Ever since I found out about the green blooded skink, I have wondered if it's possible for other blood colours to evolve among vertebrates, specifically blue in mammals.

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    $\begingroup$ Possible duplicate of Other blood colors $\endgroup$
    – dot_Sp0T
    Commented Nov 17, 2018 at 9:13
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    $\begingroup$ en.wikipedia.org/wiki/Channichthyidae $\endgroup$ Commented Nov 17, 2018 at 10:41
  • $\begingroup$ An interesting read on blood colors: compoundchem.com/2014/10/28/coloursofblood $\endgroup$
    – MTCoster
    Commented Nov 17, 2018 at 15:33
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    $\begingroup$ Not exactly a duplicate: the other query simply asks about blood colours based on metal content. This query specifies blood colour within mammalian biology. A different kettle of fish! $\endgroup$
    – elemtilas
    Commented Nov 17, 2018 at 18:15
  • $\begingroup$ I'd argue it's the same thing just more specific. While I can see this as its own question, I see it more as a duplicate. $\endgroup$ Commented Nov 17, 2018 at 20:32

7 Answers 7


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For endotherms, iron is in the heme groups binding oxygen, yielding a reddish color.

For ectotherms, copper is in the heme group binding oxygen, yielding a bluish color.

In order to bind oxygen, each protein chain binds to one heme group, allowing a maximum of four oxygen molecules to bind per one hemoglobin molecule.

At heme's center sits an iron molecule. The iron makes heme look red-brown. But what if the iron is swapped for a different metal?

...in cold-blooded animals, blood appears blue because copper atoms sit at the center of the ring and bind to oxygen.


It is probably a good bet that binding affinities for the iron-heme-O2/CO2 complex at 98F are better attuned to optimal gas exchange relative to the copper-heme-O2/CO2 complex, since this is precisely the substitution which occurred during mammalian evolution, thru many intermediate steps, doubtless, including modification of the carrying protein/heme platform.

Which means blue blood is not favored relative to red blood, for mammals.

Impossible though? Such an experiment could perhaps be run by depleting iron and raising the availability of suitable copper sources. That would be something for a molecular biologist to grapple with. I expect toxicity would likely be a large experimental hurdle, the process likely including reactivation of hemocyanin biosynthesis, per Molot.

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    $\begingroup$ so in addition to all the other physiological modifications which went into the transition from ectothermy to endothermy, there was a wholesale swap of the metallic cofactor mediating gas transfer in the blood from copper to iron - amazing. the fact that iron is vastly more prevalent in the crustal distribution probably eased that, somewhat. $\endgroup$
    – theRiley
    Commented Nov 17, 2018 at 6:05
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    $\begingroup$ Just in case this is confusing people: the precipitate colour, not the flame test colour, determines the colour of blood; copper (II) forms a blue ionic precipitate, but emits green light in a flame test. Medical News Today is being wrong. $\endgroup$
    – wizzwizz4
    Commented Nov 17, 2018 at 11:11
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    $\begingroup$ " Such an experiment could perhaps be run by depleting iron and raising the availability of suitable copper sources." — this, plus bringing back metabolic paths for hemocyanin, instead of or along the hemoglobin ones. $\endgroup$
    – Mołot
    Commented Nov 17, 2018 at 11:33
  • $\begingroup$ @wizzwizz4 -edited to remove the potential for confusion. $\endgroup$
    – theRiley
    Commented Nov 17, 2018 at 13:41
  • $\begingroup$ the swapping of how oxygen is transported occurred in a basil species that was probably far less complex than modern mammals. it may need many generations of slight genetic alterations to adjust, not just how the blood is made, but how the organs utilize hemoglobin. it may take a million years or more to make the change. By that time, they would cease being mammals $\endgroup$
    – Sonvar
    Commented Sep 30, 2021 at 19:08

I can't see why not. Octopi have blue blood. Apparently its due to a protein called Hemocyanin that binds with copper. So its clearly physically possible in complex Earth life.

