Animal Senses #4: Color Vision

Color vision doesn’t come from just the ability to detect different wavelengths, writes Ed Yong in Immense World.

“It is about comparing them.”

The signals from the three types of cones (Red, Green, Blue aka RGB) are added and subtracted by a complex of neurons. This “arithmetic” is called oppomency and is the basis of all color vision. Since different animals do the arithmetic differently, their sensation of color varies.

 

Remember those super-colorful fish from movies like Finding Nemo? Why on earth are they so colorful? Wouldn’t they be conspicuous to their predators? Aha, while it may be uber-conspicuous to us humans, they are perfectly camouflaged wrt their predators. Remember, the sensation of color is subjective.

 

Scientists often use this subjectivity of color perception in two fascinating ways. Say, Species 1 preys on Species 2. The colors of Species 2 would probably be colors that Species 1 cannot see. Or if Species 3 wants to attract Species 4 (e.g. flowers want to attract bees), then it very likely that the colors of Species 3 can be seen by Species 4.

 

Some species like armadillos don’t have any cones (those RGB detectors), so obviously they can’t see in color. Others have only one cone, and since color vision depends on addition and subtraction, with one cone they can only see in one color (monochromats). Octopuses are one such example. Ironically then:

“They can rapidly change the colors of their skin yet are unable to see their own shifting hues.”

The existence of so many monochromats is proof that color vision isn’t necessary. Why then do so many species have color vision at all?

 

One theory is that color vision evolved to solve the problem that sunlight doesn’t fall on things at a constant rate. Light flickers in the water; shadows fall on objects; the clouds can block the sun… you get the idea.

“Monochromatic eyes that only deal in brightness and darkness would struggle.”

But add color vision and you see the benefit – a leaf still looks green, even with reduced light. A strawberry still looks red. And so on.

“Color – and specifically color vision with oppomency – offers consistency.”

Knowing what hasn’t changed is very important – it allows the animal to focus on what did change. A predator’s movement. A prey’s scurrying.

 

Not surprisingly, we’ve found species that are dichromats (two colors) and trichromats (three colors). The benefit of seeing another color is multiplicative – 100 shades of grey (monochromat) X 100 shades of blue (dichromate) = 10,000 colors of vision. A trichromat, by the same logic, can see 10,000 X 100 = 1,000,000 (million).

“Each extra (color) increases the visual palette exponentially.”

You’d think seeing more colors is better than seeing fewer. Not necessarily. Yes, you can spot and differentiate more things. But, on the other hand, more colors can be distracting and lead to wrong interpretations at times. Ironically, camouflage is less effective if the predator can see fewer colors.

 

Birds and bees are tetrachromats (four) – they can see UV (ultraviolet). Flowers use “dramatic UV patterns to advertise”. A sunflower looks uniform to us (trichromats), but it has multiple colors from a bee’s perspective. Advertising aside, the extra colors indicate where the nectar lies.

 

People often wonder if we can engineer goggles to “see” the extra UV colors (remember, with UV added, the number of colors increases to 1,000,000 X 100 = 10,000,000 – that’s 100 times more colors than we can see). Sadly, no, that’s not possible. Because while the goggle could capture the UV signal, it would still need to render the 10,000,000 colors onto the 1,000,000 color palette that we see.

“Four into three just won’t go.”

 

Can any species see X-rays? Unlikely, feel scientists, because most of those rays are absorbed by the atmosphere and wouldn’t even reach the earth to be reflected. From an evolutionary point, there would be no point of seeing X-rays. How about the other end of the spectrum, say microwaves? They would have very little energy, which means the photoreceptors would have to be uber-sensitive to even tiny triggers of activation. While possible, the side-effect would probably make it very unappealing – if activated by such triggers, the photoreceptors would trigger too many false alarms at other wavelengths.

“There’s only a narrow Goldilocks zone of wavelengths that are useful for vision.”

 

Remember polarized light? Simple explanation – as light moves, it oscillates in any direction that is perpendicular to the direction of travel. But when the oscillation is restricted to only one plane of oscillation, it’s called polarized light. We humans can’t see polarized light, but most insects, crustaceans and cephalopods can see it. (They have photoreceptors that can detect light on two mutually perpendicular axis, say, horizontal and vertical. By comparing the signal from the two, they detect polarized light). Then there’s circular polarization, where the light is polarized but its plane of polarization rotates as the light moves. Some species can even detect circular polarization, though it is very rare.

 

No wonder then that color vision merited a separate chapter in Yong’s book.