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WHEN your gaze meets someone else’s across a crowded room, you can tell
they’re looking at you by the whites of their eyes. But you’d be struggling with
other primates, as they have totally dark eyes that are intended to camouflage
their gaze from predators, according to researchers in Japan.

Our eyes lack pigmentation in a region around the iris called the
sclera—which is why we call it the “white”. But other primates make
pigments to darken the same region of their eyes.

This costs them energy, and to find out why the animals go to all this
trouble, Hiromi Kobayashi and Shiro Kohshima of the Tokyo Institute of
Technology compared the coloration of the sclera, the iris and the surrounding
face in humans and 81 species of primates at the Japan Monkey Centre in
Inuyama.

They found that all primates apart from humans have scleras that are a
similar colour to their irises, faces or both. As a result, the outlines of
either the iris—which indicates where the animal is looking—or the
eye itself are effectively camouflaged. The study examined about half of all
species of living non-human primates. “It is very likely that the others also
have ‘gaze camouflage’ eyes,” says Kobayashi.

Humans are the only species with a large sclera paler than the face and the
iris. This feature draws attention to the iris and makes our eyes uniquely “gaze
signalling” rather than “gaze camouflaging”. “Human eyes have one of the optimal
shapes to signal eye direction,” says Kobayashi.

Biologists had suggested that the dark eyes of non-human primates reduce
glare. But the team showed that several nocturnal species, which shouldn’t have
a problem with glaring sunlight, also have dark eyes with dark surrounds.
Instead, the researchers propose that the colour camouflage may help to fool
predators.

At a distance, the blank stare of a gaze-camouflaging primate may trick any
prowling predators into thinking they have been spotted even if they haven’t.
This may be enough to deter some predators from ambushing their prey, Kobayashi
and Kohshima suggest.

Humans, however, may benefit more from eyes that signal where they are
looking, according to the researchers. Being able to tell what someone else is
looking at is an important part of communication and may help us to develop
highly sophisticated cooperative behaviour. “Eye direction serves as an
important cue to detect others’ attention,” says Kobayashi. The researchers also
point out that our large body size and arsenal of tools make us less vulnerable
to predators. “I think that ‘gaze signalling’ eyes emerged with the genus Homo,”
Kobayashi says.

He and Kohshima argue that humans move their eyeballs to
look somewhere else far more readily than other primates. These rely heavily on
other ways of signalling what they are looking at. “Our study suggests that
smaller primates mainly used head movement,” says Kobayashi.

“The evidence for non-human primates following gaze direction is scarce,”
agrees Nathan Emery of the University of Cambridge. He has modelled gaze
signalling in non-human primates and suggests that the dark-eye deception may
even extend to their peers, allowing them to conceal their desires. “With
non-human primates, it could be that they are actually interested in something,
but others can’t tell from just their eyes,” he says. So if your eyes meet a
monkey’s at a party, they may keep you guessing.