Craniodental Sexual Dimorphism Among Hylobatids

Sexual dimorphism has been widely documented in primates, however craniodental sexual dimorphism among hylobatids has not been well researched. In this study, I investigate interspecific differences in the presence and pattern of craniodental sexual dimorphism among gibbons and siamang using data taken from eight species representing all four hylobatid genera (Hoolock leuconedys, Hylobates agilis, Hy. klossi, Hy. lar, Hy. muelleri, Hy. pileatus, Nomascus gabriellae, and Symphalangus syndactylus). I sampled 17-30 cranial specimens for each species. I quantified craniofacial and upper canine crown height dimorphism using morphometric data taken from 3D surface models and directly from dry specimens to examine the presence and pattern of craniodental sexual dimorphism among hylobatids. Hoolock leuconedys shows statistically significant sex differences across all craniofacial shape and form measures investigated. Although Hy. lar, Ho. leuconedys, and S. syndactylus all show statistically significant cranial form dimorphism, there are interspecific differences in how this dimorphism is expressed. Hoolock leuconedys, Hy. lar, and S. syndactylus are unique in showing upper canine crown height dimorphism, and Ho. leuconedys show a high level of browridge dimorphism in which white fur highlights this region in males, in contrast to their black body and facial pelage. The finding of male-biased sexual dimorphism in only some hylobatid taxa suggests that although male craniofacial morphology of some gibbon and siamang species may be associated with sex-specific agonistic interactions, this effect is not ubiquitous among hylobatids. Further research is required to understand these findings in the context of the socioecology of individual hylobatid taxa.