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Reading this question, Why are there no wheeled animals?, I wondered why no organisms seem to make use of the tensile and other strengths of metal, as we do in metal tools and constructions. I am obviously not talking about the microscopic uses of metal, as in human blood etc.

Why are there no plants with metal thorns? No trees with "reinforced" wood? No metal-plated sloths? No beetles with metal-tipped drills? Or are there?

I can think of some potential factors why there are none (or few), but I do not know whether they are true:

  1. Is metal too scarce near the surface?
  2. Are there certain chemical properties that make metal hard to extract and accumulate in larger quantities?
  3. Is metal too heavy to carry around, even in a thin layer or mesh or tip?
  4. Can metal of high (tensile etc.) strength only be forged under temperatures too high to sustain inside (or touching) organic tissue, and is crystallised metal too weak?
  5. Are functionally comparable organic materials like horn, bone, wood, etc. in fact better at their tasks than metal, and do we humans only use metal because we are not good enough at using e.g. horn to make armour or chitin to make drills?

As a predator, I would like to eat a lot of vertebrates and save up the metal from their blood to reinforce my fangs...

A bonus question: are there any organisms that use the high electric conductivity of metal? Animals depend upon electric signals for their nervous system, but I do not think nerves contain much metal. The same applies to the few animals that use electricity as a weapon.

Cerberus
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    one possible answer is that metals have to be molten and forged.. they dont crystallize by deposition.. – WYSIWYG Jul 25 '13 at 02:58
  • @WYSIWYG: Never? So things like pyrite either do not exhibit the desirable properties of metals, or they crystallise under high temperatures or pressure? – Cerberus Jul 25 '13 at 03:10
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    There are magnetotacti bacteria. One problem would be oxidation. Also spider silk has greater tensile strength per unit mass than steel. Metals like magnesium and iron are (each) more than five times less common in the human body than sulfur much less carbon or even calcium, hinting at general biological availability. –  Jul 25 '13 at 04:23
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    Homing pigeons? – Oreotrephes Jul 25 '13 at 06:21
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    Oh, and caddisfly larvae, in symbiosis with French artists – Oreotrephes Jul 25 '13 at 06:25
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    On chemistry SE, I've asked a similiar question (or rather a question inspired by this train of thought): http://chemistry.stackexchange.com/questions/4009/is-it-possible-to-crystallize-a-solid-ingot-of-metal-from-an-acidic-aquaous-so – mart Jul 29 '13 at 12:45
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    @mart: Ah, cool! – Cerberus Jul 29 '13 at 21:23
  • Link to a previous Q on magnetic bacteria – Oreotrephes Aug 01 '13 at 04:11
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    Almost all metals corrode, and non-living parts of the body (as horn), cannot be renewed. That's all I think. – Tomáš Zato Nov 03 '15 at 16:11
  • @TomášZato: Hmm corrosion is an interesting factor indeed. But there are plenty of metals that don't corrode very much, like gold, silver, aluminium, stainless steel... Horn can be renewed, as in our finger nails and hair. Couldn't metal body parts be renewed in a similar way? Even teeth can be renewed in a way, as sharks do. – Cerberus Nov 04 '15 at 01:08
  • Gold and silver are rare, aluminium reacts with water upon acidic conditions and stainless steel oxides to produce highly toxic waste. – Tomáš Zato Nov 04 '15 at 09:31
  • @TomášZato: You may be onto something; care to post that as an answer? – Cerberus Nov 04 '15 at 16:38
  • Steel really isn't something that is going to happen naturally... :p – theforestecologist Mar 11 '16 at 23:17
  • @theforestecologist: Why not? – Cerberus Mar 12 '16 at 00:29
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    Beaver teeth are heavily impregnated with iron, which makes them stronger and tougher. http://www.dentalproductsreport.com/dental/article/new-study-finds-beaver-teeth-could-provide-insight-human-tooth-decay – John Jan 13 '17 at 15:46
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    Some molluscs incorporate iron into their radual https://www.researchgate.net/publication/44120196_Radula_synthesis_by_three_species_of_iron_mineralizing_molluscs_Production_rate_and_elemental_demand – John Jan 13 '17 at 15:51
  • @John: Ah, very interesting; hence their reddish colour! But the article mentions resistance to corrosion, not tensile strength. – Cerberus Jan 13 '17 at 16:39
  • In molluscs and beavers it also results in increased hardness, which makes them more wear resistance. Incorporating iron in small quantities will not do much for tensile strength. Bone already has a quite high strength, comparable to magnesium alloys. Metallic iron is not very chemically resistant and would quickly breakdown. If it did evolve I image a defensive enzyme from plants to encourage corrosion would be easy to evolve. – John Jan 13 '17 at 17:03
  • @WYSIWYG At risk of getting really off-topic now: metals don't have to be forged into a shape, they can be cast or machined instead. (Sorry, I couldn't stop myself and yes, I realise I have a problem.) – arboviral May 09 '17 at 14:45
  • Related: How to form biological metallic tissue? – Always Confused Jun 15 '17 at 17:55
  • Your bones are largely made of metal, calcium is a metal as is potassium sodium and a dozen other elements in the body. – John Jun 18 '19 at 19:52
  • @John: All right, but I was looking for ways in which we use the tensile strength of metal, as when metal is present in the form of a grid/roster; I believe that is not the case in bones? See my full Question above. – Cerberus Jun 19 '19 at 01:16
  • The hemoglobin in your blood contains four iron atoms per protein, whereas the hemocyanin in a number of invertebrates contains two copper atoms. – Serge Stroobandt Sep 05 '21 at 20:27
  • Ad "reinforced wood": cellulose fibers have comparable tensile strength to steel "cabels" of equal diameter. Cellulose fibers in wood are just really really thin (1 fiber = 36 molecules) and arranged in a way that allow cell growth. – BagiM Sep 20 '21 at 06:43
  • @BagiM: Hmm but could those be used in the specially useful ways that metal can, in tools and weaponry? – Cerberus Sep 20 '21 at 15:53

