Ability to detect directional gaze is not unique to humans

Chimpanzee
Credit: CC0 Public Domain

The ability to detect the direction of someone's gaze is not unique to humans, as had been previously thought, according to new research.

The contrasting color patterns of our eyes, which help us see where others are looking, was thought to be unique in humans but has been found to be present in chimpanzees and bonobos by scientists working at the University of St Andrews, the Department of Biological Sciences National University of Singapore, and Leiden University in the Netherlands.

It has been suggested that the difference between the white of our eyes, the sclera, and our colorful irises allows others to detect the direction of our —something that many of our other skills, such as social learning, seem to depend on. The sclera of apes' eyes is often darker, and because of this, researchers have long argued that their gaze was 'hidden' or cryptic and that other apes would not be able to see where they are looking.

The new study, "Scleral pigmentation leads to conspicuous, not cryptic, eye morphology in chimpanzees," published today (Monday 2 September 2019) in Proceedings of the National Academy of Sciences of the United States of America found that ape eyes have the same pattern of color differences we do, suggesting that they could also follow each other's gaze.

Before humans had language, our ancestors may have relied on the gaze of those around them to help communicate dangers or other useful information. "They couldn't say 'look over there' but a look in the direction of a predator might be enough, as long as it was possible to follow the direction of their gaze," said lead author Juan O Perea-García of the Department of Biological Sciences National University of Singapore.

Senior author Dr. Cat Hobaiter, a field primatologist and lecturer in the School of Psychology and Neuroscience at the University of St Andrews, said: "Understanding where someone is looking seems to be key to understanding what they're interested in, what they're thinking about. For a long time researchers have suggested that the colour of other apes' eyes means that they hide this information; we've shown that's not the case."

Bonobos, like humans, have paler sclera and darker irises, but chimpanzees have a different pattern: very dark sclera, and paler irises. Both color patterns show the same type of contrast seen in and could help other apes find out where they are looking.

Dr. Hobaiter said: "The idea that chimpanzees couldn't see where other were looking always puzzled me. I've spent years working with wild apes and I find it quite easy, and I'm sure they're much better at it than I am."


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More information: Juan Olvido Perea-García el al., "Scleral pigmentation leads to conspicuous, not cryptic, eye morphology in chimpanzees," PNAS (2019). www.pnas.org/cgi/doi/10.1073/pnas.1911410116
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Sep 02, 2019
Ravens and cockatoos can as well.

Sep 02, 2019
The article is long on background, followed by an expert opinion, and short on the research paper's content itself.

Sep 02, 2019
Apes aren't always staring at each other's eyes, so that there are other behaviours and sounds that must be depended on. For example, screeching and growling could immediately cause the other apes to look around for danger. They are not going to search the eyes of their mate who is doing the screeching or growling to explain from which direction the danger is coming from. The eyes are only a small part of the story. That is also true in humans.

Sep 02, 2019
"Bonobos, like humans, have pal**er** sclera and dark**er** irises..." "...but chimpanzees have a **different pattern: very dark sclera, and pal**er** irises.**"... "the same type of contrast..."... "could help...".

Sounds like a lot of fudging going on here, shown by the added emphasis. "same type" is vague, "could help" means they're just guessing. The human "contrast pattern" is generally to have very large areas of very pale sclera showing.

Dr. Hobaiter said: "The idea that chimpanzees couldn't see where other chimpanzees were looking always puzzled me. I've spent years working with wild apes and I find it quite easy, and I'm sure they're much better at it than I am."

Oh dear, it looks like Dr. H let anthropomorphizing tendencies and wishful thinking influence this statement. If humans are especially tuned to eye movements due to their HIGH contrast and our high-level, face-to-face communications, then that final "I'm sure" is a non sequitur.

Sep 03, 2019
The article is long on background, followed by an expert opinion, and short on the research paper's content itself.
Between the end of the article and the comment block where you posted the above is a link to the study itself:
Juan Olvido Perea-García el al., "Scleral pigmentation leads to conspicuous, not cryptic, eye morphology in chimpanzees," PNAS (2019). www.pnas.org/cgi/...11410116

Sep 03, 2019
The article is long on background, followed by an expert opinion, and short on the research paper's content itself.


It is an easy read, and the details should be in the paper. More annoying to me is that the paper is not released yet. If it even exists (can't find it through name, doi or pnas search for author).

Sep 03, 2019
"Bonobos, like humans, have pal**er** sclera and dark**er** irises..." "...but chimpanzees have a **different pattern: very dark sclera, and pal**er** irises.**"... "the same type of contrast..."... "could help...".

Sounds like a lot of fudging going on here, shown by the added emphasis.


I am not sure if you haven't read science or press releases on science before. But they are in general, and should be, precise in the extent of their knowledge. It is famously the remaining uncertainty that makes science (and technology) work, since it can be retested and improved on (even if only to discover previous errors).

So to point. If the article is published in peer review (but see my comment above), it would be consistent with what we know. Your "anthropomorphic" trait (but see the expert opinion) has evolved to fixation in humans so either is drift - unlikely since it is a useful trait - or evolved by selection from a common ape template of color differences.

E.g. humans are apes.

Sep 03, 2019
When I look off into the distance, my dog tends to look into the same distance as me for a moment, before turning back to me. Similar perhaps?

Sep 03, 2019
When I look off into the distance, my dog tends to look into the same distance as me for a moment, before turning back to me. Similar perhaps?


As I recall, there have been studies indicating domestic dogs do "read" human gestures, expressions, etc. It wouldn't be surprising since they've been with us for thousands of years.

Sep 03, 2019
I am not sure if you haven't read science or press releases on science before. But they are in general, and should be, precise in the extent of their knowledge. ...

... If the article is published in peer review (but see my comment above), it would be consistent with what we know. Your "anthropomorphic" trait (but see the expert opinion) has evolved to fixation in humans so either is drift - unlikely since it is a useful trait - or evolved by selection from a common ape template of color differences.

E.g. humans are apes.


I've read a lot of science papers and press releases; it's precisely the lack of precision here that I noted. That humans are apes is irrelevant -- even between species there can be significant differences. Humans are further separated and clearly unique in a number of ways, and I think it is healthy us to be careful not to jump to conclusions that minimize our differences. The history of science is filled with experts who voiced very wrong opinions.

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