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Chromatic and achromatic ∆S values for contrasts among the eye regions shaded in the top-right inset. A ∆S value greater than 3 at some distance suggest that the two ROI can be discriminated from each other. Shaded regions around lines are Bayesian 94% Highest Posterior Density intervals around ∆S estimates. Tick intervals on the x-axis are log-scaled.

Chromatic and achromatic ∆S values for contrasts among the eye regions shaded in the top-right inset. A ∆S value greater than 3 at some distance suggest that the two ROI can be discriminated from each other. Shaded regions around lines are Bayesian 94% Highest Posterior Density intervals around ∆S estimates. Tick intervals on the x-axis are log-scaled.

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Chimpanzee (Pan troglodytes) sclera appear much darker than the white sclera of human eyes, to such a degree that the direction of chimpanzee gaze may be concealed from conspecifics. Recent debate surrounding this topic has produced mixed results, with some evidence suggesting that (1) primate gaze is indeed concealed from their conspecifics, and (...

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... Importantly, chimpanzees are able to move their eyes independently of their head similarly to humans 26 and have been shown to follow others' iridal direction at ecologically relevant distances 26,27 , though typically do not utilise glance cues to ascertain referential information in object-choice tasks 16,[28][29][30][31][32] . Similarly, rhesus macaques (Macaca mulatta) have been shown to be reflexively sensitive to glance stimuli 33 , though it is not known whether they can derive referential information from this, and capuchins (Sapajus apella) 34 also do not appear able to discern referential cues from glance stimuli. ...
Article
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The drivers of divergent scleral morphologies in primates are currently unclear, though white sclerae are often assumed to underlie human hyper-cooperative behaviours. Humans are unusual in possessing depigmented sclerae whereas many other extant primates, including the closely-related chimpanzee, possess dark scleral pigment. Here, we use phylogenetic generalized least squares (PGLS) analyses with previously generated species-level scores of proactive prosociality, social tolerance (both n = 15 primate species), and conspecific lethal aggression (n = 108 primate species) to provide the first quantitative, comparative test of three existing hypotheses. The ‘self-domestication’ and ‘cooperative eye’ explanations predict white sclerae to be associated with cooperative, rather than competitive, environments. The ‘gaze camouflage’ hypothesis predicts that dark scleral pigment functions as gaze direction camouflage in competitive social environments. Notably, the experimental evidence that non-human primates draw social information from conspecific eye movements is unclear, with the latter two hypotheses having recently been challenged. Here, we show that white sclerae in primates are associated with increased cooperative behaviours whereas dark sclerae are associated with reduced cooperative behaviours and increased conspecific lethal violence. These results are consistent with all three hypotheses of scleral evolution, suggesting that primate scleral morphologies evolve in relation to variation in social environment.
... We adopted a visual modelling approach that allowed us to estimate the maximum distances at which grackles could discriminate conspecific eyes under daylight and bright ALAN. This visual modelling approach has previously been used to examine gaze discriminability in nonhuman primates (Whitham et al., 2022a;b) as well as discriminability in a wide range of other contexts and species (e. g., Nokelainen et al., 2021;Rodríguez-Morales et al., 2021;Feldmann et al., 2021). We then used low-light visual modelling to estimate the minimum artificial light level at which grackles could discriminate conspecific eyes, using techniques inspired by recent work modelling color discrimination by nocturnal hawkmoths under ALAN (Briolat et al., 2021). ...
Article
Eyes convey important information about the external and internal worlds of animals. Individuals can follow the gaze of others to learn about the location of salient objects as well as assess eye qualities to evaluate the health, age or other internal states of conspecifics. Because of the increasing prevalence of artificial lighting at night (ALAN), urbanized individuals can potentially garner information from conspecific eyes under both daylight and ALAN. We tested this possibility using a visual modeling approach in which we estimated the maximum distance at which individuals could detect conspecific eyes under daylight and high levels of ALAN. We also estimated the minimum light level at which individuals could detect conspecific eyes. Great-tailed grackles (Quiscalus mexicanus) were used as our study species because they are highly social and are unusual among birds in that they regularly gather at nocturnal roosts in areas with high levels of ALAN. This visual modelling approach revealed that grackles can detect conspecific eyes under both daylight and ALAN, regardless of iris coloration. The grackles could detect conspecific eyes at farther distances in daylight compared to ALAN. Our results highlight the potential importance of lighting conditions in shaping social interactions.
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Homogeneously depigmented sclerae have long been proposed to be uniquely human - an adaptation to enable cooperative behaviour by facilitating interpersonal coordination through gaze following. However, recent evidence has shown that deeply pigmented sclerae also afford gaze following if surrounding a bright iris. Furthermore, while current scleral depigmentation is clearly adaptive in modern humans, it is less clear how the evolutionarily intermediate stages of scleral pigmentation may have been adaptive. In sum, it is unclear why scleral depigmentation became the norm in humans, while not so in sister species like chimpanzees, or why some extant species (presumably as our ancestors did at some point) display intermediate degrees of pigmentation. We created realistic facial images of 20 individually distinct hominins with diverse facial morphologies, each face in the (i) humanlike bright sclera and (ii) generalised apelike dark sclera version. Participants in two online studies rated the bright-sclera hominins as younger, healthier, more attractive and trustworthy, but less aggressive than the dark-sclera hominins. Our results support the idea that the perceptual affordances of more depigmented sclerae increased perceived traits that fostered trust, increasing fitness for those individuals and resulting in depigmentation as a fixed trait in extant humans.
Article
The cooperative eye hypothesis posits that human eye morphology evolved to facilitate cooperation. Although it is known that young children prefer stimuli with eyes that contain white sclera, it is unknown whether white sclera influences children’s perception of a partner’s cooperativeness specifically. In the current studies, we used an online methodology to present 5-year-old children with moving three-dimensional face models in which facial morphology was manipulated. Children found “alien” faces with human eyes more cooperative than faces with dark sclera (Study 2) but not faces with enlarged irises (Study 1). For more human-like faces (Study 3), children found human eyes more cooperative than either enlarged irises or dark sclera and found faces with enlarged irises cuter (but not more cooperative) than eyes with dark sclera. Together, these results provide strong support for the cooperative eye hypothesis.