Gaze following has been argued to be uniquely human, facilitated by our depigmented, white sclera [M. Tomasello, B. Hare, H. Lehmann, J. Call, J. Hum. Evol. 52, 314–320 (2007)]—the pale area around the colored iris—and to underpin human-specific behaviors such as language. Today, we know that great apes show diverse patterns of scleral coloration [J. A. Mayhew, J. C. Gómez, Am. J. Primatol. 77, 869–877 (2015); J. O. Perea García, T. Grenzner, G. Hešková, P. Mitkidis, Commun. Integr. Biol. 10, e1264545 (2016)]. We compare scleral coloration and its relative contrast with the iris in bonobos, chimpanzees, and humans. Like humans, bonobos’ sclerae are lighter relative to the color of their irises; chimpanzee sclerae are darker than their irises. The relative contrast between the sclera and iris in all 3 species is comparable, suggesting a perceptual mechanism to explain recent evidence that nonhuman great apes also rely on gaze as a social cue.

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Comparative examinations of external eye morphology in primates initially focused on communicative functions of the eye. Subsequent work has failed to find consistent associations between specific eye morphologies and communicative functions. In this article, we review the field of primate external eye morphology and inspect publicly available and unpublished photographs. We identify and describe five commonly occurring traits that have not received attention so far. We cross-examined the clinical and psychological literature to propose potential adaptive functions. These potential adaptive functions include communicative functions, but also photoregulatory functions and photoprotective functions.

External eye appearance across primate species is diverse in shape and colouration, yet we still lack an explanation for the drivers of such diversity. Here we quantify substantial interspecific variation in eye shape and colouration across 77 primate species representing all extant genera of anthropoid primates. We reassess a series of hypotheses aiming to explain ocular variation in horizontal elongation and in colouration across species. Heavier body weight and terrestrial locomotion are associated with elongated eye outlines. Species living closer to the equator present more pigmented conjunctivae, suggesting photoprotective functions. Irises become bluer in species living further away from the equator, adding to existing literature supporting a circadian clock function for bluer irises. These results shift the current focus from communicative, to ecological factors in driving variation in external eye appearance in anthropoid primates. They also highlight the possibility that similar ecological factors contributed to selection for blue eyes in ancestral human populations living in northern latitudes.

Self-reports and questionnaires have been the preferred research methods in the criminological field of Fear of Crime (FOC) since its rise in the 1960s. Our study had two main goals: 1) to measure the physiological indicators of fear in real time, and 2) to compare these data with those obtained through self-reports, designed also to measure the emotion of fear. Methods An experimental study conducted in the course of a week during late February 2016 in Aarhus (Denmark), in which the focus was on traditional environmental variables in the field of FOC (i.e., poor lighting conditions). Results Our results support the ideas that 1) the absence of good luminosity in an open public space in an urban setting elicits physiological reactions of arousal that can be taken as indicators of experiences of fear, and 2) heart rate appears to capture aspects of the emotion of fear that are not reflected in data obtained through self-report questionnaires. Conclusions This study, introducing a pioneering approach to the study of FOC, presents great potentials in complementing traditional methods in the crime sciences. The many challenges we faced are significant and reported with the hope the subsequent literature to build on. We propose that traditional methods and new measurements could be combined to advance the research in the field by allowing researchers to more unambiguously constrain the interpretation from their data. This becomes particularly relevant in a field like FOC, that has long suffered from irreconcilable results stemming from different investigations.

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.

Understanding the adaptive function of the unique morphology of the human eye, in particular its overexposed white sclera, may have profound implications for the fields of evolutionary behavioural science, and specifically the areas of human interaction and social cognition. Existing hypotheses, such as the cooperative eye hypothesis, have attracted a lot of attention but remain untested. Here, we: (i) analysed variation in the visible sclera size in humans from different ethnic backgrounds and (ii) examined whether intraspecific variation of exposed sclera size is related to trust. We used 596 facial photographs of men and women, assessed for perceived trustworthiness, from four different self-declared racial backgrounds. The size of the exposed sclera was measured as the ratio between the width of the exposed eyeball and the diameter of the iris (sclera size index, SSI). The SSI did not differ in the four examined races and was sexually monomorphic except for Whites, where males had a larger SSI than females. In general, the association between the SSI and trustworthiness was statistically insignificant. An inverted U-shaped link was found only in White women, yet the strength of the effect of interaction between sex and race was very small. Our results did not provide evidence for the link between exposed sclera size and trustworthiness. We conclude that further investigation is necessary in order to properly assess the hypotheses relating to the socially relevant functions of overexposed sclera.

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