Colors of late summer at Queens Botanical Garden
But are those leaves really purple? I've started reading Ed Yong's An Immense World: How Animal Senses Reveal the Hidden Realms Around Us and am losing confidence that anything is just as it appears to us. His discussion of vision is amazing. He lets us feel more connected than the many animals whose eyes only have two cones ("dichromats") where we have three ("trichromats") - although before that we've learned about how much more they get from the sense of smell than we can even imagine. But then he laconically observes that there are many other animals - such as birds - who see colors we cannot on the UV part of the lght spectrum. Some even have four cones.
[Birds] don't just have human vision plus ultraviolet, or bee violet plus red. Tetrachromacy doesn't just widen the visible spectrum at its margins. It unlocks an entirely new dimension of colors. Remember that dichromats can make out roughly 2 percent of the colors that trichromats see—tens of thousands, compared to millions. If the same gulf exists between trichromats and teatrchromats then we might be able to see just 1 percent of the hundreds of millions of colors that a bird can discriminate. Picture trichromatic human vision as a triangle, with the three corners representing our red, green, and blue cones. Every color we can see is a mix of those three, and can be plotted as a point within that triangular space. By comparison, a bird’s color vision is a pyramid, with four corners representing each of its four cones. Our entire color space is just one face of that pyramid, whose spacious interior represents colors inaccessible to most of us.
If our red and blue cones are stimulated together, we see purple—a color that doesn’t exist in the rainbow and that can’t be represented by a single wavelength of light. These kinds of cocktail colors are called non-sectral. Hummingbirds, with their four cones, can see a lot more of them, including UV-red, UV-green, UC-yellow (which is red + green + UV), and probably UV-purple (which is red + blue + UV). At my wife’s suggestion, and to [hummingbird researcher Mary Caswell “Cassie”] Stoddard’s delight, I’m going to call these rurple, grurple, yurple and ultrapurple. Stoddard found that these non-spectral colors and their various shades account for roughly a third of those found on plants and feathers. To a broad-taled hummingbird, the bright magenta feathers of the male’s bib are actually ultra-purple.
(Random House, 2022), 97
