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Caspar KR, Hüttner L, Begall S. Scleral appearance is not a correlate of domestication in mammals. ZOOLOGICAL LETTERS 2023; 9:12. [PMID: 37248525 DOI: 10.1186/s40851-023-00210-z] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/09/2023] [Accepted: 05/15/2023] [Indexed: 05/31/2023]
Abstract
Numerous hypotheses try to explain the unusual appearance of the human eye with its bright sclera and transparent conjunctiva and how it could have evolved from a dark-eyed phenotype, as is present in many non-human primates. Recently, it has been argued that pigmentation defects induced by self-domestication may have led to bright-eyed ocular phenotypes in humans and some other primate lineages, such as marmosets. However, it has never been systematically studied whether actual domesticated mammals consistently deviate from wild mammals in regard to their conjunctival pigmentation and if this trait might therefore be part of a domestication syndrome. Here, we test this idea by drawing phylogenetically informed comparisons from a photographic dataset spanning 13 domesticated mammal species and their closest living wild relatives (n ≥ 15 photos per taxon). We did not recover significant differences in scleral appearance or irido-scleral contrast between domesticated and wild forms, suggesting that conjunctival depigmentation, unlike cutaneous pigmentation disorders, is not a general correlate of domestication. Regardless of their domestication status, macroscopically depigmented conjunctivae were observed in carnivorans and lagomorphs, whereas ungulates generally displayed darker eyes. For some taxa, we observed pronounced intraspecific variation, which should be addressed in more exhaustive future studies. Based on our dataset, we also present preliminary evidence for a general increase of conjunctival pigmentation with eye size in mammals. Our findings suggest that conjunctival depigmentation in humans is not a byproduct of self-domestication, even if we assume that our species has undergone such a process in its recent evolutionary history.
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Affiliation(s)
- Kai R Caspar
- Institute of Cell Biology, Heinrich Heine University Düsseldorf, Düsseldorf, Germany.
- Department of General Zoology, University of Duisburg-Essen, Essen, Germany.
- Department of Game Management and Wildlife Biology, Faculty of Forestry and Wood Sciences, Czech University of Life Sciences, Praha, Czech Republic.
| | - Lisa Hüttner
- Department of General Zoology, University of Duisburg-Essen, Essen, Germany
| | - Sabine Begall
- Department of General Zoology, University of Duisburg-Essen, Essen, Germany
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2
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Clark IR, Lee KC, Poux T, Langergraber KE, Mitani JC, Watts D, Reed J, Sandel AA. White sclera is present in chimpanzees and other mammals. J Hum Evol 2023; 176:103322. [PMID: 36706647 PMCID: PMC9998187 DOI: 10.1016/j.jhevol.2022.103322] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2022] [Revised: 12/22/2022] [Accepted: 12/23/2022] [Indexed: 01/27/2023]
Affiliation(s)
- Isabelle R Clark
- Department of Anthropology, University of Texas at Austin, 2201 Speedway, Austin, TX 78712, USA
| | - Kevin C Lee
- School of Human Evolution and Social Change and Institute of Human Origins, Arizona State University, 900 S. Cady Mall, Tempe, AZ 85281, USA
| | - Tucker Poux
- Tufts University, 419 Boston Ave, Medford, MA 02155, USA
| | - Kevin E Langergraber
- School of Human Evolution and Social Change and Institute of Human Origins, Arizona State University, 900 S. Cady Mall, Tempe, AZ 85281, USA
| | - John C Mitani
- Department of Anthropology, University of Michigan, 1085 S. University Ave., Ann Arbor, MI 48109, USA
| | - David Watts
- Department of Anthropology, Yale University, P.O. Box 208277, New Haven, CT 06520, USA
| | - James Reed
- Underdog Films Ltd, 2 St. Paul's Road, Bristol, BS8 1LT, UK
| | - Aaron A Sandel
- Department of Anthropology, University of Texas at Austin, 2201 Speedway, Austin, TX 78712, USA.
