key: cord-0885532-b7zmcnmn authors: Rostovtseva, Victoria V.; Mezentseva, Anna A.; Windhager, Sonja; Butovskaya, Marina L. title: Second-to-fourth digit ratio and facial shape in Buryats of Southern Siberia date: 2020-07-22 journal: Early Hum Dev DOI: 10.1016/j.earlhumdev.2020.105138 sha: 7153c60e4bd8af7443735f47c9455b154d54fab3 doc_id: 885532 cord_uid: b7zmcnmn Abstract Background The 2nd-to-4th digit ratio (2D: 4D) is a putative predictor of a prenatal exposure to sex hormones. 2D:4D is sexually dimorphic (males < females). Studies, linking digit ratio and full facial shapes among Europeans, show that a low 2D:4D is associated with a set of male-specific facial features. Buryats – Mongolian people from Southern Siberia – demonstrate a different pattern of facial sexual dimorphism than Europeans (narrower and more vertically elongated faces in men as opposed to women). Aim The aim of the present study was to investigate the association between facial shape and the 2D:4D in comparison to the pattern of facial sexual dimorphism in Buryats. Subjects Buryats: 88 men and 80 women aged 20 ± 2 years. Outcome measures To assess relationship between facial shape and 2D:4D we used a geometric morphometric approach based on standardized full-face frontal photographs and direct measurements of the digit lengths among right-handed individuals. Results The results revealed that 2D:4D was associated with facial morphology in Buryat men, and to a lesser extent in women. Narrower faces, elongated in the vertical direction, and a narrower lower facial outline, were characteristic of Buryat men with low 2D:4D ratios, which corresponded to the male-like facial shapes in Buryats. Conclusions In Europeans, such facial features were reported for men with high 2D:4D, which corresponded to female-like European facial shapes. Hence, our results show that sex-specific morphogenesis in humans is multidirectional, and that digit ratio is capable of predicting sex-specific facial traits even in populations with differing sexually-dimorphic morphology. J o u r n a l P r e -p r o o f Buryats -Mongolian people of Southern Siberiamay be considered as a population of interest for comparison with Europeans. According to the previous studies, sexual dimorphism in facial parameters of Buryats are similar to that reported for other Asian populations (higher relative lower facial width, and thinner upper lips in females), but alsohas a distinctive feature, such as a lowerfWHR in men [47, [50] [51] [52] .These facial parameters distinguish representatives of the Buryat population from European subjects, studied earlier with regard to association between 2D:4Dratio and facial shape [22, 25, 26, 46] . Assuming that 2D:4D is related toprenatal androgenization, we hypothesized thatthe expression of more male-specific facialtraitsin Buryats will be associated with lower digit ratios in both sexes.At the same time, this association is expected todemonstrate apartly reversedpatterncompared toEuropeans, which is in line with the observed differences in the facial sexual dimorphism patterns. Of course, none of this precludes additional effects of post-pubertal steroid action on the adult sexual dimorphism. To test this hypothesis,we conducted a study among modern Buryats, investigating association between 2D:4D ratio and full facial shape using geometric morphometrics. This is, to our knowledge, the first study to address this issue in Siberian (and Mongolian) populations. Participants of our study were 187 Buryats (88 female, 99 male)representatives of a Mongolian population living in Southern Siberia (Baikal Lake area).Participants were aged 20 ± 2 years (from 17 to 25 years, with two male outliers, who were 16 and 28 years of age).No significant sex differences in age were observed. All of the participants were residents of Ulan-J o u r n a l P r e -p r o o f The 2 nd and 4 th digit lengths were measured directly [3, 22] using an electronic caliper with a precision of 0.01 mm. Each participant's digits were measured twice, and the mean value of two measurements was used for further analysis.The Intraclass Correlation Coefficient (ICC; [55] ) between two measurements (two-way mixed model, absolute agreement, single measurement) was 0.99 (p<0.001). In the current study, we focused on the right-hand digit ratio, as it has been previously reported to show a stronger relationship with sex-specific traits [3, 11, 17, 56] .Only self-reported right-handed participantswere selected for the analysis, since they were the majority, and it is known that handedness, as a presumable consequence of specific cerebral lateralization,may be associated with exposure to prenatal sex hormones [57] [58] [59] [60] . Therefore, the final sample consisted of 168 individuals (80women, 88 men).Participants had no congenital or acquired damage to the hand or fingers on the right hand. The facial morphology was analyzed using geometric morphometrics [61] based on standardized facial photographs. Participants had no congenital or acquired facial deformations. Each participant was photographed in full-face, with a neutral facial expression, sitting on a fixed chair, with a natural head position (Frankfort horizontal plane). The camera was positioned at eyes' height. The distance between camera and participant was 170 cm. Every photograph included a scale bar (in cm). Seventy-one digital landmarks and semi-landmarks [48] were placed on each photograph using tpsDig2 2.17 [62] . To test for inter-rater agreement, themanual landmark digitization was made by two independent observers on 40 randomly selected photographs (20 male and 20 female faces). According to the ICC estimates (based on single measurement, absolute agreement, two-way mixed-effects model), the inter-observer agreement was almost perfect (ICC=0.98, p<0.001). The method was deemed reliable enough to proceed with manual digitalization of landmarks by one of the observers. The facial configurations were standardized for position, orientation and scale using