Craniofacial Shape and Nonmetric Trait Variation in Hybrids of the Japanese Macaque (Macaca fuscata) and the Taiwanese Macaque (Macaca cyclopis)
It has become apparent that natural hybridization is far more common and may play a much greater role in evolution than has historically been recognized. The skeletal morphology of hybrid primates is notoriously variable and difficult to predict. Indeed, before the advent of genetic sequencing techniques, many wild hybrid populations went undetected. Though many species of primates are now known to hybridize naturally and are likely to have done so for millions of years, anthropogenic alterations to the environment are increasingly restricting or altering primate species ranges and contact zones and driving hybridization between populations that may otherwise never have come into contact. The case of hybridizing Japanese and Taiwanese macaques (Macaca fuscata and Macaca cyclopis) is an excellent example of this, as these two island species could not have come into contact without human interference. Here we apply 3D geometric morphometrics and nonmetric trait analysis to the crania and dentition of hybrid macaques (N = 70) and their parental species, M. fuscata (N = 57) and M. cyclopis (N = 51). The exploration of 3D shape variation identifies mildly transgressive morphology in the hybrids and a general tendency toward the M. fuscata morphotype overall, but less variability in the hybrid morphotype than has been identified in previous studies of primate hybrids. We also identify a small number of nonmetric traits that differentiate the hybrids from the parental species, although the power of these traits to distinguish between groups is weak and their relationship with hybridity is unclear. We conclude that the relatively short divergence time between the parent species is likely to help explain the observed differences in hybrid morphotype, and that further exploration of the relationship between degree of evolutionary divergence and hybrid morphology may help us to better explain and predict hybrid morphology in other taxa.
KeywordsCraniofacial shape Developmental abnormalities Hybrid phenotype Hybridization Macaca cyclopis Macaca fuscata
This research was conducted as part of PhD research undertaken at the University of New South Wales, supported by an Australian Postgraduate Award and Research Excellence Award (UNSW School of Biological, Earth and Environmental Sciences). This research was chiefly funded by the Kyoto University Primate Research Institute Cooperative Research Program (awarded to C. Boel). We would like to thank Associate Professor Yoshi Kawamoto and Dr. Tsuyoshi Ito of the Kyoto University Primate Research Institute (KU-PRI) for providing access to genetic data and CT scans relevant to the hybrid sample, Professor Toshio Mouri for his generous guidance and assistance during data collection at the KU-PRI, and the Working Group of Hybrid Macaques who studied the demography and caught hybrids and Taiwanese macaques. We would also like to acknowledge the effort of the editor and reviewers, and thank them for the valuable contributions they have made to this manuscript.
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