Was facial width-to-height ratio subject to sexual selection pressures? A life course approach

Abstract

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.

Tracking sexual dimorphism of facial width-to-height ratio across the lifespan: implications for perceived aggressiveness

“…There was a significant main effect of age (F 9,17587 = 88.73, p < 0.001, h 2 p ¼ 0:043, 95% confidence interval (CI) [0.037, 0.049]), with an overall tendency for FWHR to decrease with age. This result supports the previous findings of Hehman et al [20] and Hodges-Simeon et al [13], where the authors found that FWHR decreased with age. The main effect of sex was also significant (F 1,17587 = 17.05, p < 0.001, h 2 p ¼ 0:001, 95% CI [0.0003, 0.002]), with females (M = 2.19, s.d.…”

“…For women, there is no clear prediction for the sizes of the FWHR across the lifespan or relationship with perceived aggressiveness as the FWHR may be a sexually selected trait in men, but not women. Studies of male and female faces showed a slow decline in FWHR from the age of 20 until the age of 40 for both men and women [ 13 ]. The authors attributed this result to the lengthening of the face with age.…”

Facial and body sexual dimorphism are not interconnected in the Maasai

“…One of the goals of our study was to address the debates on sex-typical variation in fWHR and its’ evolutionary origin. Recently published paper by Hodges-Simeon with co-authors [ 72 ] based on data from Caucasian origin (from 3 to 40 years) and Bolivian Tsimane (from 7 to 21 years) samples of men and women tested the applicability of four variants of fWHR measurements and demonstrated that fWHR lower (including lower jaw) exhibited both adult sex differences, and the classic pattern of ontogeny for human secondary sexual characteristics (greater lower-face growth in male adolescents relative to females). In our study, we calculated six variants of fWHR (including four variants of upper fWHR, total, and lower fWHR), but none of them revealed significant sexual dimorphism.…”

“…Classical facial indices, which are known to demonstrate certain degree of sexual dimorphism in humans, were calculated based on coordinates of facial landmarks after Procrustes superimposition. These indices included upper width-to-height ratio (upper fWHR); total width-to-height ratio (total fWHR); lower width-to-height ratio (lower fWHR); cheekbone prominence; and mandibular and nasal indices, mouth shape, mouth-face index, and height-to-width ratio of the eye [ 37 , 66 , 72 ]. The locations of the landmarks used for calculation of these indices are presented in Fig.…”

“…One of the facial parameters, which has attracted a special attention of anthropologists and behavioral scientists today, is the facial width-to-height ratio (fWHR), defined as the ratio of the bizygomatic width to the so-called upper facial height (measured from Nasion to Prosthion, or their soft-tissue approximations) [ 33 , 66 ]. This index associates with facial masculinity and has been viewed as sexually dimorphic, being higher in males than females [ 33 , 67 – 72 ]. Weston and colleagues [ 73 ] speculated that sexual dimorphism in the fWHR evolved via female choice as an attractive trait.…”