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

2019 ◽  
Vol 116 (39) ◽  
pp. 19248-19250 ◽  
Author(s):  
Juan Olvido Perea-García ◽  
Mariska E. Kret ◽  
Antónia Monteiro ◽  
Catherine Hobaiter
Keyword(s):  
Great Apes ◽  
Gaze Following ◽  
Eye Morphology ◽  
Social Cue ◽  
Relative Contrast ◽  
Perceptual Mechanism ◽  
Scleral Pigmentation ◽  
Human Specific

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.

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

2021 ◽  
Author(s):  
Kai R. Caspar ◽  
Marco Biggemann ◽  
Thomas Geissmann ◽  
Sabine Begall
Keyword(s):  
Social Cognition ◽  
Social Communication ◽  
Great Apes ◽  
Human Condition ◽  
Gaze Following ◽  
Communicative Functions ◽  
Cryptic Coloration ◽  
Visible Part ◽  
Eye Pigmentation ◽  
Scleral Pigmentation

AbstractPigmentation 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. In the context of the cooperative eye hypothesis, the white sclera of humans has been viewed as a derived adaptive trait, enhancing communication via glance cueing. Here, we provide a comparative analysis of ocular pigmentation patterns in 15 species of hominoids (humans, great apes & gibbons) representing all extant ape genera, based on photographs and literature data. Additionally, we quantify hominoid scleral exposure on the genus level during different glancing situations. Our data reveals a continuum of eye pigmentation traits among the studied taxa. Gibbons display darker, more uniformly colored eyes than great apes and expose less sclera, particularly during averted glancing. 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 great apes and that does not enhance glance cueing or gaze following. We critically evaluate the evidence for links between social cognition and eye pigmentation in primates, concluding that the cooperative eye hypothesis cannot convincingly explain the patterns observed. Although the human eye exhibits unique traits that are likely linked to social communication, high iridoscleral contrast is not one of them. Differences in scleral pigmentation between great apes and humans are gradual and might have arisen via genetic drift and sexual selection.

scholarly journals What is unique about the human eye? Comparative image analysis on the external eye morphology of human and nonhuman great apes

2021 ◽  
Author(s):  
Fumihiro Kano ◽  
Takeshi Furuichi ◽  
Chie Hashimoto ◽  
Christopher Krupenye ◽  
Jesse G Leinwand ◽  
...  
Keyword(s):  
Image Analysis ◽  
Eye Gaze ◽  
Great Apes ◽  
Gaze Direction ◽  
Human Eye ◽  
The Gaze ◽  
Eye Morphology ◽  
New Findings ◽  
Heated Debate ◽  
Signaling Hypothesis

The gaze-signaling hypothesis and the related cooperative-eye hypothesis posit that humans have evolved special external eye morphology, including exposed white sclera (the white of the eye), to enhance the visibility of eye-gaze direction and thereby facilitate conspecific communication through joint-attentional interaction and ostensive communication. However, recent quantitative studies questioned these hypotheses based on new findings that humans are not necessarily unique in certain eye features compared to other great ape species. Therefore, there is currently a heated debate on whether external eye features of humans are distinguished from those of other apes and how such distinguished features contribute to the visibility of eye-gaze direction. This study leveraged updated image analysis techniques to test the uniqueness of human eye features in facial images of great apes. Although many eye features were similar between humans and other species, a key difference was that humans have uniformly white sclera which creates clear visibility of both eye outline and iris; the two essential features contributing to the visibility of eye-gaze direction. We then tested the robustness of the visibility of these features against visual noises such as darkening and distancing and found that both eye features remain detectable in the human eye, while eye outline becomes barely detectable in other species under these visually challenging conditions. Overall, we identified that humans have distinguished external eye morphology among other great apes, which ensures robustness of eye-gaze signal against various visual conditions. Our results support and also critically update the central premises of the gaze-signaling hypothesis.

scholarly journals Northern bald ibises follow others’ gaze into distant space but not behind barriers

2009 ◽  
Vol 6 (1) ◽  
pp. 14-17 ◽  
Author(s):  
Matthias-Claudio Loretto ◽  
Christian Schloegl ◽  
Thomas Bugnyar
Keyword(s):  
Experimental Evidence ◽  
Great Apes ◽  
Gaze Following ◽  
Visual Orientation ◽  
Cognitive Differences ◽  
Geronticus Eremita ◽  
Stark Contrast ◽  
First Time

Gaze following is the ability to use the visual orientation of others as a trigger to look in the same direction. Thereby, animals may either align their head and eye orientation with others (gaze following into distant space) or may even reposition themselves to look behind barriers impairing their perception (geometrical gaze following). It has been proposed that these two different modes are functionally and cognitively distinct, but experimental evidence for this claim is lacking. We here, to our knowledge, demonstrate for the first time, that adult animals may be capable of following gaze into distant space, but not geometrically around barriers. We tested Northern bald ibises ( Geronticus eremita ) for their ability to follow a conspecific's gaze in two standard tasks. The birds readily looked up after seeing a model bird looking up; however, when seeing a model looking behind a barrier, they responded by looking at the barrier instead of walking around. These findings are in stark contrast to results obtained with great apes and corvids and provide the first experimental evidence, to our knowledge, for cognitive differences in gaze following tasks.

