Invisible Colors

Originally published on May 22, 2011 in The Artful Brain under Nature Publishing Group (see original version)

Although our visual system can paint a vibrant portrait of the world, its palette of colors is actually quite limited, as we only see between 390 to 750 nm of the full electromagnetic spectrum while the remaining trillion wavelengths escape our view. Within these wavelengths exists other colors, normally invisible to the human eye. However, birds, bees, and some humans retinal genetic mutations, can see nature's other shades.

Humans are trichromats, meaning that we experience color through three types of photoreceptors tuned to different wavelengths: short (blue), medium (green), and long (red); and the combinations of activity of these receptors give us the perception of color. However, it turns out that the tuning curve of the red receptor in bees is shifted up such that they are red-blind, but see ultraviolet light. This means that UV light is their version of red (try to imagine). That change in color gives nature another way to evolve its marketing campaign and attract more business. For instance, flowers have evolved to provide bright UV petals surrounding a dark region contrasting containing glowing UV pollen. Although we can't see it, bees must find this irresistible!

From R-L: The same flower with human vision, only UV vision (bright = UV), simulated bee vision (UV+G+B), simulated bird vision (tetrachromatcic: UV+R+G+B). (Photos: (c) Dr. Klaus Schmitt, Weinheim, Germany, uvir.eu)

The picture above shows a flower with different light receptors. The first image is what humans see. The second image is only UV light, and as you can see, the center target is vastly larger than the version we see and you can make out a faint UV glow in the center. Being red-blind, bees would see the third image, which is composed of UV, blue, and green (UV+B+G).

So why would nature select for this? If the UV colors and patterns in a flower helps guide bees to pollen, then more flowers with UV features would procreate. But grass and foliage also reflect a lot of UV. How does the flower stand out? Dr. Klaus Schmitt of Weinheim, Germany is an expert in tetrachromatic photography and has taken all of the photos in this article. He explains: "Some flowers found a tricky way, they create a landing platform for bees and bumble bees, consisting of concentric rings with a bright UV reflective center highlighted by contrasting petals around it. But why is all of this invisible to us humans? Simple, we don't pollinate flowers, so the flower has evolutionary incentive to attract us towards it's pollen." To see a stunning view of the concentric rings in flowers, check out Dr. Schmitt's finalist photo of a gazania through bee-vision in the Carl Zeiss photography contest.

But what about the fourth image with all four colors (UV+B+G+R)? It turns out that some species, such as birds, along with most reptiles, have four types of photoreceptors (UV+R+G+B) making them tetrachromats! Although it would be impossible to show exactly how this extra receptor affects the way a bird sees the flower, other colors can be used as a metaphor for UV to produce the stunning simulation of the tetrachromatic flower on the far right. But for birds, this extra receptor is not just for food, but also for mating. As you can see from the images below, the birds, like the flowers, have evolved to display a stunning set of colors invisible to the unaided eye. A study by Bennett, Cuthill, Partridge, and Lunau (1997) showed that ultraviolet reflecting plumage in starlings had profound effects on observed mating preferences, while plumage in the human visible spectrum did not predict choice. Their ultraviolet feathers are part of their mating call! Even the eggs have UV coloration.

Bird (Top) and Egg (Bottom), R-L: through human vision (R+G+B), only UV vision (bright = UV), simulated bird vision (tetrachromatic: UV+R+G+B) (the egg on the bottom right is not true bird vision, but UV simulated vision). (Photos: (c) Dr Klaus Schmitt, Weinheim, Germany, uvir.eu)

In humans, the genes that dictate photoreceptor color tuning are type 2 opsin genes, OPN1MW and OPN1MW2, and are located on the X chromosome. Since women have two different X chromosomes in their cells, it has been suggested that some of them could carry a variant cone cell tuned to a wavelength between the red and green, thereby having tetrachomatic vision and discrimination across 100 million different colors. Having more color receptors provides better color discrimination for quick decisions over food and mating. Although some species, such as butterflies and pigeons, have five types of photoreceptors and can distinguish over 10 billion colors, the advantage to having more receptors reaches a ceiling around three to five. In fact, many human males have color blindness leaving them with only two color receptors (dichromacy).

The point to take away from this is that our sensory and perceptual scope is narrow in comparison to all of the energy available to us. Despite our limitations, we are able to use tools, such as cameras, as sensory metaphor to expand our view. And although we can't experience these physical phenomena first-hand, we can use these metaphors and reason to extrapolate our senses and obtain an educated glimpse of the physical world. And what we find there can be compelling as well as humbling, as we realize that nature was not constructed solely for the benefit and pleasure of mankind.

References:

Photography: Dr. Klaus Schmitt of Weinheim, Germany (http://photographyoftheinvisibleworld.blogspot.com/), (c) Dr Klaus Schmitt, Weinheim, Germany, uvir.eu

Bennett, D., Cuthill, I.C., Partridge, L.K. (1997) Ultraviolet plumage colors predict mate preferences in starlings. Proc. Natl. Acad. Sci. USA Vol 94,,pp. 8618-8621

Emmerton, J. & Delhis, J.D. (1980). Wavelength discrimination in the ‘visible’ and ultraviolet spectrum by pigeons. Journal of Comparative Physiology 141(1).

Jameson, K.A., Highnote, S.M., & Wasserman, L.M. (2001). Richer color experience in observers with multiple photopigment opsin genes. Psychonomic Bulletin and Review 8 (2): 244–261.