by Michelle Frank
figures by Abagail Burrus
It’s a classic kindergarten icebreaker: which do you like better, blue or green? Would you rather wear pink or orange? What’s your favorite color? While these preferences might seem like markers of human personality, Homo sapiens aren’t the only animal to have a preferred hue. When given a choice, even insects show a preference for one shade of colored light over another.
The discovery of color preferences
Color vision in insects was first described 100 years ago by Karl von Frisch, a German scientist—and eventual Nobel prize winner—who specialized in honeybee behaviors. Von Frisch managed to train honeybees to fly to particular colors by placing small bowls of sugar syrup on colored pieces of paper. When he moved the colorful paper to the middle of a group of gray cards, the bees still navigated to the colored papers. Their ability to hone in on the island of color told von Frisch that the bees were able to distinguish its particular hue from the sea of gray.
Over the next several decades, researchers realized that other types of insects can perceive colors, as well. Moreover, many insect species have innate preferences for some colors over others. In other words, they possess inherited “favorite colors” that affect the way they behave. One species of moth has an inborn preference for the color blue, although it can eventually learn to prefer other colors if they’re associated with food. When given a choice, fruit flies will head toward ultraviolet (UV) light rather than green light. Water fleas (which are actually a kind of crustacean, more closely related to lobsters than flies), on the other hand, swim away from UV light and toward blue or green light.
How does color vision work?
Humans, flies, and other animals all see colored light using the same basic machinery. Light travels as a wave, an undulating series of peaks and troughs. The distance between two adjacent peaks—the wavelength—determines the color of the light (Figure 1). When the peaks are a certain distance apart, the light appears blue. A bit farther apart, and the light looks red. A bit closer together, and it looks violet.
When light enters your eye, it’s absorbed by cells in your retina called photoreceptors. Unless you’re colorblind, your eye contains three different types of color-sensitive photoreceptors, and each one is particularly sensitive to certain colors of light. Ultimately, these photoreceptors determine which wavelengths of light your eye can perceive: if the wavelength is too long or too short, your eye won’t be able to capture the light. Human eyes, for example, can’t perceive UV light (the wavelength is too short) or infrared light (the wavelength is too long).
Once light hits a photoreceptor in your retina, it triggers a series of reactions that send signals ricocheting throughout your brain. To turn these signals into color vision, cells in your brain need to compare and compile information from each of the different types of photoreceptors. This process is roughly analogous to combining three different colors of paint together to create many different shades, and it allows the brain to transform the cues from only three kinds of photoreceptors into the abundant assortment of colors we can see (Figure 2).
Although human and insect eyes look very different, the general pattern of events is the same in both: light enters the eye and gets absorbed by one kind of photoreceptor but not another. Comparisons across the photoreceptors then allow the organism to see in color. Not all insects have three kinds of photoreceptors, though, and many can also see in UV: fruit flies have four kinds of photoreceptors, cockroaches have only two, and some kinds of butterflies have as many as fifteen.
Still, this explanation leaves a bigger question unanswered: why do animals like the colors they like? Scientists have come up with a few hypotheses. Maybe fruit flies like UV light because open skies—which contain a lot of UV light—are safer for them than enclosed spaces. Maybe water fleas like the color green because green light suggests the presence of the algae they eat. These evolutionary explanations help make sense of why organisms like the colors they do, but they can’t clarify how the brain drives these creatures toward some colors and away from others. For that, we need to take a closer look at the way the brain handles the color information emanating from photoreceptors.
Guided by the light: how fruit flies are drawn to UV
Neuroscientists like to think of sensory perceptions as a kind of ladder. As you go up each rung and pass through each processing level in the brain, you add something new to the perception. At first, you have only a bit of light hitting a single photoreceptor and a bit of knowledge about that light’s color. From there, you add information from other photoreceptors: the light is not just blue/green-ish, but more specifically a light seafoam green. After that, you might reach a part of the brain that adds other information—that seafoam green smear is part of a hat. And not only is it part of a hat, but it’s a very fashionable hat that you would like to buy.
This kind of multi-level, hierarchical process is how you perceive most of your surroundings. But in certain, special cases, the brain takes shortcuts. For example, many kinds of animals, including flies and fish, have a hardwired escape reflex that’s triggered when they see certain kinds of moving objects. When animals encounter these threatening sensations, special, extra-fast processes kick in, bypassing typical neural circuits in order to activate muscles as quickly as possible. Humans have something like this, too: think of the last time you automatically jumped when you were startled by a loud sound, before you quite realized what had happened.
For some insects, favorite colors fall into this special, shortcut category. In fruit flies, as in humans, color vision arises from comparisons among different kinds of photoreceptors. But a few recent studies have found that flies also have a special brain circuit that exists only to guide them toward UV light. Although flies have two different kinds of photoreceptors that are tuned to slightly different colors of UV light, both of these photoreceptor types feed indiscriminately into this special circuit. So, while some information from these UV-sensitive photoreceptors is being shuttled into the normal circuit for comparing photoreceptor signals and identifying different colors, other information is being trafficked into a separate, color-blind pathway. Instead of trying to figure out what colors the fly is looking at, this circuit is asking a different question entirely: is there UV light present or not? It’s as if light entering the eye is a color palette; some of the pigments are mixed together to form new shades that go into making a painting, but when certain, special colors (in this case, UV) appear on the palette, a bit of that pigment is set aside before it gets mixed with anything. And when that special pigment is around, the fly is drawn toward it.
Scientists still don’t know how common this shortcut pathway for identifying colors is. However, it’s unlikely to be exclusive to flies. Mantis shrimp, for example, don’t seem to make any comparisons between their photoreceptors, instead relying exclusively on shortcut circuits for all of their color vision. Scientists also remain in the dark about what exactly makes humans like the colors they do, but it will probably be more complicated than the hardwired circuits of insects; after all, all fruit flies like UV, but humans have idiosyncratic color preferences. Still, a specialized brain circuit, like the one that draws flies toward UV light, may be at play in our responses to things that humans find intrinsically scary, like sudden, loud sounds or unexpected movements. Regardless, understanding the way flies perceive the world might teach us something about the way the brain weaves together packets of information to form the tapestry that is our visual awareness of the world.
Michelle Frank is a Ph.D. Candidate in Neurobiology at Harvard Medical School.
Abagail Burrus is a third-year Organismic and Evolutionary Biology Ph.D. student at Harvard University who studies elaiophore development.