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Morphological variation and sexual dimorphism of the modern human sacrum

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author KrennV, FornaiC, HaeuslerM,
Project Birth and human evolution - implications from computer-assisted reconstructions
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Original article (peer-reviewed)

Journal Proc Europ Soc Hum Evol
Volume (Issue) 8
Page(s) 98
Title of proceedings Proc Europ Soc Hum Evol

Open Access

Type of Open Access Publisher (Gold Open Access)


Sexual dimorphism of the human pelvis has been explained by the obstetrical dilemma hypothesis as the result of antagonistic se- lection pressures for a narrow, biomechanically efficient pelvis in both sexes and a spacious birth canal in females. The details of how pelvic sexual dimorphism evolved are, however, largely unknown. The pattern and degree of sexual dimorphism in early hominins is obscured by geographic, temporal and taxonomical variation as well as the fragmentary condition of the fossil pelvic record [1, 2]. As part of the pelvic girdle, it is reasonable to think that the sacrum contributes to pelvic sexual dimorphism. The female sacrum is generally described as broader, shorter and more curved than in males, contributing to a wider birth canal. However, the liter- ature provides contrasting results and it is not entirely clear which factors drive sacral variation beside sex [3, 4]. Based on linear measurements and angles as well as landmark-based data, classification accuracies for sexual determination range between 60 and 90% depending on the studied population. Here, we investigate the morphological variation of the modern human sacrum using qualitative, linear and geometric morphometric (GM) data from a worldwide sample of male and female individuals from diverse populations to account for sexual and geographical variation. Our sample comprised 150 individuals of known sex from Central Europe, South-East Asia, South America and Africa, including pygmies and Khoesan. The 3D GM analysis was based on 44 land- marks and 66 semilandmarks. Interlandmark distances were calculated to assess the classical corporo-basal index [5]. Furthermore, a qualitative sex determination was carried out by each of the co-authors and repeated three times. We observed a vast overlap of male and female sacra in the PCA plot of the GM analysis in shape space. The warps showed no sexual dimorphism in sacral cur- vature. Nevertheless, we confirmed a pattern in the height-to-width proportion and relative expansion of the alae. Classification accuracies for the linear measurements ranged from 50-75% and for the qualitative investigations from 65-75%. All analyses were repeated for the Central Europeans only (n=58), yielding a clearer pattern of sexual dimorphism. Males and females separated with minor overlap along PC2 and the classification accuracies increased by 10-15%, depending on the analysis. On a worldwide perspective, sexual dimorphism therefore seems to be lower than previously suggested and to be confounded by other factors, in- cluding geographical origin and body size, though regional groups such as Central Europeans remained more dimorphic. Thus, the accuracy for sex determination of the human sacrum appears to be highly population-dependent, which explains the heterogeneous outcomes reported so far. Nevertheless, the sexual dimorphism of the sacrum remains lower than demonstrated by studies of the human hipbone. Although the sacrum forms part of the birth canal, it might be under lower selective pressure than the pelvis as a whole. Perhaps, factors such as the sacro-iliac joint mobility during birth act as compensatory mechanisms, but this needs to be investigated. Learning more about modern human sexual dimorphism represents a step towards a better understanding of hominin morphology. Our outcomes suggest that sexing fossil hominin sacra remains problematic because besides taxonomy, geographic and temporal variation, it presents additional challenges for the interpretation of fossil remains.