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.