Department of Anthropology, Boston University, Boston, Massachusetts, United States of America.
School of Human Services, University of Cincinnati, Cincinnati, Ohio, United States of America.
PLoS One. 2021 Mar 12;16(3):e0240284. doi: 10.1371/journal.pone.0240284. eCollection 2021.
Sexual selection researchers have traditionally focused on adult sex differences; however, the schedule and pattern of sex-specific ontogeny can provide insights unobtainable from an exclusive focus on adults. Recently, it has been debated whether facial width-to-height ratio (fWHR; bi-zygomatic breadth divided by midface height) is a human secondary sexual characteristic (SSC). Here, we review current evidence, then address this debate using ontogenetic evidence, which has been under-explored in fWHR research. Facial measurements were collected from 3D surface images of males and females aged 3 to 40 (Study 1; US European-descent, n = 2449), and from 2D photographs of males and females aged 7 to 21 (Study 2; Bolivian Tsimane, n = 179), which were used to calculate three fWHR variants (which we call fWHRnasion, fWHRstomion, and fWHRbrow) and two other common facial masculinity ratios (facial width-to-lower-face-height ratio, fWHRlower, and cheekbone prominence). We test whether the observed pattern of facial development exhibits patterns indicative of SSCs, i.e., differential adolescent growth in either male or female facial morphology leading to an adult sex difference. Results showed that only fWHRlower exhibited both adult sex differences as well as the classic pattern of ontogeny for SSCs-greater lower-face growth in male adolescents relative to females. fWHRbrow was significantly wider among both pre- and post-pubertal males in the Bolivian Tsimane sample; post-hoc analyses revealed that the effect was driven by large sex differences in brow height, with females having higher placed brows than males across ages. In both samples, all fWHR measures were inversely associated with age; that is, human facial growth is characterized by greater relative elongation in the mid-face and lower face relative to facial width. This trend continues even into middle adulthood. BMI was also a positive predictor of most of the ratios across ages, with greater BMI associated with wider faces. Researchers collecting data on fWHR should target fWHRlower and fWHRbrow and should control for both age and BMI. Researchers should also compare ratio approaches with multivariate techniques, such as geometric morphometrics, to examine whether the latter have greater utility for understanding the evolution of facial sexual dimorphism.
性选择研究人员传统上专注于成年性别差异；然而，特定性别个体发育的时间表和模式可以提供仅关注成年人无法获得的见解。最近，人们一直在争论面部宽高比（fWHR；双颧骨宽度除以中面部高度）是否是人类的次要性特征（SSC）。在这里，我们回顾了当前的证据，然后使用个体发育证据来解决这一争论，而这在 fWHR 研究中尚未得到充分探讨。在研究 1（美国欧洲裔，n=2449）中，我们从 3 至 40 岁的男性和女性的 3D 表面图像中收集了面部测量值，从 7 至 21 岁的男性和女性的 2D 照片中收集了面部测量值（玻利维亚的提斯曼人，n=179），用于计算三种 fWHR 变体（我们称之为 fWHRnasion、fWHRstomion 和 fWHRbrow）和两种其他常见的面部男性比例（facial width-to-lower-face-height ratio，fWHRlower 和颧骨突出度）。我们测试了所观察到的面部发育模式是否表现出指示 SSC 的模式，即青春期男性或女性面部形态的差异生长导致成年性别差异。结果表明，只有 fWHRlower 表现出成年性别差异以及 SSC 的经典发育模式——青春期男性的下脸生长大于女性。在玻利维亚的提斯曼人样本中，fWHRbrow 在青春期前和青春期后的男性中均明显更宽；事后分析表明，这种影响是由眉骨高度的巨大性别差异驱动的，在所有年龄段，女性的眉骨都比男性高。在两个样本中，所有 fWHR 测量值都与年龄呈负相关；也就是说，人类面部生长的特征是中面部和下脸相对于面部宽度的相对伸长。这种趋势甚至持续到中年。BMI 也是各年龄段大多数比例的正预测因子，BMI 越大，脸越宽。收集 fWHR 数据的研究人员应将 fWHRlower 和 fWHRbrow 作为目标，并控制年龄和 BMI。研究人员还应将比率方法与几何形态测量等多元技术进行比较，以检查后者是否更有助于理解面部性别二态性的进化。
