1Department of Anthropology, Humboldt State University, 2Department of Human Evolutionary Biology, Harvard University, 3School of Biological Sciences, Washington State University, Pullman, 4Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, 5Department of Anatomical Sciences, State University of New York, Stony Brook, 6Department of Evolutionary Anthropology, Duke University, 7Depto Ciências Biológicas, Universidade Federal de São Paulo, Campus Diadema, 8Departments of Anthropology and Biological Sciences, Dartmouth College
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A prevailing concept in the literature is that body size constrains hearing sensitivity and vocal communication. Among primates, larger species tend to send and receive low-frequency vocal signals, whereas smaller species utilize higher frequencies. In studies of vocal communication, this phenomenon is known as the motivation-structural (MS) rule, can account for the suboptimal acoustic structures many of vocal signals. The New Worldmonkeys represent a model system for testing this scaling concept, yet few data exist on the auditory sensitivities of platyrrhine primates. Here we explore this premise by presenting new data on the auditory sensitivities of nine species: A. caraya, A. clamitans, A. palliata, Ateles belzebuth marginatus, A. geoffroyi, Brachyteles arachnoides, Calicebus cupreus, Cebus apella and Lagothrix lagotricha. We found that the high-frequency hearing limit was negatively related to interaural distance; however, this relationship was influenced by phylogenetic relatedness and did not extend to the frequency of best auditory sensitivity. In addition, several conventional parameters of vocal acoustics (mean values for the vocal repertoire) were unrelated to body mass; for each species, there were vocalizations that corresponded closely with auditory sensitivity (often calls emitted by infants), but the associated motivational states varied. Taken together, these findings challenge the idea that acoustic communication varies primarily as a function of size. Rather, a complex array of phylogenetic, anatomical and ecological factors appears to have exerted a selective pressure on the platyrrhine auditory system.