum number of plucks for a given food item tend to scale with body mass and thumb length. Given that intrinsic hand proportions (IHP) were relatively similar between grazing fossil theropithecine species, we suggest that IHP evolution early in the Theropithecus lineage played a key role in the success of the genus.
JTK and VVV were supportedby National Science Foundation Graduate Research Fellowship Awards, VVV was further supported by the American Society of Primatologists, and a National Geographic Young Explorers grant. PJF and NNN were supported by the Pittsburgh Zoo, the Margot Marsh Biodiversity Fund, and the Cleveland Metroparks Zoo.