Spatially dense morphometrics of craniofacial sexual dimorphism in 1‐year‐olds

Abstract

Recent advances in the field of geometric morphometrics allow for powerful statistical hypothesis testing for effects of biological and environmental variables on anatomical shape. This study used partial least-squares regression (PLSR) and the recently developed bootstrapped response-based imputation modelling (BRIM) algorithm to test for sexual dimorphism in the craniofacial shape of 1-year-old humans. We observed a recession of the forehead in boys relative to girls, and differences in the nose, consistent with adult dimorphism. Results also suggest that the degree to which individuals express dimorphic traits is continuous throughout the population. This is also seen in adult dimorphism but in 1-year-olds the amount of overlap between groups is much higher, indicating the strength of dimorphism between sexes is lower. Our results demonstrate early sexual dimorphism that is not attributable to the influx of sex hormones at puberty. This highlights the need to look at very early ontogeny for the origins of sexual dimorphism. We suggest that future work look at potential mediating effects of this early dimorphism on the later impact of puberty. The subtle shape differences we have detected, may also be applied to sexing fossilised crania. A common artefact in 3D images of faces of young children is that they often have their mouths open to varying degrees, introducing variability in the data unrelated to anatomy. We describe two PLSR-based methods of correcting this. These methods may facilitate surgical planning and assessment of young children based on 3D images.

Facial masculinity does not appear to be a condition-dependent male ornament in humans and does not reflect MHC heterozygosity

“…Little is known about the genetic and environmental factors contributing to differences in facial shape between males and females, or the variation in facial masculinity within the sexes. Differences in facial shape exist between male and female children as young as three years old (70,71) , and are likely defined, in part, during gestation by the intrauterine environment (72)(73)(74) . Sexual dimorphism in the face increases dramatically at the onset of puberty, implicating sex hormones and other endocrine processes underlying general growth occurring around this period (70,(75)(76)(77)(78)(79) .…”

A broad autism phenotype expressed in facial morphology

“…Given the possible links between prenatal testosterone and the masculinisation of the brain and face during fetal development, investigations motivated by the extreme male brain theory have been extended to studying facial masculinisation in children with ASD. Although facial structures are known to be most sexually dimorphic after the onset of puberty 18 , recent advances in three-dimensional (3D) photogrammetry have allowed for facial masculinity to be defined in prepubescent infants and children [19][20][21] .…”

Association Between Prenatal Alcohol Exposure and Craniofacial Shape of Children at 12 Months of Age

“…A neutral facial expression was not ensured at all times, and some images were taken with the mouth open and others with the mouth closed. Mouth position in the images was subsequently standardized individually using a previously published method 16 to ensure that each image had a neutral expression.…”

“…Covariates Regression covariates included risk factors known to be, or that could plausibly be, associated with craniofacial shape: child's sex, which is known to be associated with early craniofacial dimorphism 16 ; maternal age, a risk factor for malformations 21 ; and maternal smoking in pregnancy, a risk factor for malformations of the head and face. 22 The child's birth weight and maternal prepregnancy body mass index were included as factors potentially resulting in greater fat deposition around the cheeks.…”