![]() More recent experimentation suggests that experimentally-enculturated chimpanzees are capable of discriminating computerized images of averted chimpanzee eyes from images of chimpanzee eyes directed forward, albeit with less success than when making the same discrimination of human eye images or chimpanzee eye images with reversed polarity 13. However, it is unclear in both of these cases whether chimpanzees use conspecific eyes specifically, rather than the head or body, to guide gaze following. Chimpanzees have relatively dark sclera and follow the gaze of conspecifics in their social group 11 as well as the gaze of a computerized conspecific on a computer monitor 12. The extent to which dark sclera in nonhuman primates are cryptic is less clear. Human adults detect gaze information more easily when sclera are naturally coloured, rather than darker or the same colour as the iris, across multiple light environments 8, 9, 10. By 18-months-of-age, humans infants use gaze cues functionally to find hidden items 7. Newborn infants attend preferentially to individuals who gaze directly at them 5, and exhibit rudimentary gaze following 6. In support of this hypothesis, natural human sclera colours are indeed highly perceptible, communicative cues. The hypothesis suggests that these divergent phenotypes are the product of evolutionary pressures to cooperate and communicate prosocially (with highly discriminable gaze) or to avoid doing so (with eye colouration that conceals intraspecific cues), and that eye colouration thus provides a window into the evolutionary history of primate social ecology. According to this hypothesis, lighter primate sclera, like those of humans, are discriminable, conspicuous, and useful for gaze perception whereas darker primate sclera, like those of chimpanzees, are dark, obscured, and camouflaged in ways that impede gaze perception. The cooperative eye hypothesis suggests that primate eyes are adapted to reveal or conceal gaze information 4. The sizes, shapes, and colours of primate eyes are highly variable across species in ways that potentially affect their perceptibility and utility for communication 3. Eye colours appear exemplary of this relationship between morphology and social ecology. For example, more varied, complex, and colourful facial patterning is most common in Old World primates that live in larger social groups 2. The colours and patterns of primate faces are remarkably diverse, and are related to primate social ecology in ways that suggest adaptive, communicative functions 1. Chimpanzee sclera colour does not effectively conceal gaze, and we discuss this result with regard to the cooperative eye hypothesis, the evolution of primate eye colouration, and methodological best practices for future primate visual ecology research. We also found that chimpanzee gaze is discriminable to the visual system of primates that chimpanzees prey upon, Colobus monkeys. ![]() Our visual modeling of chimpanzee eye coloration suggests that chimpanzee gaze is visible to conspecifics at a range of distances (within approximately 10 m) appropriate for many species-typical behaviours. ![]() We considered biases of cameras, lighting conditions, and commercial photography software along with primate visual acuity, colour sensitivity, and discrimination ability. We photographed the faces of captive chimpanzees and quantified the discriminability of their pupil, iris, sclera, and surrounding skin. Using a visual modelling approach that properly accounts for specific-specific vision, we reexamined this topic to estimate the extent to which chimpanzee eye coloration is discriminable. Recent debate surrounding this topic has produced mixed results, with some evidence suggesting that (1) primate gaze is indeed concealed from their conspecifics, and (2) gaze colouration is among the suite of traits that distinguish uniquely social and cooperative humans from other primates (the cooperative eye hypothesis). 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.
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