Balolia, KL; (2015) Sexual dimorphism, growth and development beyond dental maturity in the cranium of extant hominoid primates. Doctoral thesis , UCL (University College London).

Abstract

Sexual size dimorphism is a useful predictor of sociality in primates based on the association between male intrasexual competition and male size, relative to female size. Less considered ideas include the relationship between sexual dimorphism of facial traits and socioecological variability and the notion that females may obtain reproductive advantages from larger body size or facial morphology, similar to what is observed in males. In the present research, I investigate the morphological correlates of social and non-social variables to understand whether the facial skeleton, including the sagittal crest, carries a social signal. I adopt the heterochronic framework advocated by Shea (1986) to understand how sexual dimorphism is attained. I examine craniofacial size and shape dimorphism, growth and development in five extant hominoid primate taxa (<i>Homo sapiens</i>, <i>Pan t. schweinfurthii</i>, <i>Gorilla g. gorilla</i>, <i>Pongo p. pygmaeus</i> and <i>Hylobates lar</i>) to understand whether sex-specific patterns of facial dimorphism, growth and development are associated with social and life history variables. I use 3D surface data to quantify cranial size and shape using 3D co-ordinate landmarks, surface area data and linear measurements. Geometric morphometric techniques are used to calculate size and shape variables, including Procrustes distances between male and female average shapes, and visualisation of shape differences using Principal component scores. Marital system explains a small, but significant, amount of craniofacial size and shape variation in modern humans, and patterns of shape dimorphism in the brow ridge of <i>Pan t. schweinfurthii</i> and <i>Gorilla g. gorilla</i>, and in the mid-face of <i>Gorilla g. gorilla</i> and <i>Pongo p. pygmaeus</i>, indicate that sex-specific shape of these regions may be morphologically conserved. Sagittal crest emergence in <i>Gorilla g. gorilla</i> and <i>Pongo p. pygmaeus</i> males cannot be explained by mastication alone and is likely to be, in part, a result of sexual selection. Future studies, adopting a heterochronic approach to sexual dimorphism, are likely to afford detailed inferences about the relationship between morphological and behavioural variables and may have applications in reconstructing extinct hominoid social behaviour.

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