Biochemical plausibility of blue(-gray) fur?

Question

I want some critters to have gray-blue fur (specifically, colors like Alice blue, cadet gray and marengo).

I know there are blue schemochromes, but I'd prefer getting there by absorption rather than light-scattering "trickery". (Absorption is more consistent under a wider array of light conditions. Structural colors can "disappear" or look totally different depending on how they are lit, even more so than absorptive pigments.) Are blue biochromes possible?

Please ignore how/why they came to be (this will be hand-waved with "genetic engineering" or some such); I just want to know whether such pigments could exist without violating the laws of physics, or necessarily incorporating elements that a living organism could never obtain in the requisite quantities, or would necessarily be harmful to an organism that has them, etc.

Other notes:

• The critters are mostly identical to Earth mammals. Similar gross traits, similar basic biology. (This is roughly an alternate reality setting that is Earth-like aside from some minor differences... such as mammals with blue(ish) fur.)
• I'd prefer the color to not be subject to dietary variation. (In other words, I want my blue critter to be always blue, not sometimes blue and sometimes some other color depending on what it's been eating recently.)
• I don't need vibrant blues, greens, or purples. If there's a logical reason why those wouldn't occur, even better, but it's okay if I just need to just hand-wave that they don't.
• Environmental factors (e.g. lots of organic copper floating around?) are okay as long as the environment can still appear to be Earth-like to a layperson.

Answer 1

Answering this on the clarification in comments by OP that:

the setting is an alternate reality and more fantasy than sci-fi; "Earth-like except as noted". The animals in question are "mostly" identical to real animals except for specific, limited differences; sapient, bipedal... and sometimes blue(ish gray). Less Star Wars, more Zootopia.

Other people have already pointed out that real world birds, plants, etc produce blue pigments.

I'll add an alternative Earth spin to this: way back in primordial history, the ancestors of the vertebrates didn't use hemoglobin (iron based) as the respiratory pigment in their blood, they used Hemocyanin, which is copper-based.

Hemocyanin is not as good as hemoglobin for transporting oxygen around the body, so you cover that with the fantasy aspect of your setting, and hand-wave it away.

Hemocyanin is blue when oxygenated and colourless when deoxygenated. If you google 'horseshoe crab blood', you'll see the blue-grey colour of hemocyanin. I've seen fresh squid blood and it is a similar, but paler, grey-blue.

Also have a look at copper-based minerals to get an idea of the range of possible colours of copper-based substances. The exact shade is about oxidation state of the copper and what other elements it is combined with, so some of the colours will be possible to duplicate in biological pigments. (Others may be hideously toxic!)

You can then get blue fur by these methods:

1. Because copper is dangerous as well as beneficial. Your body needs it, even if you don't have hemocyanin blood. But it needs to regulate the amount very carefully, as excess copper is toxic. So what to do if with something that you can't survive without, yet will cause problems if you have too much just slopping about your body? Answer: store the excess somewhere that is biologically inert. Dump it into hair follicles to be incorporated into fur and hair. You can also put it into claws/nails. Then if you need some more copper, store less in the fur and/or lick yourself and don't spit up the hairballs.
2. Having blue blood will alter the colour of your skin, unless the skin is chock full of pigment to hide the colour of the underlying blood. I'm white. If my blood turned blue/clear instead of its normal bright red/dark red, then my normal shade of pinkish-beige would be drastically altered. So if any of your animals have sparse fur, which lets the colour of the underlying skin show through, then you might be able to generate a range of other shades. This one, obviously, isn't blue fur as such, but is fur which looks blue, because the observer sees the combination of the blood pigment, skin pigment (if any) and fur pigment.

I'm hoping this doesn't violate your "not in the diet" rule, because they have the hemocyanin every day of their lives, not just on a special diet. So there is a dietary aspect, just like you will become anemic if you don't have enough iron in your diet.

EDIT: to include thoughts about individual variation in fur colour within a species.

We know from real world domestic mammals such as horses, rabbits, dogs and cats that a multitude of coat colours (and patterns) is possible. So mammals can do that. BUT mankind has interfered in the evolution of those species, and many of the coat colours only exist because when a mutation cropped up, some human went "Ooooh, pretty! Mum, I waaaaaaant one!" :-)

I've been racking my brains trying to come up with wild animals which have a lot of variation in fur colour (not pattern), and come up with... the brown bear (Ursus arctos). And, um, the black bear (Ursus americanus). Both of which can be a range of shades from a creamy white to a very dark brown (brown bear) or black (black bear).