a Generalized Procrustes superimposition, which was performed iteratively together with the sliding of the semi-landmarks in tpsRelw 1.67 [62] . In addition, the facial shape information was symmetrized in Mathematica 11 [63] . Thereafter, the facial shape coordinates were regressed upon sex of the participants (male, female)and the 2D:4D ratio using tpsRegr 1.45 [62] . To test for statistical significance permutation testswith 10,000 permutationswereused [64] . Visualization of facial shape differences was realized with thin-plate deformation grids using tpsRegr 1.45 [62] . Geometric morphometric morphs' visualization was conducted by unwarping and averaging the images in tpsSuper 2.04 [48, 62] . Within the given age range,there was nosignificant effect of age on male facial shape variation (var. expl. < 1%, p=0.524), and a weak trend only in women (var. expl. 2.43%, p=0.063).Also there was no association between age and 2D:4D,neither in men (R 2 <0.001, p=0.968), nor in women (R 2 <0.001, p=0.889).Female Buryats generally had higher digit ratios on the right hand (0.96 ± 0.03), than male Buryats (0.95 ± 0.03), but the difference was not statistically significant(Student's t-test: t=-0.590, p=0.556). To test for possible allometric effects, which suggest a negative relationship between 2D:4D and general finger lengths (as well as general hand size) [27] [28] [29] , we have regressed 2D:4D ratio upon mean digits length (sum of the lengths for 2 nd and 4 th digits divided by two). Linear regression analysis for the whole sample (Beta=0.029, R 2 =0.001, p=0.705), as well as for men (Beta=0.033, R 2 =0.001, p=0.762), and women (Beta=0.118, R 2 =0.014, p=0.298) separately did not reveal any significant associations. Hence, lower 2D:4D ratios in our study were not associated with longer digits, which does not support the allometry hypothesis. To assess sexual dimorphism in facial shape, we regressed facial shape coordinates upon the sex of participants.Apermutation test revealed significant differences in facial shape between righthanded men and women (var. expl. 20%; p<0.001). Figure1a displaysthe average female and male faces with an exaggeration factor of 2.5 (to facilitate visual perception of the observed Journal Pre-proof J o u r n a l P r e -p r o o f differences). Next, we regressed the facial shape coordinates upon 2D:4D of the right hand, for men andwomen separately.In terms of facial shape variation, the permutation test revealed a significant association only formen (N=88, var. expl. 3%, p=0.036), whereas 2D:4D did not significantly contribute to female facial variation(N=80, var. expl. <1%, p=0.773).Still, the correlation between facial shape scores and 2D:4D was significant in both sexes (men: r=0.36, p=0.0005; women: r=0.27, p=0.017), which means that there is a weak, but significant association between facial shape and 2D:4D in both sexes. Figure 1 displays the effect ofthe 2D:4D on facial shape within each sex, exaggerated for +/-5 SD from the male (Fig. 1b) and the female (Fig. 1c) averages, respectively. The amount of exaggerationwas chosenarbitrarily to facilitate visual perception of shape differences. [23, 32] . Still, in our study of Buryats (Mongolian origin) the patternwassomewhat reversed, which at the same time corresponded to the general direction offacial sexual dimorphism in Buryats (Fig. 1a) .Part of the sex-specific facial traitswas not associated with the male2D:4D patternboth in our study, and in the studies by our colleagues conducted in Europe [23, 26] . This indicates the existence of further mechanisms for sex-specific facial morphogenesis, such asthe impact of sex hormone exposure on later stages of ontogenesis (perinatal [40] , pubertal hormones [45] ), allometric effects [65] ,genetics [66, 67] , andsexual selection [39, [68] [69] [70] [71] to result in the observable pattern of sexual dimorphism in young adulthood. The lack of association between 2D:4D and some facial regionsmightcause null results in studies where only some discrete indexes or incomplete facial shapes are considered. We suppose that this could be one of the reasons fornull association between 2D:4D and facial shape reported by Whitehouse andcolleagues [40] , who run the analysis based on21 facial landmarks (and no information on the full-face outline and visible eye shape).With regard to the study by Whitehouse and others it is also important to note, that the lack of association between T concentrations in umbilical cord upon delivery and the 2D:4D ratios, which has been alsoreported by several studies earlier [72, 73] ,cannot evidence that 2D:4D is a poor predictor of prenatal androgenization. The reason is that umbilical cord method deals with the level of hormones in the perinatal period, when T is significantly decreased compared to early prenatal levels [74] . Whitehouse and colleagues [40] obtained relatively large correlation coefficients between perinatal hormone levels and adult facial scores (from a discriminant function analysis of adult sex differences based on a number of facial distances). It remains to be determined whether the perinatal hormonal effects replicate in studies like ours, where facial shape is studied more holistically by preserving the geometry of the morphological structures, and independently of absolute size differences. The lack of a significant association of facial shape variation and 2D:4D in women is in line with the findings of Fink and colleagues [23] for European faces. In sum, our results suggest that,if 2D:4D ratios can beconsidered as a J o u r n a l P r e -p r o o f markerof prenatal androgenization, the impact of prenatal hormones can at least partly explain the male-specific morphogenesis characteristic of a particular population.Our findingsalso indicate that population homogeneity of studied subjectsshould be strictly enforced in researchdealing withandrogenization and its consequences, to avoid false null or contradictive results. 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