Sexual differences in human cranial morphology: Is one sex more variable or one region more dimorphic?

Abstract

The quantification of cranial sexual dimorphism (CSD) among modern humans is relevant in evolutionary studies of morphological variation and in a forensic context. Despite the abundance of quantitative studies of CSD, few have specifically examined intra-sex variability. Here we quantify CSD in a geographically homogeneous sample of adult crania, which includes Italian individuals from the 19th and 20th centuries. Cranial morphology is described with 92 3D landmarks analyzed using Procrustean geometric morphometrics (PGMM). Size and shape variables are used to compare morphological variance between sexes in the whole cranium and four individual regions. The same variables, plus Procrustes form, are used to quantify average sex differences and explore classification accuracy. Our results indicate that: (a) as predicted by Wainer's rule, males present overall more variance in size and shape, albeit this is statistically significant only for total cranial size; (b) differences between sexes are dominated by size and to a lesser extent by Procrustes form; (c) shape only accounts for a minor proportion of variance; (d) the cranial base shows almost no dimorphism for shape; and (e) facial Procrustes form is the most accurate predictor of skeletal sex. Overall, this study suggests developmental factors underlying differences in CSD among cranial regions; stresses the need for population-specific models that describe craniofacial variation as the basis for models that facilitate the estimation of sex in unidentified skeletal remains; and provides one of the first confirmations of "Wainer's rule" in relation to sexual dimorphism in mammals specific to the human cranium.

A human craniofacial life‐course: Cross‐sectional morphological covariations during postnatal growth, adolescence, and aging

“…Gender differences of size were established well before puberty and our study suggests that males experienced subtly different patterns of brain and of cranial aging as well as growth. These findings are consistent with recent work published by Smith et al (2020;Smith, Duncan, Pears, Profico & O'Higgins, 2021) as well as Milella et al (2021). We find no evidence that the aged adult cranium is more variable due to, for example, fragility nor that covariations were any weaker or stronger compared with middle aged adults.…”

“…More recently, Smith et al's (2020) analyses of surface landmarks has suggested that craniofacial sexual dimorphism is mostly size related and that males follow an extended growth trajectory (hypermorphosis). This size offset has also been confirmed in a recent study of skeletal landmarks (Milella, Franklin, Belcastro, & Cardini, 2021) with males, on average, more prognathic and dolichocephalic compared with females. In the present study, we test for sexual dimorphism in the size and shape of the brain as well as of the cranium across a large population, including during ontogeny and adulthood.…”

Exploring sexual dimorphism of human occipital and temporal bones through geometric morphometrics in an identified Western‐European sample

“…This finding support previous studies that found that cranial morphology is largely influenced by size, with males having larger cranial size (Franklin et al, 2006;Gonzalez et al, 2011;Green & Curvoe, 2009;Kimmerle et al, 2008;Rosas & Bastir, 2002). This has also been observed in earlier studies analyzing the occipital (Chovalopoulou et al, 2013;Guyomarc'h & Bruzek, 2010) and the temporal bones (Guyomarc'h & Bruzek, 2010;Milella et al, 2021), as it has also been seen in other skeletal regions (Bastir et al, 2014;García-Martínez et al, 2016). On the regression graph (Figure 8), it appears that the size range of females is substantially comprised into the size range of males, suggesting that the highly significant differences could in fact be driven by a handful of particularly large males.…”

“…Nevertheless, those two specific PCs only represent 22.74% of the overall variation. Previous authors noted intraspecific variations on either both temporal bones (Guyomarc'h & Bruzek, 2010) or on the left one (Milella et al, 2021). Franklin also observed in a South‐African sample, a larger mastoid process in length and height, in males, with considerable antero‐inferior projection (Franklin et al, 2006).…”

“…There has been little attention given to the occipito-temporal region, particularly the mastoid process, which is yet a feature that is included in well-known visual scoring methods (Acsádi & Nemeskéri, 1970;Buikstra & Ubelaker, 1994;Walker, 2008;Walrath et al, 2004). Previous studies found significant size and shape differences in temporal bones between males and females while investigating 19th and 20th European documented populations (Guyomarc'h & Bruzek, 2010;Milella et al, 2021). Besides, significant sex variations were either highlighted in the cranial base shape data (i.e., occipital and/or sphenoid bones) (Chovalopoulou et al, 2013) or not found at all (Bigoni et al, 2010;Guyomarc'h & Bruzek, 2010;Milella et al, 2021).…”