scholarly journals Great apes are sensitive to prior reliability of an informant in a gaze following task

PLoS ONE ◽  
2017 ◽  
Vol 12 (11) ◽  
pp. e0187451 ◽  
Author(s):  
Benjamin Schmid ◽  
Katja Karg ◽  
Josef Perner ◽  
Michael Tomasello
Keyword(s):  
Great Apes ◽  
Gaze Following

scholarly journals Head turning is an effective cue for gaze following

2021 ◽  
Author(s):  
Paula Rubio-Fernandez ◽  
Vishakha Shukla ◽  
Vrinda Bhatia ◽  
Shlomit Ben-Ami ◽  
Pawan Sinha
Keyword(s):  
Target Object ◽  
Referential Communication ◽  
Gaze Following ◽  
Head Turning ◽  
Motion Cues ◽  
New Words ◽  
Social Cue ◽  
Before And After ◽  
Improved Performance ◽  
Gaze Cues

In referential communication, gaze is often interpreted as a social cue that facilitates comprehension and enables word learning. Here we investigated the degree to which head turning facilitates gaze following. We presented participants with static pictures of a man looking at a target object in a first and third block of trials, while they saw short videos of the same man turning towards the target in the second block. In Experiment 1, newly sighted individuals (recently treated for congenital cataracts) benefited from the motion cues, both when comparing their initial performance with static gaze cues to their performance with head turning, and their performance with static cues before and after the videos. In Experiment 2, neurotypical school children (ages 5-10 years) and adults also revealed improved performance with motion cues, although most participants had started to follow the static gaze cues by the end of the first block. Our results confirm that head turning is an effective social cue when interpreting new words, offering new insights for a pathways approach to development.

scholarly journals Rhesus monkeys show human-like changes in gaze following across the lifespan

2016 ◽  
Vol 283 (1830) ◽  
pp. 20160376 ◽  
Author(s):  
Alexandra G. Rosati ◽  
Alyssa M. Arre ◽  
Michael L. Platt ◽  
Laurie R. Santos
Keyword(s):  
Rhesus Monkeys ◽  
Sexual Maturity ◽  
Rhesus Macaques ◽  
Gaze Following ◽  
Cognitive Mechanisms ◽  
Juvenile Period ◽  
Free Ranging ◽  
Human Social Behaviour ◽  
Human Primate ◽  
Human Specific

Gaze following, or co-orienting with others, is a foundational skill for human social behaviour. The emergence of this capacity scaffolds critical human-specific abilities such as theory of mind and language. Non-human primates also follow others' gaze, but less is known about how the cognitive mechanisms supporting this behaviour develop over the lifespan. Here we experimentally tested gaze following in 481 semi-free-ranging rhesus macaques ( Macaca mulatta ) ranging from infancy to old age. We found that monkeys began to follow gaze in infancy and this response peaked in the juvenile period—suggesting that younger monkeys were especially attuned to gaze information, like humans. After sexual maturity, monkeys exhibited human-like sex differences in gaze following, with adult females showing more gaze following than males. Finally, older monkeys showed reduced propensity to follow gaze, just as older humans do. In a second study ( n = 80), we confirmed that macaques exhibit similar baseline rates of looking upwards in a control condition, regardless of age. Our findings indicate that—despite important differences in human and non-human primate life-history characteristics and typical social experiences—monkeys undergo robust ontogenetic shifts in gaze following across early development, adulthood and ageing that are strikingly similar to those of humans.

scholarly journals Reliance on head versus eyes in the gaze following of great apes and human infants: the cooperative eye hypothesis

2007 ◽  
Vol 52 (3) ◽  
pp. 314-320 ◽  
Author(s):  
Michael Tomasello ◽  
Brian Hare ◽  
Hagen Lehmann ◽  
Josep Call
Keyword(s):  
Great Apes ◽  
Gaze Following ◽  
Human Infants ◽  
The Gaze

scholarly journals What is unique about the human eye? Comparative image analysis on the external eye morphology of human and nonhuman great apes

2021 ◽  
Author(s):  
Fumihiro Kano ◽  
Takeshi Furuichi ◽  
Chie Hashimoto ◽  
Christopher Krupenye ◽  
Jesse G. Leinwand ◽  
...  
Keyword(s):  
Image Analysis ◽  
Great Apes ◽  
Human Eye ◽  
Eye Morphology

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

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Kai R. Caspar ◽  
Marco Biggemann ◽  
Thomas Geissmann ◽  
Sabine Begall
Keyword(s):  
Social Cognition ◽  
Great Apes ◽  
Human Condition ◽  
Congeneric Species ◽  
Mediated Communication ◽  
Communicative Functions ◽  
Cryptic Coloration ◽  
Visible Part ◽  
Eye Pigmentation ◽  
Scleral Pigmentation

AbstractPigmentation 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.

Sign in / Sign up

Export Citation Format

Share Document