enter image description here

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    $\begingroup$ Hemocyanin is less effective than hemoglobin, and mammals require a lot of oxygen. We mammals have a fine tuned, extremely efficient metabolism at the price of being prone to even slightly lower oxygen supply. That's why not. $\endgroup$
    – Mołot
    Commented Nov 17, 2018 at 11:26
  • $\begingroup$ @Mołot mammals have lacatate which makes them survive and thrive in situations where the body can't get enough oxygen, not to be confused with lactic acid. $\endgroup$
    – user56803
    Commented Nov 17, 2018 at 14:12
  • $\begingroup$ @Eries yes, mammals have support mechanisms that helps. Still, as pointed here, worldbuilding.stackexchange.com/questions/130546/… switching to more efficient base mechanism waa important part of evolution. Part that was only followed by further improvements, improvements you don't find in hemocyanin organisms. $\endgroup$
    – Mołot
    Commented Nov 17, 2018 at 14:37

It's not only possible, it has been documented, in humans, within the past century. Google for the "blue Fugates" -- they were an inbred family/community in Kentucky who, due to a mutated gene, had a much higher than normal level of methemoglobin. This altered hemoglobin doesn't carry oxygen efficiently, but it is quite blue in color -- blue enough to overpower the normal pink skin color in Caucasian humans.

The belief is that the Fugate family had a couple members who were born with this mutation, and as a result they were shunned by others. Add this to geographic isolation, and you get inbreeding. Over a period of a couple centuries, this led to everyone in their extended family (which was their entire community) having blue complexion. After the family broke their isolation in the late 20th century, physicians found a simple treatment: injection of methylene blue dye converted the methemoglobin to common hemoglobin, and their skin color changed from blue to pink in minutes. Periodic treatments are needed, and the mutation is still present in their family/community, so the Blue Fugates aren't gone, they're just hiding among us.

  • $\begingroup$ I'm not sure that this is correct. en.wikipedia.org/wiki/Methemoglobinemia states that "Instead of being red in color, the arterial blood of met-Hb patients is brown. This results in the skin of Caucasian patients gaining a bluish hue" [emphasis mine]. (I don't really understand why this is, though.) $\endgroup$
    – ruakh
    Commented Nov 18, 2018 at 4:28
  • $\begingroup$ @ruakh The brown is for highly oxygenated arterial blood so it is a mix of the blue and oxygenated (red) normal hemoglobin. The blood near the skin is less oxygenated and normal hemoglobin is much better at releasing oxygen so it rapidly turns blue after leaving arteries. (<- just what I got from the same article you linked, do not really know) $\endgroup$ Commented Nov 18, 2018 at 9:52
  • $\begingroup$ Why not actually link to articles? I'm a bit confused by "google for X" when you can just post a link. $\endgroup$
    – FuzzyChef
    Commented Nov 18, 2018 at 21:26
  • $\begingroup$ @FuzzyChef At the time I posted this, I was in a hurry, and now I'm at work. I'll try to come back and add the links to the article and the YouTube video where I first heard about the Blue Fugates. $\endgroup$
    – Zeiss Ikon
    Commented Nov 19, 2018 at 12:12


blue sea squirt and structure of amavadin

image of ascidians: http://frontiersmagazine.org/post-11/

Amavadin: from https://en.wikipedia.org/wiki/Amavadin

Vanabins are vanadium containing molecules found in sea squirts and some other organisms. Depicted is amavadin which is from a mushroom but which I think is more hemoglobin-like than the true "hemovanadins" from ascidians. They were called hemovanadins because they were thought to participate in oxygen transport but according to what I read that is now in doubt because these creatures have hemocyanin too. Oxygen transport still seems likely to me.

In any case: they are hemoglobin analogs, they can do oxygen transport and they are awesome colors of blue and green. Hemovanadin blood could be a fine blue color.