This lack of colour variants in most species is a simple one: if you stand out from the crowd or from the background, you die. Especially if you are prey and what is hunting you has fantastic colour vision (birds of prey or people); or if you are the predator and you stand out like a sore thumb. These Kermode white bears are better at catching salmon than their black comrades, because the salmon can see the black bears more clearly.

However, the the OP mentioned Zootopia in comments as an inspiration. If his critters are intelligent and civilised, all bets are off. If civilisation, even a stone age one, has reduced your change of being nailed by a predator if you stand out, or has let you invent sneaky ways of catching prey (like fishing nets), then what colour your fur is becomes a lot less important. Local variation, family variation and individual variation can become the norm. If some colours are considered sexy, or lucky, or religiously significant, then bears with those fur colours may become more common.

Fur colour variation in individuals can happen for this reason: many coat colours, like black, are not created from a single pigment. Black pigments are expensive to make, so a black animal's blackness is actually a combination of various black, brown, red and yellow pigments. The black animal has the genes to make all of these pigments. Some of its descendants inherit some of the genes but not the others, so the descendants can make hair colour all of the natural shades of human or bear hair. This is called polymorphism.

The same could be true for the copper-based pigments: Lots of genes for lots of different ones.

So you could have a population of alternative-world bears who have collectively:

1. Up to 10 different 'normal' pigment genes to make fur from albino (none) to black (all 10 pigments).
2. Up to 10 different 'copper' pigment genes to make fur from albino (none) to intense blue-green (all 10).

Any individual animal will have inherited some of these 20 genes from its parents, perhaps all of them. Some genes will be dominant, others recessive, others will show incomplete dominance (blend the colour effect). Some genes will be common, others will be rare. The mix of who gets what will result in a range of coat colours in the alternative-world bears.

Plus you can get other effects like roan animals which have white and another colour of hairs mixed together.

There is however the issue of what the animals themselves can see. Humans and other primates are unusual in that we have good colour vision. Most mammals - particularly nocturnal species - have limited colour vision. Horses, for instance are fantastic at distinguishing different shades of yellow and green, so will be able to spot subtle differences in just how 'cream', 'beige' or 'yellow' a palomino horse is. But they might be utterly rubbish at telling shades of chestnut, bay and brown apart, and thus also bad at telling apart fantasy horse colours like shades of purple.

Answer 2

No reason why not - Well, sorta:

So I'm assuming you are specifically talking about Mammals. Mammals have great reasons not to be colorful - they are mostly colorblind compared to other species. Look HERE for a great explanation why. Primates tend to be less colorblind than most mammals. Just because mammals mostly use pigment (and then only melanin), doesn't mean they can't use diffraction and other alternate pigments like bird feathers. There is just little reason for them to, and mostly they lack genes to make them (but given evolutionary pressure, they could evolve them or have the genes inserted). Hair and feathers both derive from scales. All the visual tricks allowing FEATHERS to be colorful can be used by mammals as well.

Blue pigment is super-rare but not impossible, mostly by being a rare butterfly or maybe eating them (even then, not often). Mandrills achieve blue but not with a pigment. Even bird use tricks.

Answer 3

There is a really simple way for aliens (I'm assuming you're not talking about future humans) to have evolved colored fur: have them be tri- or tetra-chromats. Most mammals are dichromates (that is, they have only two kinds of color receptors in their eyes), so they only see a limited range of colors. Primates (including humans) evolved tricromatic vision only fairly recently (in evolutionary terms).

Coat colors serve two primary functions: camoflage and sexual advertising. If you're a dichromat, the various shades & patterns that can be produced by melanin &c are sufficient for camoflage. Brilliant colors would be of no use for sexual advertising, since your preferred sex partners couldn't see them.

If the aliens have a long evolutionary history of tri- or tetra-cromatic vision, though, they will be able to see multiple colors, and plausibly could evolve various fur colors in response. We need only look at birds to see this in action. Most have tetrachromatic vision (being able to see ultraviolet as well as the red-green-blue that humans see). So while you have a number of rather drab brown birds, you also have a large number - from peacocks to parrots - that have evolved brilliant colors. Given their UV vision, even colors we can't see. Some of those drab brown birds are not all that drab in the UV spectrum: https://academic.oup.com/bioscience/article/50/10/854/233996

Answer 4

EDIT: this was posted before the question was changed to rule out dietary effects.

