Scleral pigmentation leads to conspicuous, not cryptic, eye morphology in chimpanzees

Abstract

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.

Sclera and Iris Color Interact to Influence Gaze Perception

“…Because depigmented sclera likely evolved at a time when ancestral humans exhibited dark irises (Eiberg et al, 2008 ), the depigmented sclera would probably have been particularly important for efficient and accurate gaze perception in these ancestral humans. Some non-human primates exhibit relatively dark irises with minimally exposed sclera (Kobayashi and Kohshima, 2001 ; but see Mayhew and Gómez, 2015 ; Perea-García et al, 2019 ), suggesting that gaze perception may be difficult for these species. Other non-human primates, however, exhibit light irises with minimally exposed sclera (Meyer et al, 2013 ) and may therefore be able to easily discriminate gaze.…”

“…The faces with pigmented sclera represent a phenotype that is similar to many primate species in which the sclera has a similar color as the iris color or the sclera is minimally exposed (Kobayashi and Kohshima, 2001 ). However, there are some primate species with more conspicuous sclera (Mayhew and Gómez, 2015 ; Perea-García et al, 2019 ). Our previous results found that sclera color influences gaze perception (Yorzinski and Miller, 2020 ) but the current study expands upon those results by examining the impact of both sclera color and iris color on gaze perception.…”

Ocular pigmentation in humans, great apes, and gibbons is not suggestive of communicative functions - having an eye on the ‘cooperative eye hypothesis’

“…It has become increasingly apparent that ocular pigmentation in most great ape species is far more variable than assumed by Kobayashi and Kohshima (2001), who predominately studied few individuals per species in their sample (e. g. n = 2 for bonobos, n = 5 for orangutans). In Western gorillas (Gorilla gorilla) and bonobos (Pan paniscus), scleral pigmentation appears to be particularly plastic, ranging from plain black to fully white (Mayhew & Gómez, 2015;Perea García et al, 2019). A small-scale study also found predominantly light sclerae in Sumatran orangutans (Pongo abelii) (Perea García, 2016).…”

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

“…An influential study by Kobayashi and Kohshima (1997; see also Kobayashi and Kohshima 2001) compared the ocular morphology of humans and 87 other primate species and concluded that human eyes were unique in having a larger width-to-height ratio (WHR), larger surface area of the visible sclera (SSI), and an extremely depigmented sclera. The supposed morphological uniqueness and resulting unique conspicuousness of the human eye (though see Perea-García et al 2019) resulted in several proposals to explain the adaptive origins of the peculiar morphology of the human eye.…”