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Wolf W, Thielhelm J, Tomasello M. Five-year-old children show cooperative preferences for faces with white sclera. J Exp Child Psychol 2023; 225:105532. [PMID: 35988359 DOI: 10.1016/j.jecp.2022.105532] [Citation(s) in RCA: 4] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/22/2021] [Revised: 07/29/2022] [Accepted: 07/29/2022] [Indexed: 11/25/2022]
Abstract
The cooperative eye hypothesis posits that human eye morphology evolved to facilitate cooperation. Although it is known that young children prefer stimuli with eyes that contain white sclera, it is unknown whether white sclera influences children's perception of a partner's cooperativeness specifically. In the current studies, we used an online methodology to present 5-year-old children with moving three-dimensional face models in which facial morphology was manipulated. Children found "alien" faces with human eyes more cooperative than faces with dark sclera (Study 2) but not faces with enlarged irises (Study 1). For more human-like faces (Study 3), children found human eyes more cooperative than either enlarged irises or dark sclera and found faces with enlarged irises cuter (but not more cooperative) than eyes with dark sclera. Together, these results provide strong support for the cooperative eye hypothesis.
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Affiliation(s)
- Wouter Wolf
- Department of Psychology and Neuroscience, Duke University, Durham, NC 27708, USA; Department of Developmental Psychology, Utrecht University, 3584 CS Utrecht, the Netherlands.
| | - Julia Thielhelm
- Department of Psychology and Neuroscience, Duke University, Durham, NC 27708, USA
| | - Michael Tomasello
- Department of Psychology and Neuroscience, Duke University, Durham, NC 27708, USA; Department of Developmental and Comparative Psychology, Max Planck Institute for Evolutionary Anthropology, 04103 Leipzig, Germany
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Kano F. Evolution of the uniformly white sclera in humans: critical updates. Trends Cogn Sci 2023; 27:10-12. [PMID: 36229339 DOI: 10.1016/j.tics.2022.09.011] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/21/2022] [Revised: 09/19/2022] [Accepted: 09/20/2022] [Indexed: 12/24/2022]
Abstract
The human eye characteristically has exposed and uniformly white sclera, which is hypothesized to have evolved to enhance eye-gaze signaling for conspecific communication. Although recent studies have put this hypothesis into question, current morphological and experimental evidence supports its key premise, albeit with recommendations for critical updates.
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Affiliation(s)
- Fumihiro Kano
- Centre for the Advanced Study of Collective Behaviour, University of Konstanz, 78464, Konstanz, Germany; Max-Planck Institute of Animal Behavior, 78315, Radolfzell, Germany.
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Wacewicz S, Perea-García JO, Lewandowski Z, Danel DP. The adaptive significance of human scleral brightness: an experimental study. Sci Rep 2022; 12:20261. [PMID: 36424405 PMCID: PMC9691750 DOI: 10.1038/s41598-022-24403-2] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2022] [Accepted: 11/15/2022] [Indexed: 11/27/2022] Open
Abstract
Homogeneously depigmented sclerae have long been proposed to be uniquely human-an adaptation to enable cooperative behaviour by facilitating interpersonal coordination through gaze following. However, recent evidence has shown that deeply pigmented sclerae also afford gaze following if surrounding a bright iris. Furthermore, while current scleral depigmentation is clearly adaptive in modern humans, it is less clear how the evolutionarily intermediate stages of scleral pigmentation may have been adaptive. In sum, it is unclear why scleral depigmentation became the norm in humans, while not so in sister species like chimpanzees, or why some extant species display intermediate degrees of pigmentation (as our ancestors presumably did at some point). We created realistic facial images of 20 individually distinct hominins with diverse facial morphologies, each face in the (i) humanlike bright sclera and (ii) generalised apelike dark sclera version. Participants in two online studies rated the bright-sclera hominins as younger, healthier, more attractive and trustworthy, but less aggressive than the dark-sclera hominins. Our results support the idea that the appearance of more depigmented sclerae promoted perceived traits that fostered trust, increasing fitness for those individuals and resulting in depigmentation as a fixed trait in extant humans.
