TY - JOUR
T1 - What is unique about the human eye? Comparative image analysis on the external eye morphology of human and nonhuman great apes
AU - Kano, Fumihiro
AU - Furuichi, Takeshi
AU - Hashimoto, Chie
AU - Krupenye, Christopher
AU - Leinwand, Jesse G.
AU - Hopper, Lydia M.
AU - Martin, Christopher F.
AU - Otsuka, Ryoma
AU - Tajima, Tomoyuki
N1 - Funding Information:
We thank the following sanctuaries and zoos (in alphabetical order): Antwerp Zoo (especially, the Royal Zoological Society of Antwerp, Marjolein Osieck, Jeroen Stevens, Jonas Verhulst, and Sara Lafaut), Great Ape Research Institute (especially, Satoshi Hirata), Indianapolis Zoo, Kumamoto Sanctuary (especially, Toshifumi Udono, Etsuko Nogami, Melody So, and Lucy Baehren), Leipzig Zoo, Lincoln Park Zoo, Lola ya Bonobo, North Carolina Zoo (especially, Jennifer Ireland, Emily Lynch, Brooke Sides, and Chris Goldston), Olmense Zoo/Pakawi Park, Primate Research Institute (especially, Yoko Sakuraba and Yumi Yamanashi), Sepilok Orangutan Rehabilitation Centre (especially, Titol Malim, Sylvia Alsisto and Vijay S. Kumar), and Wolfgang Köhler Primate Research Center, and the following field sites: Wamba (especially Nahoko Tokuyama), Kalinzu, Bwindi, and Danum Valley (especially, Noko Kuze and Tomoko Kanamori) for kindly offering the images of great apes. We also thank the authors of Columbia Gaze Data Set for kindly offering the images of humans. We also thank Sabah Biodiversity Centre (License Ref. No. JKM/MBS.1000-2/2 JLD.11 (7) to T. Tajima) and Danum Valley Management Committee. Financial support came from Japan Society for the Promotion of Science KAKENHI Grants 19H01772 and 20H05000 to F·K, and the Lincoln Park Zoo Women's Board supported J.G.L. and L.M.H.
Publisher Copyright:
© 2021
PY - 2022/5
Y1 - 2022/5
N2 - 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 certain features of human eyes are not necessarily unique among great ape species. Accordingly, there is currently a heated debate over whether external eye features of humans are distinct from those of other apes and how such distinguishable features contribute to the visibility of eye-gaze direction. The present 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 great apes, a key difference was that humans have uniformly white sclera which creates clear visibility of both the 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 noise, such as shading 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 unique external eye morphology among other great apes, which ensures the robustness of eye-gaze signals in various visual conditions. Our results support and also critically update the central premises of the gaze-signaling hypothesis.
AB - 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 certain features of human eyes are not necessarily unique among great ape species. Accordingly, there is currently a heated debate over whether external eye features of humans are distinct from those of other apes and how such distinguishable features contribute to the visibility of eye-gaze direction. The present 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 great apes, a key difference was that humans have uniformly white sclera which creates clear visibility of both the 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 noise, such as shading 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 unique external eye morphology among other great apes, which ensures the robustness of eye-gaze signals in various visual conditions. Our results support and also critically update the central premises of the gaze-signaling hypothesis.
KW - Communication
KW - Comparative analysis
KW - Eye color
KW - Gaze detection
KW - Great ape
KW - Human evolution
KW - Sclera
UR - http://www.scopus.com/inward/record.url?scp=85121984396&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85121984396&partnerID=8YFLogxK
U2 - 10.1016/j.evolhumbehav.2021.12.004
DO - 10.1016/j.evolhumbehav.2021.12.004
M3 - Article
AN - SCOPUS:85121984396
SN - 1090-5138
VL - 43
SP - 169
EP - 180
JO - Evolution and Human Behavior
JF - Evolution and Human Behavior
IS - 3
ER -