1Department of Applied Forensic Sciences, Mercyhurst University, 2Department of Anatomy, Khon Kaen University
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Rensch’s Rule states that sexual size dimorphism increases with size and has been demonstrated in many non-human animals including primates. In humans, tests of Rensch’s Rule have been limited to stature, with no significant relationship, but tests using human craniometrics have not been investigated to our knowledge. This study tests Rensch’s Rule using craniometric data collected from geographically and temporally diverse human skeletal samples.
Thirteen cranial measurements were analyzed from 26 groups in the Howells Data Set as well as modern groups from South Africa, Thailand and the Forensic Data Bank (n=3305). Sexual dimorphism was calculated as the Mahalonobis distances (D2) between males and females of each group independently; the larger the D2, the greater the sexual dimorphism. Cranial size was represented as male and female geometric means and their logged values were analyzed using major axis regression.
Our results show that male and female cranial sizes within groups are highly correlated (r2 = 0.92), and that sexual dimorphism is greatest in cranial length and bizygomatic breadth. In general, Pacific Island groups show the largest size and the greatest sexual dimorphism; Asian groups are variable; African groups are smaller and show lower sexual dimorphism. A statistically significant positive allometric relationship (p < 0.05) exists between cranial size and sexual dimorphism, but modern groups tend to be relatively large with lower sexual dimorphism. These results suggest that human cranial sexual dimorphism follows Rensch's Rule, but also that other factors influence the relationship between size and sexual dimorphism.