10 Answers10

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There are some cases of bio-metallic materials, as hinted at by the comments. But these are relatively small amount of metal.

It's not that there is a lack of metal available. Iron in particular is the fourth most common element in the earth's crust. Most soil that has a reddish color has iron in it. There are several reasons you don't see iron exoskeletons on animals all the time.

Firstly, metallic iron (in chemistry terms, fully reduced, oxidation state 0) has a high energetic cost to create.

Iron is the second most common metal after aluminum on the earth's crust but it's almost entirely present in oxidized states - that's to say: as rust. Most biological iron functions in the +2/+3 oxidation state, which is more similar to rust than metal. Cytochromes and haemoglobin are examples of how iron is more valuable as a chemically active biological agent than a structural agent, using oxidized iron ions as they do. Aluminium, the most common metal on Earth, has relatively little biological activity - one might assume because its redox costs are even higher than iron.

As to why reduced biometal doesn't show up very often, inability of biological systems to deposit reduced (metallic) metals is not one of them. There are cases of admittedly small pieces of reduced metal being produced by biological systems. The Magnetosomes in magnetotactic bacteria are mentioned, but there are also cases of reduced gold being accumulated by microorganisms.

Bone and shell are examples of biomineralization where the proteins depositing the calcium carbonate or other minerals in the material are structured by the proteins to be stronger than they would be as a simple crystal. most of the examples here have very little or no metal, but rather minerals like the Chrysomallon squamiferum cited by @navyguymarko and @loki'sbane here. The Iron Sulfide looks metallic but it is a mineral, akin to a bone.

While iron skeletons might seem to be an advantage, they are electrochemically unstable - oxygen and water will tend to oxidize (rust) them quickly and the organism would have to spend a lot of energy keeping it in working form. Electrical conductivity sounds useful, but the nervous system favors exquisite levels of control over bulk current flow, even in cases like electric eels, whose current is produced by gradients from acetylcholine.

What's more, biological materials actually perform as well as or better than metal when they need to. Spider silk has a greater tensile strength than steel (along the direction of the thread). Mollusk shells are models for tank armor - they are remarkably resistant to puncture and breakage. Bone is durable for most purposes and flexible in addition.

The time it would take for metallized structures to evolve biologically are likely too long. By the time the metalized version of an organ or skeleton got started, the bones, shells and fibers we know probably have a big lead and selective advantage.