But why would a mammal use vanadium for oxygen transport when iron works so well? What if the iron brought trouble with it? That is actually the case for mammals - infectious organisms also want iron. When you are infected, a molecule called ferritin grabs all the iron it can, denying it to the infection.

Immune response Ferritin concentrations increase drastically in the presence of an infection or cancer. Endotoxins are an up-regulator of the gene coding for ferritin, thus causing the concentration of ferritin to rise.... Thus, the iron stores of the infected body are denied to the infective agent, impeding its metabolism.[24]

A consequence of this in real life is anemia - bound to ferritin, iron is denied to the red blood cells too.

What if there were some prevalent infection which depended on an organisms iron? An organism with minimal iron would have resistance to that infection. If infected, it would not have to get anemic because its cells are using V not Fe. Even if vanadium is less efficient at oxygen transport, the disease resistance conferred could cause hemovanadin-based blood to spread throughout the population.

How could mammals get hemovanadin? The sea squirts are our distant ancestors. Perhaps the gene for hemovanadin is still in mammalian DNA, sequestered and unused in some dusty corner of the genome. An accidental mutation restores it, and it is used shoulder to shoulder with iron hemoglobin, conferring advantages. Later an organism mutates out the hemoglobin gene leaving only hemovanadin - with consequent improved fitness.


I know you said blood, but it is documented that colloidal silver taken as a supplement causes argyria which turns the skin blue. Colloidal silver is used often to sterilize water among other things if this helps any.


I am going with probably not

This is entirely possible in the sense of why not. Its not like blood color is a factor in its function, merely a byproduct. Its just our evolution resulted in iron based hemoglobin so walla red blood.

Here's why I say probably not:

We and most mammals are pretty complicated organisms. Blood is a pretty basic functioning trait. Its kind of unlikely that evolution would seek to revise our blood color without a strong reason. Some would hat wave mating for this but I feel its pretty safe to say blood isn't going to play out in most mammals mating habits unless vampires become a thing.

Maybe our nature can find improved immunity function that changes its color. And this is really the complicated part. Finding some adaptation that requires significant change in the blood chemistry to result in change of the color.

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    $\begingroup$ Id like to hear why this got downvoted $\endgroup$
    – anon
    Commented Nov 17, 2018 at 5:20
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    $\begingroup$ I suppose because its a guess, and a wrong one. I don't like downvoting, but we shouldn't be guessing around here - speculating is ok, but not about matters which can be determined as matters of fact. I get downvoted too, and usually without explanation, sometimes I think unreasonably. I hope you don't feel that way about mine. $\endgroup$
    – theRiley
    Commented Nov 17, 2018 at 6:10
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    $\begingroup$ There are an infinite number of possible reasons why a mammal might evolve blue blood and as you acknowledge no reason why blue color would necessarily be detrimental. Take the skink example. Why does it accumulate high quantities of biliverdin? Who knows. If the question was "Can a reptile evolve green blood?" and we hadn't discovered this family of skinks you could use this same answer to say that a green-blooded reptile would be unlikely to evolve. If there existed a blue-blooded mammal no one would say that it couldn't have evolved. The question is "Is it possible?" The answer is "Yes." $\endgroup$ Commented Nov 17, 2018 at 6:23
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    $\begingroup$ This answer is correct; it's unlikely. However, it doesn't answer the question "would it be possible". $\endgroup$
    – wizzwizz4
    Commented Nov 17, 2018 at 11:12
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    $\begingroup$ Downvoted because there is another answer that proves that it exists already, so this is wrong. $\endgroup$
    – Fabby
    Commented Nov 17, 2018 at 15:51

It is possible. Hemocyanin would be what you would use and not hemoglobin. As stated, hemocyanin is worse at oxygen transport, compared to hemoglobin. But not all hemocyanins are the same, there are more effecient versions with better cooperative binding.

If you go with methods of directed evolution, we use to produce better enzymes, you could hope to produce hemocyanin, that would be good replacement for hemoglobin.

That for sure would take a lot of changes in your body to make it work well after that. But it is possible.


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