Sex differences in ocular morphology in Caucasian people: a dubious role of sexual selection in the evolution of sexual dimorphism of the human eye

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 statement It 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.

The adaptive significance of human scleral brightness: an experimental study

“…In the context of sexual preferences, it is also worth noting that the attractiveness rating may be further modified by the sexually dimorphic nature of the ocular morphology. As recent findings show, both sclera shape 12,41 and colour 36 are sexually dimorphic at least in Caucasians. However, Aggressiveness.…”

“…In conclusion, we see our study as a methodological contribution to this fast-developing field. Interest in the adaptive significance of human ocular morphology was sparked a quarter-century ago with the influential study by Kobayashi and Kohshima 1 , and has now been revived with several very recent publications 12 . This particular wealth of studies within the last few years shows not only that this is a very timely topic, but most importantly testifies to the urgency of discussion on developing methodological standards.…”

The Adaptive Significance of Human Scleral Brightness. An Experimental Study

“…Finally, we see our study as a methodological contribution to this fast-developing eld. Interest in the adaptive signi cance of human ocular morphology was sparked a quarter-century ago with the in uential study by Kobayashi and Kohshima (1997) 1 , and has now been revived with several very recent publications (Danel et al 2020) 12 . This particular wealth of studies within the last few years shows not only that this is a very timely topic, but most importantly testi es to the urgency of discussion on developing methodological standards.…”

“…In the context of sexual preferences, it is also worth noting that the attractiveness rating may be further modi ed by the sexually dimorphic nature of the ocular morphology. As recent ndings show, both sclera shape (Danel et al 2018, 2020) 12,41 and colour (Krammer & Russel, 2022) 36 are sexually dimorphic at least in Caucasians. However, the link between attractiveness and differences in ocular morphology in men and women should be inspected in more detail, as it may be mediated by age (brighter sclerae are associated with youth, and younger people are perceived as more attractive).…”

“…The transition from pigmented to depigmented sclerae in humans may have been mediated by several mutually nonexclusive proximate functions (see esp. Danel et al 2020) 12 , among which the literature suggests the signaling of attractiveness and health (e.g. through symmetrical depigmentation that affords the perception of changes in vasculature or pigmentation), signalling of reliability and trustworthiness (e.g.…”