We tend to trust our eyes. We believe what we see and tend to assume that what we don't see isn't there. Sometimes that's not true.

We hear about ultraviolet (UV) light but most of us don't have much experience with it. Besides being the name of one of Andy Warhol's film stars, ultraviolet describes light that's just beyond what people can see at the end of the spectrum (rainbow colors) where you see blue and violet (at the opposite end from red). The light receptors in human eyes which detect color cannot pick up most light in the UV region. But UV light is biologically important.

The news reports that the hole in the ozone layer over Antarctica is letting in too much UV. We know that UV is partly responsible for the formation of smog. And we know that it can damage biological molecules including proteins and DNA, causing cataracts or genetic mutations leading to skin cancers. So-called black lights give off UV, mostly of a sort that's not very dangerous, and most of us have seen museum displays with rocks under "black light" that fluoresce bright and unexpected colors (red, yellow, green, and so forth) very different than the color of the UV shining on them. Or we have seen white shirts light up in the dark under black lights because of fluorescent molecules used as brighteners in laundry detergents. But even here you aren't seeing the UV light. The minerals or the molecules of brightener absorb the UV light and glow (fluoresce), giving off new light of a color you can see (this is different than reflection where the same light just bounces off).

But many animals can see ultraviolet light. Bug zappers attract insects because the insects can see the ultraviolet light they give off and are attracted (so are they attracting more mosquitoes to those back yards from the neighbors?). Some flowers have different areas that reflect UV differently. Bees can see the difference but people cannot. This is like a pattern written in invisible (to people) ink. As a result, bees can follow the clues in that pattern to find nectar in the flowers.

Scientists are now looking for other ways in which UV is important to animals. Knowing that birds can see UV light, Bell Museum ornithologists Muir Eaton and Scott Lanyon (Director of the Bell Museum and Eaton's advisor) are looking to see how much various bird feathers reflect UV. Maybe there are important color patterns in bird plumage that the birds can see but we cannot. For all the time people have spent looking at birds, we've been unaware that birds see a much more colorful world than we do. Compared to birds, all humans are partially color blind. Eaton and Lanyon have found that there certainly are areas on many birds which reflect UV. It remains to be seen what the birds make of this, but it's a good guess that it's significant to them.

In another approach, the January 4, 2002 issue of the journal Science contains a report that some of the feathers on the heads of budgerigars (budgies or parakeets in pet shops) fluoresce under UV. As there is UV in daylight it stands to reason that that fluorescence is there whenever they're out in daylight. You won't notice it if you just look at them in the sunlight, but do the birds notice?. You bet! When the scientists put sunscreen on the heads of some birds and not on others, the sunscreen blocked the UV from getting to the feathers and so prevented the fluorescence (control animals got petroleum jelly without sunscreen). The authors say the sunscreen did not affect the reflection of UV, however, so this is quite different than what Eaton and Lanyon are looking at. Potential mates much preferred the birds without the sunscreen. This probably has no significance for people planning beach or skiing vacations, however. Wear your sunscreen and prevent skin cancer.

So seeing may be believing, but we have to learn to look carefully if we are to understand other animals because they, with their different eyes, may be seeing things quite differently.

Web Links:
1) We don't know what a bird sees when adding UV to other colors, but to get an idea how colors change as they add together (if you have a color monitor), check out the Mix-N-Match page on the Exploratorium's web site at http://www.exploratorium.edu/exhibits/mix_n_match/index.html

2) A web page that shows the spectrum of light people generally see and gives information on http://www.photo.net/photo/edscott/vis00010.htm

3) An interesting discussion of Color Perception Deficiency can be found at http://www.firelily.com/opinions/color.html

You'll get a sense of where different colors are on the spectrum and the effect of not seeing with one kind or another of our color receptors (cones). If the story seems a little complicated, that's because it is. But try it, it's interesting.