Enlarge /. One of the helpful study participants.
Noah Whiteman (University of California, Berkeley)
The expression "every color of the rainbow" is not quite as comprehensive as it sounds. On the one hand, the color chips in the color range of your hardware store contain some colors that you can hardly point out in a real rainbow. But even on a less hair-splitting level, purple is missing from this rainbow.
The "V" in "ROYGBIV" stands for violet, but it's actually not the same as purple. There is no purple light wavelength – a mix of red and blue wavelengths is required. That makes it a "non-spectral color" – in fact, it is the only non-spectral color that people see. It requires our brains to interpret signals from red and blue sensitive cones in our eyes and view them as a separate color.
While humans have three types of cones (which makes us "trichromatic"), many creatures have four that expand their visible spectrum to ultraviolet (UV) wavelengths. Theoretically, this means that they may be able to see additional non-spectral colors that we humans can hardly imagine: UV radiation mixed with red, yellow, green or purple. So … are they doing this?
More than just UV
Some research has been done on bees to show that they see UV + green as their own color (called "bee purple"), but beyond that there is not much experimental evidence. A team led by Princeton Mary Stoddard decided to test the idea by exploiting the hummingbirds' love for sugar-water feeders.
The researchers worked several summers in Colorado and set up two feeders for their experiments – one with this delicious sugar water and one with boring old water. Above it was a special LED light with UV, blue, red and green LEDs behind a diffuser, with which the researchers were able to illuminate the feeder in various non-spectral colors.
The researchers watched as wild, broad-tailed hummingbirds came to visit and recorded which feeder they had flown to first. After a set number of visits, the feed positions were switched so that the birds could not simply return to the same place once they found the sweet stuff. The idea was that they would use the color of the light to identify the feeder on return visits. They could not track individual birds separately, but due to some streaking, they estimated the local population to be 200 to 300 (depending on the year). In total, they recorded over 6,000 hummingbird visits.
Different color pairs were combined in the experiments. There were some control runs in which both lights showed exactly the same color, and some experiments in which red against green was tested. From then on, the differences became more subtle and depended on the distinction between non-spectral colors. Most were different UV blends and a different color – in the same way that we could distinguish between a reddish purple and a bluish purple.
The tests showed that the birds could see any non-spectral color that the researchers threw at them. Color pairs that were closer in shade led to more missed visits, but exceeded the 50/50 quotas of the control experiments.
Enlarge /. Results for each color pair experiment, including the four controls where both colors were the same (bottom). The blue-gray shading of the data indicates how far the two colors were apart in terms of the color space.
As an additional plausibility check, the researchers scanned databases with precisely measured colors that occur in plants and birds. These non-spectral colors are widespread in nature and account for 30 percent of the plumage colors of birds and 35 percent of the plant colors in the databases. So it would make sense that hummingbirds (and other birds) can see these colors in their environment.
And the researchers believe that this study can be generalized beyond the only broad-tailed hummingbirds that volunteered for it. Many things are poorly understood about the physiology of vision in bird species, let alone the neuronal processing of signals from these color cones in the eye, but what we do know suggests that hummingbirds are likely to be representative. "Although these experiments were done with hummingbirds," the team writes, "our results are likely to be relevant to all diurnal, tetrachromatic birds, and likely to many fish, reptiles, and invertebrates."
But you also notice that it is difficult to get into the tiny little heads of these animals and understand what the experience is like. "Even if the neural mechanisms for color vision were clear, and even if color mixing experiments confirmed the tetrachromy of birds," they write, "we still could not answer the more philosophical question of how non-spectral colors really look for birds." Does UV + Green appear to birds as a mixture of these colors (analogous to a double-stop chord played by a violinist) or as a sublime new color (analogous to a completely new tone in contrast to its components)? We cannot say that. "
PNAS, 2020. DOI: 10.1073 / pnas.1919377117 (About DOIs).