Shape of the orbital opening: individual characterization and analysis of variability in modern humans, Gorilla gorilla, and Pan troglodytes

Unique human orbital morphology compared with that of apes

Humans' and apes' convergent (front-facing) orbits allow a large overlap of monocular visual fields but are considered to limit the lateral visual field extent. However, humans can greatly expand their lateral visual fields using eye motion. This study aimed to assess whether the human orbital morphology was unique compared with that of apes in avoiding lateral visual field obstruction. The orbits of 100 human skulls and 120 ape skulls (30 gibbons; 30 orangutans; 30 gorillas; 30 chimpanzees and bonobos) were analyzed. The orbital width/height ratio was calculated. Two orbital angles representing orbital convergence and rearward position of the orbital margin respectively were recorded using a protractor and laser levels. Humans have the largest orbital width/height ratio (1.19; p < 0.001). Humans and gibbons have orbits which are significantly less convergent than those of chimpanzees/bonobos, gorillas and orangutans (p < 0.001). These elements suggest a morphology favoring lateral vision in humans. More specifically, the human orbit has a uniquely rearward temporal orbital margin (107.1°; p < 0.001), suitable for avoiding visual obstruction and promoting lateral visual field expansion through eye motion. Such an orbital morphology may have evolved mainly as an adaptation to open-country habitat and bipedal locomotion.

Gorillas with white sclera: A naturally occurring variation in a morphological trait linked to social cognitive functions

Human eye morphology is considered unique among the primates in that humans possess larger width/height ratios (WHR), expose a greater amount of visible sclera (SSI; width of exposed eyeball/width of visible iris), and critically, have a white sclera due to a lack of pigmentation. White sclera in humans amplifies gaze direction, whereas the all-dark eyes of apes are hypothesized to conceal gaze from others. This study examines WHR and SSI in humans (N = 13) and gorillas (N = 85) engaged in direct and averted gazes and introduces a qualitative assessment of sclera color to evaluate variations in sclera pigmentation. The results confirm previous findings that humans possess a larger WHR than gorillas but indicate that humans and gorillas display similar amounts of visible sclera. Additionally, 72% (N = 124) of gorilla eyes in this sample deviated from the assumed all-dark eye condition. This questions whether gaze camouflage is the primary function of darkened sclera in non-human primates or whether other functional roles can be ascribed to the sclera, light or dark. We argue that white sclera evolved to amplify direct gazes in humans, which would have played a significant role in the development of ostensive communication, which is communication that both shows something and shows the intention to show something. We conclude that the horizontal elongation of the human eye, rather than sclera color, more reliably distinguishes human from great ape eyes, represented here by gorillas.

Visual kin recognition in nonhuman primates: (Pan troglodytes and Macaca mulatta): inbreeding avoidance or male distinctiveness?

Faces provide important information about identity, age, and even kinship. A previous study in chimpanzees reported greater similarity between the faces of mothers and sons compared with mothers and daughters, or unrelated individuals. This was interpreted as an inbreeding avoidance mechanism where females, the dispersing gender, should avoid mating with any male that resembles their mother. Alternatively, male faces may be more distinctive than female faces, biasing attention toward males. To test these hypotheses, chimpanzees and rhesus monkeys matched conspecifics' faces of unfamiliar mothers and fathers with their sons and daughters. Results showed no evidence of male distinctiveness, rather a cross-gender effect was found: chimpanzees were better matching moms with sons and fathers with daughters. Rhesus monkeys, however, showed an overwhelming bias toward male-distinctiveness. They were faster to learn male faces, performed better on father-offspring and parent-son trials, and were best matching fathers with sons. This suggests that for the rhesus monkey, inbreeding avoidance involves something other than facial phenotypic matching but that among chimpanzees, the visual recognition of facial similarities may play an important role.

Oval in males and triangular in females? A quantitative evaluation of sexual dimorphism in the human obturator foramen

Among the numerous pelvic traits presenting sex differences, the obturator foramen is classically described as being oval in males and triangular in females. However, no demonstrations or detailed studies seem available in the literature. The purpose of this work was to study quantitatively this trait using Fourier analysis, because this methodological approach is particularly well adapted for discrimination between different simple shapes. Using this approach, an outline can be characterized by a series of harmonics (1 to n), each defined by two Fourier descriptors: amplitude (C(n)), describing the relative importance of the harmonic contribution to the original shape, and phase (Phi(n)), representing the orientation of the harmonic contribution. The material consisted of 104 three-dimensional CT reconstructions of adult pelves (52 males and 52 females). After size normalization, the outlines of the 104 left obturator foramens were studied. Significant differences were demonstrated with, in total, 84.6% of individuals presenting a correct inferred sex. The most discriminating descriptors were the phase of the second harmonic (C(2),) related to the oval (or elliptic) aspect and thus the elongation of the shape, and the amplitude of the third harmonic (Phi(3)), describing the triangularity of the shape. Because the trend for an outline to be more or less oval or triangular is difficult to visually assess and because there is an infinite number of transitional shapes, only a precise quantitative approach such as Fourier analysis allows for unambiguous characterization and statistical analysis.

