Why are Mammals Brown? (pt. 1)

We don’t naturally look like this

Compared to colorful fish, lizards, birds, and even ladybugs, we mammals are downright drab. I see no particular environmental reason for this–plenty of mammals live in areas with trees or grass where green fur or spots might help them blend in, or have such striking patterns–like a zebra–that I hardly think a blue stripe would result in more lion attacks.

I think there are two main reasons mammals are mostly brown, instead of showing the vibrant colors of other species:

1. Some colors are difficult to produce.

Blue, for example. Walk into the forest or a meadow on an average day, and you’ll see a lot of green. Anything not green is likely brown. Outside a garden, there are very few naturally blue or purple plants.

This guy, however, does

It’s no coincidence that early human art uses colors that could be easily produced from the natural environment, like brown, black, (charcoal,) and yellow. By the Roman era, we could produce purple dye, but it was so hard to obtain from such rare sources (shells) that it was prohibitively expensive for mere mortals, hence why it was called “royal purple.” The European tradition of painting the Virgin Mary’s cloak blue also hails from the days when blue pigments were expensive, and thus a sign of exalted status.

A purple dye cheap enough for average people to buy and wear wasn’t invented until 1856, by William Henry Perkin.

I’m not sure exactly why blue and purple are so hard to produce, but I think it’s because light toward the violent end of the spectrum is higher energy than light toward the red end. As Bulina et al state:

Pigments in nature play important roles ranging from camouflage coloration and sunscreen to visual reception and participation in biochemical pathways. Considering the spectral diversity of pigment-based coloration in animals one can conclude that blue pigments occur relatively rare (as a rule blue coloration results from light diffraction or scattering rather than the presence of a blue pigment). At least partially this fact is explained by an inevitably more complex structure of blue pigments compared to yellow-reds. To appear blue a compound must contain an extended and usually highly polarized system of the conjugated π-electrons.

Okay… So, because blue and purple are more energetic, they require molecules that have more double bonds and are less common in nature. (Why double bonds are less common is a matter I’ll leave for a chemistry discussion.)

You’re probably used to thinking of color as an inherent property of the objects around you–that a green leaf is green, or a red bucket is red, in the same way that the leaf and bucket have a particular mass and are made of their particular atoms.

Low energy to the left, high to the right

But turn off the lights, and suddenly color goes away. (Mass doesn’t.)

The colors we see are created by light “bouncing” (really, being absorbed and then re-emitted) off objects. Within the visible spectrum, red light requires the least energy to produce (because it has the widest wavelength,) and violent takes the most energy.

But nature, being creative, has come up with alternative way to produce blues and purples that doesn’t depend on electron energy levels: structure.

Unless you are a color scientist you are probably accustomed to dealing with chemical colors. For example, if you take a handful of blue pigment powder, mix it with water, paint it onto a chair, let it dry, then scrape it off the chair, and grind it back into powder, you expect it to remain blue at all stages in the process (except if you get a bit of chair mixed in with it.)

Blue Morpho butterfly

By contrast, if you scraped the scales off a blue morpho butterfly’s wings, you’d just end up with a pile of grey dust and a sad butterfly. By themselves, blue morpho scales are not “blue,” even under regular light. Rather, their scales are arranged so that light bounces between them, like light bouncing from molecule to molecule in the air. Or as Ask Nature puts it:

Many types of butterflies use light-interacting structures on their wing scales to produce color. The cuticle on the scales of these butterflies’ wings is composed of nano- and microscale, transparent, chitin-and-air layered structures. Rather than absorb and reflect certain light wavelengths as pigments and dyes do, these multi scale structures cause light that hits the surface of the wing to diffract and interfere.

The same process is at work in the peacock’s plumage and bluebird’s blue:

Male eastern bluebird

Soft condensed matter physics has been particularly useful in understanding the production of the amorphous nanostructures that imbue the feathers of certain bird species with intensely vibrant hues. The blue color of the male Eastern bluebird (Sialia sialis), for example, is produced by the selective scattering of blue light from a complex nanostructure of b-keratin channels and air pockets in the hairlike branches called feather barbs that give the quill its lift. The size of the air pockets determines the wavelengths that are selectively amplified.

When the bluebird’s feathers are developing, feather barb cells known as medullary keratinocytes expand to their boxy final shape and deposit solid keratin around the periphery of the cell—essentially turning the walled-in cells into soups of ß-keratin suspended in cytoplasm. Next, b-keratin filaments free in the cytoplasm start to bind to each other to form larger bundles. As these filaments become less water-soluble, they begin to come out of solution—a process known as phase separation—ultimately forming solid bars that surround twisted channels of cytoplasm. These nanoscale channels of keratin remain in place after the cytoplasm dries out and the cell dies, resulting in the nanostructures observed in the feathers of mature adults.

“The bluebird doesn’t lay down a squiggly architecture and then put the array of the protein molecules on top of it,” Prum explains. “It lets phase separation, the same process that would occur in oil and vinegar unmixing, create this spatial structure itself.”

The point at which the phase separation halts determines the color each feather produces.

Decades old Pollia fruit retains its structural brilliance

This kind of structural color works great if your medium is scales, feathers, carapaces, berries, or even CDs, but just doesn’t work with hair, which we mammals have. Unlike the carefully hooked together structure of a feather or the details of a butterfly’s scales, hair moves. It shakes. It would have to be essentially solid to create structural color, and it’s not.

So for the most part, bright colors like green, blue, and purple are expensive, energy-wise, to produce chemically, and mammals don’t have the option birds, fish, lizards, and insects have of producing them structurally.

To be continued…