Why no blue human hair?

Blue is actually a big deal in the animal world. Apparently blue is a hard color combination to come by. Male bower birds are always on the lookout for blue objects when building their bowers.

The biggest problem is that humans only have a single pigment: Melanin, a dark brown pigment. You can get colors from yellow to red to brown to black, but no blues, greens or purples. (Well, not entirely true. There’s a few carotid pigments that are yellowish in color. Lipochrome is the one I can think of, but I don’t know any blue or green ones.)

Even eye color is done by melanin. The irises are actually a very light reddish brown (pink almost), but appear bluish due to Rayleigh scattering. Blue and gray eyes simply have less melanin-type pigment than brown eyes which is why they’re recessive.

Melanin is an excellent pigment because it protects against UV rays which is why brown and black tones are such popular colors in the animal kingdom – especially among mammals.

Maybe another reason is that mammals lost their ability to see much color during their early evolutionary history. Early mammals were nocturnal, and if you can shove more rods cells in your eye at the cost of some of the color seeing cones, you get better night vision. Having bright colorful coats doesn’t do mammals a lot of good. They make you easy to be seen by predators (birds and dinosaurs could see color), but they don’t impress the mammal babes.

Only old world monkeys gained the ability to see color, and that may explain why some old world monkeys have bright blue and red coloring in their faces. However, it appears these pigments never made it into the great apes.

Do any mammals have green or blue fur? I can’t think of any.

Interestingly, the blue in bird feathers isn’t a blue pigment, it is also an optical trick for a darker color.

Hair color from black to platinum blond can be provided by varying the amount of melanin. That’s one pigment. Red hair is the oddity. I rather expect that knowing that would help in understanding why we don’t have blue or green.


I could have sworn I’ve seen a picture of a pygmy marmoset that was green but after searching again I can not find it so maybe it was just the particular lighting of that picture.

And here are some pictures of the cute little buggers.


Some sloths can be greenish in color, but only because they grow blue-green algae in their hair. They don’t grow the algae because they move so slow, but because it help with camouflage.

Is this true of the bright blue you see in blue jays? I know it’s true for the shimmer you see in certain birds and butterflies, but I don’t know if it’s true for the solid bright blue color.

That red color is also a form of melanin called pheomelanin.

You make it sound like they do it on purpose. :dubious:

Well, you could argue that in some anthropomorphic sense sloths don’t do anything “on purpose”, including such seemingly purposeful activities as eating or mating. I don’t suppose sloths are particularly reflective creatures, and they probably lack the intellectual capacity to formulate such abstractions as “Boy, I’m hungry. I could sure go for some of the buds, tender shoots, and leaves of a tree of genus Cecropia right about now”; or that they write sonnets (or catchy pop songs) about their desire to make sweet, sweet love to lady sloths.

However, eating and mating are clearly adaptive behaviors of the sloth, and most people recognize them as being in some sense “deliberate” on the part of the sloth (as opposed to such behaviors as being electrocuted by power lines or being eaten by jaguars). Evidently, having blue-green algae grow in their fur is also an adaptive behavior (it helps camouflage them), and their fur has thus evolved to provide a particularly cozy place for blue-green algae to grow in (as opposed to them having blue-green algae grow in their fur just because they’re too darned lazy to attend to personal hygiene in good primate fashion; or because the deadly blue-green algae has evolved to the point where it currently has the upper hand in the eternal war of algae and sloth).

But do they actively apply blue-green algae to themselves, or does it sort of just accumulate because it can? That’s the difference between “on purpose” or not.

qazwart, here you go:

Yes it is. My theory, no cite, is that the elements that are blue, and would contribute to any truly blue pigment–cobalt, copper (when oxidized) are toxic. Anyone know if there are any naturally occurring blue minerals that are not toxic?

The bluest paints are made with cobalt, but they’re very poisonous.

Cool, so that’s where “email” comes from.

How then do they manage blue food coloring?

Gotta love it when things like this interrelate. :smiley:

Of course, ingesting silver can lead to Argyria (a bluish skin), but silver is hardly abundant enough for a species to rely upon metabolising it.

When I said that the sloths grew the algae in their hair I didn’t mean the sloths did this on purpose. In fact, I never said the words on purpose. Here’s the original quote:

I wanted to make it clear that the algae grew in the sloths’ hair because it was designed that way. The sloths hair on their back and shoulders is hollow which is a great place for the algae to grow. It’s not that sloths are so slow and slothful that they get covered with spiderwebs and algae as if they were tree stumps.

And, I mean designed not that the Grand Overall Designer designed it that way, but it evolved that way through natural selection. Sloths that had algae growing in their hair and blended better with the green foliage survived better than sloths that didn’t have the greenish tint to their fur.

The sloths get the algae in rainstorms. The algae lives in the forest canopy, and is washed down upon the sloths in the rain.

Except that you don’t know that it exists because it helps with camouflage. All you know is that they grow it, and that it makes them harder for us to see. You don’t have any indication that they competed with non-algae growing sloths, even. For all you know, they’ve been growing algae since the first sloth climbed into the trees in that area. And since the algae is washed down upon the sloths in the rain, it’s washed down upon other species as well. Why are THEY not sporting agae camouflage? Because they move too fast or too vigorously for it to grow? :wink:

First off, I don’t know if anyone has come up with any clever ways to test the hypothesis that sloths have evolved to “purposefully” cultivate the algae in their fur in order to aid in camouflage. But as to the question “Why aren’t other animals sporting algae camouflage?”–you could also ask why don’t sloths just sprint really fast and outrun the jaguars, or grow big spines on their backs, or grow wings and fly away. There are lots of ways to be successful in the great game of life; sloths do so by being slow, not using much energy, being hard to see, and as a last resort having pretty powerful claws.

(You could test whether or not the algae actually aids in camouflage: Get a bunch of sloths and tag them all with radio collars so you can keep track of them. Give half of them a good shampooing. See if the now algae-free sloths are more likely to be eaten by jaguars. Probably give the control group a good rub-down with something that doesn’t actually get rid of the algae, just in case the whole experience of being captured and messed around with by scary fast bipeds freaks out sloths so badly they become suicidally depressed and deliberately seek out jaguars in order to end it all. Whether nor not anyone has bothered to do this I don’t know, and even if experiment showed that algae actually aid in camouflage I guess you could still argue over whether this is just a lucky break for the sloths or something they have actually evolved to encourage.)

lissener said:

:confused: What does email have to do with my post or this thread?

DSYoungEsq said:


qazwart said:

I don’t know why you’re quoting me, but DSYoungEsq is the one who brought up “on purpose”, indicating that your wording suggested such. I was actually responding to MEBuckner.

Did you actually read the JSTOR article you linked to? (Which, by the way, is not a very good idea – normal human beings have to pay to use JSTOR, unless they have access to a university library.)

Ah. I read the first paragraph, which answered the question asked.

John W. Kennedy said:

I did a google search on the cause of blue color in birds and posted the page that displayed that stated that cause. There’s no need to actually read the article, is there?

You are joking, right? You do realize that reading a summary paragraph in an abstract of an article is hardly the equivalent of reading the article to understand what it actually concludes, the basis for that conclusion, and the validity of making that conclusion? :smack: