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Craniodental Sexual Dimorphism Among Hylobatids

Abstract

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.

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Acknowledgments

This article is dedicated to Charles A. Lockwood and to Colin Groves, whom I thank for their mentorship and for valuable discussions surrounding sexual dimorphism and gibbon biology respectively. I thank the museum curators whose institutions house the hylobatid crania used in this study and J. Michael Plavcan for sharing his cranial and dental hylobatid data (NSF SBR 9616671) used in this study. I also thank the Smithsonian's Division of Mammals (Dr. Kristofer Helgen) and Human Origins Program (Dr. Matt Tocheri) for the scans of USNM specimens used in this research (http:// humanorigins.si.edu/evidence/3d-collection/primate). These scans were acquired through the generous support of the Smithsonian 2.0 Fund and the Smithsonian’s Collections Care and Preservation Fund. I thank Bernard Wood, Alison Behie, and Eric Lewitus for comments on an earlier draft of this article, and Jo Setchell, J. Michael Plavcan, and an anonymous reviewer for comments and feedback that improved the manuscript.

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KLB conceived and designed the study, analyzed the data, interpreted the results, and wrote the article.

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Correspondence to Katharine L. Balolia.

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Handling Editor: Joanna M. Setchell

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ESM 1

Supporting Information is available online: Table SI: Summary of primary research to date documenting hylobatid craniofacial sexual dimorphism. Table SII: Summary of primary research to date documenting hylobatid canine sexual dimorphism. Table SIII: Descriptive statistics, index of sexual dimorphism (ISD) values, and Student’s t-test results for individual cranial measurements for seven hylobatid taxa (XLSX 46.2 kb)

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Balolia, K.L. Craniodental Sexual Dimorphism Among Hylobatids. Int J Primatol 42, 737–758 (2021). https://doi.org/10.1007/s10764-021-00233-3

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Keywords

  • Gibbon
  • Hoolock
  • Hylobates
  • Nomascus
  • Sexual selection
  • Siamang
  • Symphalangus