Those Rose Hips are Really Red!

Imagine working on Zoerb Prairie and seeing a few dazzling, bright red objects out of the corner of your eye. They stand out boldly in the mostly browns of the withering grasses near the ground and as you approach, the red becomes even more vivid. They turn out to be rose hips of the prairie rose or Rosa arkansana :

Seeing this stunning display of red color can make one think of how some color-blind people, and most mammals, cannot see red at all. However, for those of us who can see it, what a treat it is! Yet, could this vibrant red color be even more red? How red can red truly be? Surprisingly, more than you might expect or even imagine!

First, some evolutionary history of vision. The retina consists of rods and cones. Rods are far more numerous than cones and are distributed throughout the retina except at a small area called the fovea. Here, cones predominate allowing us to see fine detail and perceive color. Our earliest vertebrate ancestors were probably tetrachromats meaning that they had four types of cones in their retinas, each capable of sensing a different part of the optical spectrum. These cones contain different opsin proteins that are sensitive to different wavelengths allowing them to perceive light from ultra violet to red. Tetrachromatic vision persists in many reptiles, birds, fish, and some insects. These tetrachromats can see all the colors visible to humans, but also ultraviolet light. Honeybees and many other pollinators, however, are trichromatic, with vision that includes ultraviolet but excludes red. This ultraviolet sensitivity helps pollinators find nectar because flowers have evolved to reflect ultraviolet light in patterns called "nectar guides", which function to direct these insects to the nectar which is located strategically for pollination to occur.

Most mammals, including dogs and cats, are dichromatic, being unable to sense ultraviolet or red light. It is believed that mammals lost their tetrachromatic vision about 100-150 million years ago during the Cretaceous period while they were primarily living underground and were nocturnal, making the ability to detect the full optical spectrum unnecessary. Humans, and some other primates, regained the ability to see red and are trichromatic, possessing S-cones, which are sensitive to blue light, M-cones to green, and L-cones to red. One theory for the reemergence of red sensing cones in primates was that it gave them an advantage in finding red fruits and berries.

Now back to those rose hips. They are red, but could they be even redder? An object like the rose hip can have a "saturated" color meaning its color is pure, vivid, and intense, reflecting only red while absorbing all other colors. However, even if the object is fully saturated, the actual process of perception can cause the color signals to our brains to become diluted, making the object appear less red than it otherwise could be. When red light from an object enters our eye, even from a saturated object, our L-cones are primarily stimulated sending a signal to our the brain which then interprets it as red. But, a few green and blue-sensing cones are also stimulated by the red light causing the red to be slightly diluted by blue and green, making it not a "pure"red.

Is it possible to "see" pure red? A team from the University of California, Berkeley has devised a system whereby individual types of cones can be identified and then precisely stimulated by a laser delivering a tiny pulse of light of a selected wavelength or color. By selectively stimulating only one type of cone, this produces "unprecedented saturation" or "supersaturation" causing the participant's brains to "see" something never seen before, a color beyond standard human visual capabilities. The team chose a blue-green color, but they could have just as easily chosen red. And that red would have been out-of-this-world!

So the next time you see a bright red rose hip...or a cardinal or scarlet tanager...and it looks really red, know that there is a red beyond that which is hard to even imagine! And that, yes, "a rose hip is a rose hip is a rose hip" but maybe that rose hip could be even redder!