Variations of the mandibular shape in extant hominoids: Generic, specific, and subspecific quantification using elliptical fourier analysis in lateral view

While a number of studies have documented the mandibular variations in hominoids, few focused on evaluating the variation of the whole outline of this structure. Using an efficient morphometrical approach, i.e. elliptical Fourier analysis, mandibular outlines in lateral view from 578 adult hominoids representing the genera Hylobates, Pongo, Gorilla, Pan, and Homo were quantified and compared. This study confirms that elliptical Fourier analysis provides an accurate characterization of the shape of the mandibular profile. Differences in mandibular shape between hominoid genera, species, subspecies, and to a lesser extent between sexes were demonstrated. Mandibles in great apes and hylobatids subspecies were generally less distinct from each other than were species. However, the magnitudes of differences among subspecies of Gorilla and Pongo approached or exceeded those between Pan troglodytes and P. paniscus. The powerful discrimination between taxa from the genus down to subspecific level associated to the relatively low level of intrageneric mandibular polymorphism in great apes provides strong evidences in support of the taxonomic utility of the shape of the mandibular profile in hominoids. In addition, morphological affinities between Pongo and Pan and the clear distinction between Homo and Pan suggest that the mandibular outline is a poor estimate of phylogenetic relationships in great apes and humans. The sexual dimorphism in mandibular shape exhibits two patterns of expression: a high degree of dimorphism in Gorilla, Pongo, and H. s. syndactylus and a relatively low one in modern humans and Pan. Besides, degree of mandibular shape dimorphism can vary considerably among closely related subspecies as observed in gorillas, arguing against the use of mandibular shape dimorphism patterns as characters in phylogenetic analyses. However, the quantification of the mandibular shape and of the variations among hominoids provides an interesting comparative framework that is likely to supply further arguments for a better understanding of the patterns of differentiation between living hominoids.

Sex differences in face gender recognition in humans

Human faces are ecologically-salient stimuli. Face sex is particularly relevant for human interactions and face gender recognition is an extremely efficient cognitive process that is acquired early during childhood. To measure the minimum information required for correct gender classification, we have used a pixelation filter and reduced frontal pictures (28,672 pixels) of male and female faces to 7168, 1792, 448 and 112 pixels. We then addressed the following questions: Is gender recognition of male and female faces equally efficient? Are male and female subjects equally efficient at recognising face gender? We found a striking difference in categorisation of male and female faces. Categorisation of female faces reduced to 1792 pixels is at chance level whereas categorisation of male faces is above chance even for 112 pixel images. In addition, the same difference in the efficiency of categorisation of male and female faces was detected using a Gaussian noise filter. A clear sex difference in the efficiency of face gender categorisation was detected as well. Female subject were more efficient in recognising female faces. These results indicate that recognition of male and female faces are different cognitive processes and that in general females are more efficient in this cognitive task.

The human mandible in lateral view: elliptical fourier descriptors of the outline and their morphological analysis

An original procedure based on new developments in elliptical Fourier methods associated with image analysis technics was applied to 117 human mandibles in lateral view (69 males, 48 females) in order to analyze interindividual variability and sexual dimorphism. Original parameters, called elliptical Fourier descriptors, allowed for the quantification of the shape of an outline irrespective of its morphological complexity, and provided a precise individual characterization. The use of step by step reconstructions with an increasing number of harmonics allowed for demonstration of the morphological contributions of the elliptical Fourier descriptors, and relationships to precise anatomical features were established. The first harmonics (1st to 8th) described the general mandibular shape, and the later harmonics (8th to 14th) characterized accessory and finer morphological features. The proportion of individuals presenting significant sexual dimorphism was 97.1% in males and 91.7% in females, and after size normalization 84.1% in males and 81.2% in females. Even after removal of the size component, these results in regard to mandibular dimorphism remained better than those found by classical approaches; this could be due to the use of elliptical Fourier analysis which takes into account all the information of the mandibular outline. Compared to the size component, the shape component seemed to be more important in the sexual dimorphism of the human mandible than previously recognized. The present approach opens interesting anthropological and clinical perspectives for the characterization of mandibular morphology.

Curve analysis of the aging orbital aperture

It was hypothesized that skeletal aging results in curve distortion of the orbital aperture. Data were compiled from a cross-sectional study of the Robert J. Terry human skull collection at the Smithsonian Institution, Washington, D.C. Statistical analyses suggest that postadult differential growth results in progressive distortion of the orbital aperture. These changes may have both cosmetic and functional consequences.