In orangutans, scleral exposure during averted glancing is markedly lower, approximating that of humans during forwardly directed glancing 11. Additionally, it has been shown that the amount of visible sclera does not differ significantly between gorillas and humans in averted gaze situations, which are of particular communicative value, demonstrating a greater than previously assumed continuity in this ocular trait as well 7. Bornean orangutans ( Pongo pygmaeus), chimpanzees ( Pan troglodytes) and Eastern gorillas ( Gorilla beringei) were instead reported to almost consistently display dark sclerae 7– 9, but see e.g., 10 for exceptions), so that great ape scleral coloration does not appear to follow a clear phylogenetic pattern. A small-scale study also found predominantly light sclerae in Sumatran orangutans ( Pongo abelii) 9. In Western gorillas ( Gorilla gorilla) and bonobos ( Pan paniscus), scleral pigmentation appears to be particularly plastic, ranging from plain black to fully white 7, 8. It has become increasingly apparent that ocular pigmentation in most great ape species is far more variable than assumed by Kobayashi and Kohshima 4, who predominately studied few individuals per species in their sample (e.g., n = 2 for bonobos, n = 5 for orangutans).
However, the assumption of such a clear dichotomy between ape and human eyes has been contested. Given that, human and non-human primate pigmentation patterns would serve contrary adaptive purposes. Darker, less conspicuous eyes on the other hand would conceal glance direction in order to mask intentions, which was hypothesized to be advantageous in the more competitive social environments assumed for apes 4. 3 who characterized the human eye as a social tool to convey intentions, guide actions and mediate joint attention (see also 6). This idea was further developed and termed the cooperative eye hypothesis by Tomasello et al. Kobayashi and Kohshima 4 emphasized how ocular morphology differs between humans and non-human apes (from here on “apes”, if not specified otherwise) and argued that the traits of the human eye would be uniquely suited to enable effective glance-based communication. The latter characteristic might have originally evolved to facilitate wide-angle glancing and thereby to extend humans’ visual field in terrestrial habitats 4. Beside its conspicuous coloration, large portions of scleral surface are exposed in the human eye due to its marked horizontally extended outline. Humans exhibit an almost complete depigmentation of the sclera and overlying conjunctiva, creating the white of the eye, which contrasts with the darker iris. In an influential paper, Kobayashi and Kohshima 4 argued that this difference in communicative behavior is mirrored by the morphology of the human eye, a hypothesis popularized beforehand by Morris 5. It is commonly assumed that glance-mediated communication is far more sophisticated in humans than it is in other primates, if present there at all. Glancing (i.e., eye orientation/eye gaze opposed to gazing, i.e., head orientation) in particular can facilitate social communication, for instance as a referential cue to inform observers about one’s attentional focus 2, 3. Differences in scleral pigmentation between great apes and humans are gradual and might have arisen via genetic drift and sexual selection.Įyes are importantly involved in human non-verbal communication 1. We reevaluate the evidence for links between social cognition and eye pigmentation in primates, concluding that the cooperative eye hypothesis cannot explain the patterns observed.
Contrary to recent discussions, we found chimpanzee eyes to exhibit a cryptic coloration scheme that resembles gibbons more than other apes. Iridoscleral contrasts in orangutans and gorillas approach the human condition but differ between congeneric species. Gibbons display darker eyes than great apes and expose less sclera. Our data reveals a continuum of eye pigmentation traits in hominoids which does not align with the complexity of gaze-mediated communication in the studied taxa. Here, we provide a comparative analysis of ocular pigmentation patterns in 15 species of hominoids (humans, great apes & gibbons) that show marked differences in social cognition and quantify scleral exposure at the genus level. The cooperative eye hypothesis suggests the white sclera of humans to be a derived adaptive trait that enhances eye-mediated communication. Pigmentation patterns of the visible part of the eyeball, encompassing the iris and portions of the sclera, have been discussed to be linked to social cognition in primates.
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