LAUSR

Abstract The rise and spread of tropical grasslands was a signal event in the Cenozoic, causing many ungulates to evolve adaptations to a diet of graminoid tissues, or graminivory. In parallel, a lineage of monkeys (Theropithecus) is distinguished among primates for its large size and commitment to graminivory, a trait expressed by species throughout the Plio-Pleistocene fossil record and T. gelada, the sole surviving species today. An open question concerns the mechanics of how fossil species of Theropithecus handled graminoid tissues. They might have exhibited preference, selecting tissues within a given tuft, or they might have practiced indiscriminate bulk-feeding in a manner similar to large grazing ungulates. To differentiate between these handling behaviors, we used time- and graminivore-calibrated carbon stable isotope values to show progressive reliance on high-throughput bulk-feeding graminivory. Variation in this behavior explains a significant amount of variation in body mass through time, and we describe these covarying traits, which peaked during the Pleistocene, as evolutionary traps. To support this characterization, we report evidence of temporal increases in strontium isotope variability among North African theropiths, a result that suggests greater lifetime travel and energetic costs in response to diminishing food resources, a probable factor in the extinction of T. oswaldi, the largest monkey that ever lived.