Prompted by a question about the impact of dietary iron in elves (How would Fair Folk-type elves deal with dietary iron?), I wondered whether haemocyanin could be an alternative to haemoglobin in creatures (for example elves, who have a weakness to iron - although probably not haemoglobin).

Although - as far as I know - only some arachnids, arthropods, and molluscs have haemocyanin, I'm making the initial assumption that it is in fact possible for human-sized beings to have copper-based blood. With that in mind, what sort of impacts (positive or negative) would this blood chemistry have on humanoids?

My thoughts so far:

  1. Haemocyanin is second only to haemoglobin in frequency as an oxygen transport molecule. Is it any more/less effective? If it is less effective, would that result in these creatures living at lower altitudes with greater atmospheric oxygen?
  2. The obvious colour difference: deoxygenated blood is clear, and oxygenated blood is blue.
  3. Diseases like anaemia would probably have parallels - could this be treated easily with supplementary dietary copper (in mild cases)?
  4. Cross-breeding with humans would become difficult, I expect - half-elves wouldn't exist, and heterozygosity could be compromised.


I'm not too bothered about why they might have copper-based blood, but more about the implications if they do have it. Thank you for pointing out that haemoglobin wouldn't actually be toxic/harmful to an elf - it's not exactly iron filings floating around in the bloodstream. ;)

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    $\begingroup$ Haemocyanin is more efficient than haemoglobin at lower temperatures and at lower partial pressures of oxygen; for example, at the bottom of the sea. At higher temperatures (such as the 37° C of the human body) and higher partial pressures of oxygen (such as in air) haemoglobin is much more efficient than haemocyanin. And anyway, in our evolutionary lineage haemoglobin is very very old, dating from the Silurian; it's a shared attribute of jawed vertebrates. $\endgroup$
    – AlexP
    Commented Sep 8, 2017 at 17:30
  • $\begingroup$ Thank you! If these elves/humanoids were to live in the air then, could this be achieved by them having significantly lower body temperatures? And/or by increasing the amount of oxygen in the air? $\endgroup$ Commented Sep 8, 2017 at 17:32
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    $\begingroup$ Elves, as commonly imagined, are mammals. Real-life mammals use iron-based haemoglobin. However, I think that you are committing the falacy of mistaking the components of a chemical substance with the substance itself. Elves may have a weakness to iron, but haemoglobin is not iron. Table salt is a compound of sodium (a metal with self-ignites in contact with water) and chlorine (a very toxic gas): humans cannot ingest sodium or chlorine and live, but we must eat table salt or we die. High doses of iron are toxic for humans too. $\endgroup$
    – AlexP
    Commented Sep 8, 2017 at 17:38
  • $\begingroup$ Good point - I'll edit my question a bit, if that helps. I'm more interested in the consequences of having copper-based blood, rather than necessarily why. Elves and iron-weakness seemed a logical start, but I didn't really think it through fully. Cheers. $\endgroup$ Commented Sep 8, 2017 at 17:41
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    $\begingroup$ It's not all down to the metal. Fetal hemoglobin has different oxygen-binding kinetics than adult hemoglobin. This is because fetuses acquire oxygen from their mother's blood, and so it must of necessity bind the oxygen more efficiently than adult hemoglobin. You have latitude for where your elves live, it's a matter of the protein portion of the molecule. $\endgroup$
    – SFWriter
    Commented Sep 8, 2017 at 17:49

3 Answers 3


It would probably effect the daily level or iron and copper needs for health. according to https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3690345/ Recommended daily allowance (?) of Iron is 18 mg while that of copper is 0.9 mg. Just a guess but creatures with copper based blood would probably need to eat more copper "rich" foods and less iron "rich" ones.

adding a bit. The cycle of iron as a nutrient in the environment is well studied (https://www.nature.com/scitable/knowledge/library/earth-s-ferrous-wheel-15180940) as well as the storage and use of iron in the body of mammals (http://www.chemistry.wustl.edu/~edudev/LabTutorials/Ferritin/Ferritin.html). Iron cycling out of the body is even what gives poop it's brown color. Copper as a nutrient is less well understood (just above http://ajcn.nutrition.org/content/88/3/826S.full#sec-4).

If there is a species that needs a lot of copper in their diet there is probably a whole ecosystem of plant and maybe animals the provide a good source of copper. So this species would need to hunt or cultivate these. For Iron based species to eat these plant in the same quantities would probably be toxic. So now your have potentially two species that cannot really share food. There could be extra complexities in trying to have sets of agriculture in the same place and this could drive them to different regions.

If there were not a whole ecosystem based on getting enough copper then the species may have to work extra hard to cultivate or hunt food that gave them enough copper. Again perhaps restricting population centres geographically and perhaps limiting the size of those population centres.

Hopefully a better answer

Bonus note: Hemocyanin ref: https://en.wikipedia.org/wiki/Hemocyanin

  • $\begingroup$ I wonder if enough copper could be found by eating large numbers of certain arthropods, seeing as they use haemocyanin themselves. $\endgroup$ Commented Mar 25, 2018 at 17:01

The human body does need some copper, but we are talking very small amounts. Chronic ingestion of large amounts of copper can cause a range a organ damage/death. The first to be impacted would be the liver as normally when you ingest extra copper it filters and then releases it into pile in the gull bladder where you eventually poop it out (so the gall bladder would also be one of the first organs to be effected).

Copper also has strong anti-microbial effects, and I would speculate this would cause all kinds of havoc with your natural microbes in your digestion system which would cause severe diarrhoea. There is a genetic disorder known as Wilson's disease, those who suffer from it are unable to process and expel copper. Symptoms can include itching, muscle cramps, psychosis, organ failure... though it does come with kind of a cool symptom called Kayser-Fleischer rings, copper rings that form around either the iris or pupil.

There are some benefits to using hemocyanin over haemoglobin, but not many. Hemolymph, which is what hemocyanin is usually found in, has some natural antimicrobial properties because of the copper, though to what degree and how effective this can be is not fully understood. Another is that metabolic cost for hemolymph and hemocyanin is much lower, as hemocyanin isn't a cell but just a protein, unlike red blood cells which make the protein haemoglobin.

The life span of red blood cells is 3-4 months which means your body is constantly making new red blood cells to replace the dying ones, every 4 months you have a new set of cells. This is not the case for hemocyanin as it's protein structure can remain stable for years. Generally, haemoglobin is about 4 times as efficient as hemocyanin for transporting oxygen, except in very cold environments with low oxygen pressure such as low depths in the arctic ocean.

  • $\begingroup$ Some of your points are about changing humans to run on copper, but AFAICT the question is about a species that's always been that way and thus adapted to it. $\endgroup$ Commented Jan 5, 2019 at 15:45

Due to high viscosity and the fact that all creatures with haemocyanin have it loose in the blood not in blood cells the concentration must be kept low. Maybe elves have haemocyanin blood cells to solve the problem. I've read that haemocyanin from horseshoe crabs is more efficient and nears the efficiency of haemoglobin. Something about cooperative binding versus non-cooperative binding of oxygen. The advantages of haemocyanin are an ability to function at lower temperatures and lower oxygen concentrations. You will note that Star Trek's blue skinned Andorians come from an icy world. For elves it would aid survival in deep, cold, and oxygen poor caverns. On another note haemocyanin would be an ideal for a race of deep water mermaids for the same reasons.

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    – Gryphon
    Commented Jan 5, 2019 at 1:44

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