Abstract
The horizontal size of the exposed depigmented sclera in Caucasians has been previously suggested to be sexually dimorphic, and the significance of this phenomenon remains unclear. Here we build on a previous study and extend it by (i) examining sex differences in other measures of ocular morphology and (ii) exploring the link between eye morphology and biometric markers of facial attractiveness. We used facial photographs of 100 Caucasians (50 men) from Eastern-Central Europe and digitally measured four ocular features. Eye measurements were tested for sex differences and associations with morphometric data on facial averageness and sexual shape dimorphism. We found that sclera surface is more horizontally exposed in men, even though the total surface area is similar in both sexes. We also found that eye fissures are rounder (less rectangular) in women than in men and that irises are brighter in women. We did not find any relationship between the examined eye features and two aspects of facial attractiveness: facial averageness and sexual dimorphism in facial shape. Despite being sexually dimorphic, eye features may be loosely linked with the development of facial sexual ornamentation. The role of sexual selection in the evolution of the observed phenomena is disputable.Significance statementIt is often argued that because of their physical appearance, human eyes are crucial to interpersonal and social interactions. In many aspects, however, the significance of the human eye architecture is unclear. In this study, we examine sex differences in eye morphology and explore the link between ocular features and biometric measures of facial attractiveness in Caucasian men and women. We found that despite being sexually dimorphic, eye features may be loosely linked with biometric markers of facial attractiveness. We argue that the role of sexual selection in the evolution of the observed sex differences is disputable.
Highlights
All great apes differ markedly from other mammals, including other primates, in presenting a range of functional and morphological adaptations for increased reliance on vision, such as a less protruding snout and more exposed eyeballs (Emery 2000)
Our study showed that neither averageness nor facial sexual dimorphism was correlated with the examined characteristics of eye morphology
We examined sex differences in ocular morphology in Caucasians
Summary
All great apes differ markedly from other mammals, including other primates, in presenting a range of functional and morphological adaptations for increased reliance on vision, such as a less protruding snout and more exposed eyeballs (Emery 2000). The greater importance of vision is shown by the more varied ways in which great apes actively use the gaze of others in social contexts; for example, eye contact and gaze alternation in great apes have been observed to play a role in bonding (de Waal and van Roosmalen 1979; Silk 2002), communication (Gómez 1996) and competitive foraging (Bräuer et al 2005, Kano and Call 2014; Hall et al 2017) This great ape trend is further accentuated in humans (Homo sapiens), who rely on visual inputs to an even greater extent and make use of eyes as cues in a wider array of socially relevant functions (Kleinke 1986). The supposed morphological uniqueness and resulting unique conspicuousness of the human eye (though see Perea-García et al 2019) resulted in several proposals to explain the adaptive origins of the peculiar morphology of the human eye
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