March 2, 2006

All the better to see you blush, my dear…

WASHINGTON (Reuters) - Primates may have evolved color
vision not to find the ripest, tastiest fruit but to detect
that tell-tale blush on someone else's rump, U.S. researchers
reported on Thursday.

The cone structures in the eye that help detect color seem
exquisitely tuned to skin tones, the team at the California
Institute of Technology reports.

"For a hundred years, we've thought that color vision was
for finding the right fruit to eat when it was ripe," Mark
Changizi, a theoretical neurobiologist and postdoctoral
researcher at Caltech who led the study, said in a statement.

"But if you look at the variety of diets of all the
primates having trichromat (three-color) vision, the evidence
is not overwhelming."

Instead, Changizi and colleagues report in the current
issue of the journal Biology Letters, the system seems adapted
especially to find the colors prevalent in primate skins --
notably changes due to how much oxygenated hemoglobin is in the

In contrast, bees have four color cones that are evenly
spread across the visible spectrum, with the high-frequency end
extending into the ultraviolet. Birds have three color cones
that are also evenly distributed in the visible spectrum.

And the three-cone system can help a primate tell not only
if a potential partner is having a rush of emotion in
anticipation of mating, but also if an enemy's blood has
drained out of his face due to fear.

"Also, ecologically, when you're more oxygenated, you're in
better shape," Changizi said. That may be why humans value rosy
cheeks, he said.

The clincher -- Changizi said old-world primates that have
the three-cone vision are also all bare-faced and bare-butted.

"There's no sense in being able to see the slight color
variations in skin if you can't see the skin," Changizi said.

"This could connect up with why we're the 'naked ape,"' he