Ocular pigmentation in humans, great apes, and gibbons is not suggestive of communicative functions

GND
1233063405
ORCID
0000-0002-2112-1050
LSF
60490
Affiliation
Department of General Zoology, University of Duisburg-Essen, Essen, Germany
Caspar, Kai R.;
Affiliation
Department of General Zoology, University of Duisburg-Essen, Essen, Germany
Biggemann, Marco;
GND
123947472
Affiliation
Anthropological Institute, University Zurich-Irchel, Zurich, Switzerland
Geissmann, Thomas;
GND
22312724
ORCID
0000-0001-9907-6387
LSF
5703
Affiliation
Department of General Zoology, University of Duisburg-Essen, Essen, Germany
Begall, Sabine
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. The cooperative eye hypothesis suggests the white sclera of humans to be a derived adaptive trait that enhances eye-mediated communication. 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. 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. Gibbons display darker eyes than great apes and expose less sclera. Iridoscleral contrasts in orangutans and gorillas approach the human condition but differ between congeneric species. Contrary to recent discussions, we found chimpanzee eyes to exhibit a cryptic coloration scheme that resembles gibbons more than other apes. 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. Differences in scleral pigmentation between great apes and humans are gradual and might have arisen via genetic drift and sexual selection.

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