Why are Mammals Brown? (pt. 2)

Rainbow leaf beetle
Rainbow leaf beetle

As I was saying in part 1, compared to colorful fish, lizards, birds, and even ladybugs, we mammals are downright drab. Blue and purple fur are non-existent because these colors are difficult to produce as pigments, and so most animals with these colors produce them structurally rather than chemically, but hair is not a good medium for structural color. We are limited to pigments.

But this only explains blue and purple. Why are there so few mammals with bright red, pink, orange, or green fur? Wouldn’t green offer convenient camouflage for tree-dwelling sloths or lemurs? So on to the second reason we’re drab:

mantis_shrimp_12. Compared to other animals, mammals have bad color perception.

For example, according to the guy who writes The Oatmeal, which is totally a reputable scientific source, dogs can only see two colors, blue and green. Humans can see three colors–green, blue, and red–which we combine to make the rest of the colors we see. Butterflies, non-mammals, can perceive 5 colors–we have no idea what that actually means, since we can’t even imagine the colors they see. And the mantis shrimp perceives an incredible 16 different colors.

The majority of mammals run closer to dogs than humans in color-perception.

But this only inspires a new question: why do we have bad eyesight?

The original mammals were small, shrew-like creatures that tried to avoid being eaten by dinosaurs back in the Triassic, about 200 million years ago.

Read the full comic over at The Oatmeal
Read the full comic over at The Oatmeal

Lizards, being mostly cold-blooded, are forced to be active primarily during the day, when it’s warm. Our warm-blooded ancestors therefore probably found it easy to avoid reptilian predators by doing their hunting and foraging at night.

According to Wikipedia:

The nocturnal bottleneck hypothesis is an hypothesis to explain several mammal traits. The hypothesis states that mammals were mainly or even exclusively nocturnal through most of their evolutionary story, starting with their origin 225 million years ago, and only ending with the demise of the dinosaurs 65 millions years ago. While some mammal groups have later evolved to fill diurnal niches, the 160 million years spent as nocturnal animals has left a lasting legacy on basal anatomy and physiology, and most mammals are still nocturnal.[1]

Between the nocturnal and the crepuscular, most mammals are only awake at times when color isn’t particularly relevant. Most mammals, therefore, have evolved eyes that aren’t very good at perceiving color, in order to optimize for seeing in dim light.

We have more rods, which perceive light; diurnal animals have more cones, which perceive colors.

Animals use their colors for three main purposes: to signal to each other, to hide, and to signal to predators.

Since most mammals can’t see many colors, even if they had a peacock’s spots, they couldn’t use them for mate selection. Few mammals are poisonous (if any,) so we don’t have the poison dart frog’s use for bright color. And you might want to be green to blend in with the trees during the day, but at night, trees are dark.

In short, we are optimized for the dark.

So even though we humans like being awake during the day, we’re unlikely to trade in our drab pelts for the macaw’s rainbow hues anytime soon.



12 thoughts on “Why are Mammals Brown? (pt. 2)

  1. Based on reading wikipedia and googling, apparently different mammals have retained different cones, starting with Red, Green and two types of blue for amniote ancestors of mammals, with early mammals losing green and then different mammals losing a different blue. Monotremes keeping one type while placental and marsupials keeping the other. Then marsupials (they may actually have never lost their green cones) and primates evolved green cones making their vision trichromatic. While ceteaceans and pinnnipeds lost their remaing blue cone making their vision monochrotic.

    I understand why marine mammals lost their colour vision (contrast to help see through murky waters) but I wonder why some primates and marsupials (?) evolved green cones again though? Some answers for primates I’ve found are they are more likely to be diurnal, it helps id fruit/leaves and abilty to see blushing.

    Still enjoyed the post, evolution is very interesting. Also I found out recently that Echidnas evolved from a semi-aquatic Platypus-like animal, so monotremes probably survived because marsupials couldn’t fill a aquatic niche due to young drowning compared with egg laying monotremes.


  2. Back in the 70s while I was at Union College, I had a biologist colleague who was interested in the vision of insects and birds. He used a tv camera to record flowers and other plants because the tv cameras then in use recorded will into the ultraviolet. Most flowers look quite different in the uv, and even some drab looking stuff (to us) really stand out in uv.

    By the way, this is why crt monitors in tv shows look blue. The tv cameras used to record the scene are responding to the uv emissions and dumping the uv output into the blue pixels.


  3. This wins the “most interesting thing I’ve read in a while” award. I wonder if we couldn’t CRISPR the ability to see more colors into ourselves and what that would do to status signaling.


  4. If you figure that most species of mammals are either rodents or bats (we’re in the bats’ clade, more or less), it makes sense that we’re adapted for smelling and listening more than seeing.


  5. Chickens have five-color vision, so instead of a one-dimensional “rainbow” of hues, they see a three-dimensional hue-space (that is, not counting brightness and saturation).

    The downside of this is that they are struck blind the moment the sun dips below the horizon. When I can still see color and read fine print, they can’t see a crippled fly hobbling around on the palm of my hand, a treat that in good light would be instantly devoured.


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