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Affiliation(s)
- Slawomir Wacewicz
- grid.5374.50000 0001 0943 6490Center for Language Evolution Studies, Nicolaus Copernicus University, Toruń, Poland
| | - Juan Olvido Perea-García
- grid.5132.50000 0001 2312 1970The Cognitive Psychology Unit, Faculty of Social Sciences, Leiden University, Leiden, The Netherlands
| | - Zdzisław Lewandowski
- grid.8505.80000 0001 1010 5103Department of Human Biology, Wroclaw University of Health and Sport Sciences, Wroclaw, Poland
| | - Dariusz P. Danel
- grid.413454.30000 0001 1958 0162Department of Anthropology, Hirszfeld Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, Wroclaw, Poland
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Yorzinski JL, Troscianko J, Briolat E, Schapiro SJ, Whitham W. A songbird can detect the eyes of conspecifics under daylight and artificial nighttime lighting. ENVIRONMENTAL POLLUTION (BARKING, ESSEX : 1987) 2022; 313:120000. [PMID: 35995296 DOI: 10.1016/j.envpol.2022.120000] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/21/2022] [Revised: 07/25/2022] [Accepted: 08/16/2022] [Indexed: 06/15/2023]
Abstract
Eyes convey important information about the external and internal worlds of animals. Individuals can follow the gaze of others to learn about the location of salient objects as well as assess eye qualities to evaluate the health, age or other internal states of conspecifics. Because of the increasing prevalence of artificial lighting at night (ALAN), urbanized individuals can potentially garner information from conspecific eyes under both daylight and ALAN. We tested this possibility using a visual modeling approach in which we estimated the maximum distance at which individuals could detect conspecific eyes under daylight and high levels of ALAN. We also estimated the minimum light level at which individuals could detect conspecific eyes. Great-tailed grackles (Quiscalus mexicanus) were used as our study species because they are highly social and are unusual among birds in that they regularly gather at nocturnal roosts in areas with high levels of ALAN. This visual modelling approach revealed that grackles can detect conspecific eyes under both daylight and ALAN, regardless of iris coloration. The grackles could detect conspecific eyes at farther distances in daylight compared to ALAN. Our results highlight the potential importance of lighting conditions in shaping social interactions.
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Affiliation(s)
- Jessica L Yorzinski
- Department of Ecology and Conservation Biology, Texas A&M University, College Station, TX, USA.
| | - Jolyon Troscianko
- Centre for Ecology & Conservation, University of Exeter, Penryn, United Kingdom
| | - Emmanuelle Briolat
- Centre for Ecology & Conservation, University of Exeter, Penryn, United Kingdom
| | - Steven J Schapiro
- Department of Comparative Medicine, The University of Texas MD Anderson Cancer Center, Bastrop, TX, USA
| | - Will Whitham
- Department of Ecology and Conservation Biology, Texas A&M University, College Station, TX, USA; Department of Comparative Medicine, The University of Texas MD Anderson Cancer Center, Bastrop, TX, USA
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Mearing AS, Burkart JM, Dunn J, Street SE, Koops K. The evolutionary drivers of primate scleral coloration. Sci Rep 2022; 12:14119. [PMID: 35982191 PMCID: PMC9388658 DOI: 10.1038/s41598-022-18275-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2022] [Accepted: 08/08/2022] [Indexed: 11/09/2022] Open
Abstract
The drivers of divergent scleral morphologies in primates are currently unclear, though white sclerae are often assumed to underlie human hyper-cooperative behaviours. Humans are unusual in possessing depigmented sclerae whereas many other extant primates, including the closely-related chimpanzee, possess dark scleral pigment. Here, we use phylogenetic generalized least squares (PGLS) analyses with previously generated species-level scores of proactive prosociality, social tolerance (both n = 15 primate species), and conspecific lethal aggression (n = 108 primate species) to provide the first quantitative, comparative test of three existing hypotheses. The 'self-domestication' and 'cooperative eye' explanations predict white sclerae to be associated with cooperative, rather than competitive, environments. The 'gaze camouflage' hypothesis predicts that dark scleral pigment functions as gaze direction camouflage in competitive social environments. Notably, the experimental evidence that non-human primates draw social information from conspecific eye movements is unclear, with the latter two hypotheses having recently been challenged. Here, we show that white sclerae in primates are associated with increased cooperative behaviours whereas dark sclerae are associated with reduced cooperative behaviours and increased conspecific lethal violence. These results are consistent with all three hypotheses of scleral evolution, suggesting that primate scleral morphologies evolve in relation to variation in social environment.
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Affiliation(s)
- Alex S Mearing
- Department of Archaeology, Fitzwilliam Street, University of Cambridge, Cambridge, CB2 1QH, UK.
| | - Judith M Burkart
- Department of Anthropology, University of Zurich, 8057, Zurich, Switzerland
| | - Jacob Dunn
- Department of Archaeology, Fitzwilliam Street, University of Cambridge, Cambridge, CB2 1QH, UK.,School of Life Sciences, Anglia Ruskin University, Cambridge, CB1 1PT, UK.,Department of Cognitive Biology, University of Vienna, 1090, Vienna, Austria
| | - Sally E Street
- Department of Anthropology, University of Durham, Durham, DH1 3LE, UK
| | - Kathelijne Koops
- Department of Archaeology, Fitzwilliam Street, University of Cambridge, Cambridge, CB2 1QH, UK.,Department of Anthropology, University of Zurich, 8057, Zurich, Switzerland
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