This is surprisingly easy once you understand “pink” flamingos, without evolving new biochromes or resorting to schemochromes.

Like the kidneys, our sweat glands, hair follicles and nail cuticles are a dumping ground for anything the body wants to get rid of. This is why long-term drug testing can be done with hair or nail samples: they contain the exact same metabolites as urine, but retained for weeks or months rather than hours or days.

So, what you need is a way for your animals to tolerate eating certain materials in high enough concentrations (which may be poisonous to us) that when they’re excreted in the hair follicles, their density is high enough to create the color you want.

This also easily delivers different colors while still being interfertile: each individual would have unique fur color/ticking based on their recent personal diet, rather than any genetic difference. In fact, they may even alter their diet for that purpose, resulting in some interesting social conventions like singles eating more colorful foods and marrieds eating less colorful foods. Or religions that require/ban certain foods and thus colors.

Why would this have evolved? If your fur adapts (however slowly) to the colors of your environment, that provides great natural camouflage even for migratory animals, rather than needing different species with different fixed camouflage for each environment.

Answer 5

It's plausible in certain cases, certainly it's possible given slightly different diets and environmental constraints. For instance, the flamingo is a bird whose pigment is white, but whose feathers turn pink when on a diet of certain shrimp that are prevalent in their ecosystem niche. Polar bears can live in warmer climates, however they sometimes suffer from mildew growth withing their normally clear hair follicles that turns them green. A prevalence of aluminum in the human diet is (poisonous) but also can impart a green color to hair. Eating plants with large amounts of beta carotene can produce a brilliant orange color in skin and hair follicles without significant negative effects.

All you'd have to do in your story is explain how alternate biology or ecological niche allows your creatures to eat foods that might be poisonous to animals today, while imparting specific colors. Alternatively, you could base their colors on iridescence, such as is seen in butterfly wings; although that line of reasoning may be more difficult to explain, due to the more sensitive nature of iridescent scales that may be rubbed off or damaged on large mammals. Either way, you have a biologically plausible explanation for various colors that could be present under the specific ecological niches of your world.

Edit: skin is more easily explained. Already there exist cases in biology, such as the wattle of a turkey or a chameleon, where skin can change color to match the surroundings, and can vary widely from blue to green to even bright red.

Answer 6

Is blue pigment possible, Yes

Carotenoprotein complex include a variety of blue pigments and are common in marine invertebrates.

Obrina Olivewing butterflies also have true blue pigments.

Is it possible to naturally get in anything we would call a mammal, No

Mammals don't have blue pigments so you either have to use magic (in which case no science matters), Genetic engineering, (in which classification starts to breakdown) or make them aliens, (in which case they are not mammals).

Answer 7

Probably?

Well... schemochromes can certainly produce vibrant blues. This isn't exactly what we're after, but maybe mixing just a little of these with black/gray would do the trick. This approach may be best for "cool white", as our target color there is essentially "the color of icebergs", which is a color that occurs due to structural effects.

But... we want biochromes. Do those exist? Well, fortunately, yes as other answers observe. Although "blue" plants are mostly colored by anthocyanin and tend a bit more toward indigo than we might like, mixing these with yellow might achieve the desired result. The same can be seen in butterflies, which can produce green. Azulene also seems very promising, particularly for lighter shades. Then there is Prussian blue, which has a chemical formula that seems at least vaguely plausible to produce biologically.

Even more interesting however is the recently developed beet blue dye. Again, whether this could be biologically produced is outside my wheelhouse, but the color it produces when used to die hair (which is what we're trying to accomplish, after all!) is almost dead on. Again, if anything, it's too vibrant, but if we postulate less of the blue pigment and hair was otherwise a darker shade of gray, this probably gets us where we want to be. (In fact, woad and indigo (the plant) also are used in the production of dyes that seem suitable. The former is not very UV-stable, which might result in interesting sun-bleaching for critters that are exposed to more sunlight, which could be interesting; tan-lines with fur! Both, despite the name of the latter, produce a dye that is very close to the desired "between blue and cyan" target, rather than being between blue and violet like the modern color known as "indigo".)

If anything, the biggest problem may be justifying why azure coloring exists while indigo coloring doesn't.