In Mammal Fashion

We humans are obsessed with our plumage. We spend hours tending our carefully styled hair, we endlessly mix and match the colorful costumes we call clothes, and we worry constantly about how other people perceive our appearance. However, underneath all these layers we are just another drab mammal.

I started thinking about today's topic for two reasons. In response to my recent newsletter on pumas, several people commented that one of the photos looked like a puma had killed another puma because the fur on the hunter and prey was the same color. And last night I gave a talk on bird coloration and how birds live in a world of bright color.

While the birds, fish, insects, reptiles, and amphibians of the world have wildly different and often brilliant colors, the mammals of the world are notable in being uniformly drab. Nearly every mammal is some shade of brown or gray, often with darker hints of black or warmer red and orange tones, created by a single pigment called melanin (compared to more than a dozen pigments that create colors in birds).

Why is this?!

A big part of the answer has to do with the fact that mammals first appeared during the time of dinosaurs. These original small and furtive mammals needed to avoid being eaten by dinosaurs that had excellent color vision and hunted in the daytime, so these mammals became nocturnal or moved underground.

This scenario lasted for around 100 million years, long enough that mammals lost much of their need or ability to see color and ultimately lost the ability to produce dynamic colors in their fur or skin. To this day the vast majority of all mammals are still nocturnal or live underground, and with few exceptions can only see two colors (red and blue).

Compare this to birds, fish, insects, reptiles, and amphibians that see four colors (red, green, blue, and ultraviolet) and live in a world of color. However, the mammals we call primates (including humans) eventually became diurnal and moved into trees to eat colorful fruits, so they regained the ability to see three colors (red, green, and blue) and now respond to color in ways that no other mammals do.

Living in the dark (or underground), and not being able to rely on color, means that mammals instead developed an excellent sense of smell and hearing to communicate with each other rather than using color.

Because mammals are mostly earthbound, with a limited ability to move quickly, they use drab colors to conceal themselves—whether they are hunters or the hunted. On the other hand, birds get away with wearing flamboyant colors because they can easily escape predators.

The limited ability to move great distances also shapes mammalian mating systems and the colors they display. Because mammals stay in relatively small areas it means that a male doesn't have to worry about a female wandering around looking at lots of fancy (i.e. colorful) males and picking the one she likes best, which means that mammalian males don't face any pressure to evolve more and more colorful displays over time.

Instead, mammals are left with the handful of colors produced by melanin pigments (the same pigments that color our own skin and hair). While offering a limited palette of colors, melanin does provide some key benefits: it absorbs many wavelengths of light and protects skin from the sun's damaging rays, it acts as an antioxidant and sequesters harmful trace elements, and it has antimicrobial properties. Melanin is also helpful because it absorbs the sun's heat and helps animals stay warm.

On a final note, humans might feel self-conscious about being just another drab mammal, but if it's any consolation, we do have one spot of true color on our bodies...

It's the spot we are instantly drawn to when we look at each other—our eyes.