shigeta
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    mineral deposition is different from free metal deposition and thats what i was implying.. deposited metals hhave to form metallic bond else they'll remain colloidal – WYSIWYG Jul 25 '13 at 22:16
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    While minerals tend to include oxides of elements and metals are different a chemical state, I'd just say that the creation of reduced metal structures could be created by similar sorts of genes. they can structure the lattice into layers and introduce defects of specific amounts. i would say that they represent a degree of control that genes could exert on the structure of deposited minerals if necessary. I see no reason that reduced metals could be built with less control over their structure. If so, then there are probably other reasons you don't find organic metal structures often. – shigeta Jul 26 '13 at 00:45
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    Thanks for your answer! So you are mainly saying the energetic cost of building and maintaining metal constructs is too high for organisms to be a good investment compared to other materials. – Cerberus Jul 26 '13 at 03:20
  • yes - I should have just said this :) I think its the cost relative to other minerals and organic compounds is high for what you get from using metal. Metal based tissue could have a hard time healing after damage too. – shigeta Jul 26 '13 at 05:40
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    Beavers and molluscs both incorporate iron into their teeth/radula for increased strength and resistance. there is a high metabolic cost however https://www.researchgate.net/publication/44120196_Radula_synthesis_by_three_species_of_iron_mineralizing_molluscs_Production_rate_and_elemental_demand – John Jan 13 '17 at 15:53
  • @John: Very interesting article. One of the other answers also mentions molluscs. But a comment says they use oxides or other compounds, not metal rosters, so I don't think metal's tensile strength is used? – Cerberus Jan 13 '17 at 16:42
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    teeth and shells are nanocomposites they exploit a mixture of materials with various properties, and many iron oxides are plenty strong, hematite comes to mind. in these cases they want hardness and chemical resistance, which metallic iron would not give you. – John Jan 13 '17 at 17:05
  • Gold is the ONLY actual example of a material that is metallic in and of itself being formed by biology, and it doesn't seem to be used. All the other examples in biology that I know of are oxidized rust essentially. Even the "copper" bloodworm teeth which are called "metallic" are just atacamite and organic polymers and maybe other minerals. – Kevin Kostlan Mar 22 '20 at 17:03
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A few minor points to add to shigeta's excellent answer:

Biological enzymes don't work well on metals. Some often incorporate metals (see chelation) but elemental atoms aren't easy to process. For one, a large molecule would be identical everywhere, so cleavage, for example, would be indiscriminate and just leave a bunch of tiny tiny atoms.

More to the point, once an organism incorporates metal there certainly isn't a lot it can do about that. A lot of shell-based organisms swap out their shells because of the inflexibility of those designs, and metal would be no different. It's difficult to grow when you're encased in a self-made iron maiden.

Amory
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    Thanks for your additions! I'm not sure I understand all the chemical terminology, but you seem to be saying that there are chemical reasons why metals are hard to work with for organisms, don't you? // As to metal shells, they could be segmented, or the metal could only cover certain vital areas in patches, or the metal could be used in joints, or other body parts... – Cerberus Jul 26 '13 at 03:27
  • good point Amory. I think the metal could be replaced/resurfaced as calcium carbonate is, but it would degrade much more readily because molecular oxygen is everywhere too. – shigeta Jul 26 '13 at 05:41
  • A self-made iron maiden would have organ-puncturing sharp spikes on the inside. A plate mail would be a better approximation, but not even these are form-fitting in the chest area. Another useful comparison would be to look at how the brain can't grow after birth because it's encased in a solid layer of bone from all sides. – John Dvorak Feb 10 '17 at 08:43
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There are good reasons why tissues/structures with a very high metal content might cause problems (I defer to the other answers here).

However, I am aware of one other example: some molluscs incorporate high concentrations of iron into the points of the radula (basically a ribbon of teeth, used for grazing). This is especially important for grazing molluscs, as they essentially make a living by scraping a thin layer of microalgae directly off the rock surface.

atrichornis
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    That is very interesting, a nice counter-example! So these molluscs use iron to make their radula stronger. Now I wonder why not many other animals or plans to this too...those molluscs don't happen to live in an en environment rich in metals? – Cerberus Jul 29 '13 at 21:26
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    Good question. These molluscs (primarily chitons) occur all over the world, so it's not a question of some local abundance of iron, but I guess it's possible that their rock-scraping habits give them quite a high dietary intake of iron (particularly on non-sedimentary rocks). However, I think it's probably more to do with their need for especially durable teeth, which outweighs any physiological/chemical costs associated with iron impregnation. It's just one of many solutions in the animal kingdom for dealing with tooth wear. – atrichornis Jul 30 '13 at 03:31
  • Right! It is interesting to note how humans have fairly recently switched away from metal teeth. I think dentists now use ceramic materials. – Cerberus Jul 30 '13 at 22:54
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    @Cerberus according the linked article, their teeth are not metallic, but of iron oxide (rust, a ceramic meterial). – Anixx Jun 16 '15 at 12:11
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    @Anixx: Hmm you're right, they use oxides, not pure crystals. Perhaps it is still relevant, if metallic oxides are still harder than most materials? – Cerberus Jun 16 '15 at 17:59
  • some beautiful pictures and the slug looks awesome. the iron in references are Iron Sulfide, which is a mineral form of Iron(II) its not really metal in the sense that the iron in our bodies is iron, nor is the calcium in our bones metallic (Calcium can form a metal, but its a mineral in our bones. ) – shigeta Apr 28 '23 at 05:27
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Well there is the common Bloodworm (Glycera dibranchiata)which people use for fishing bait. The animals are unique in that they contain a lot of copper without being poisoned. Their jaws are unusually strong since they too contain the metal in the form of a copper-based chloride biomineral, known as atacamite.

http://www.sciencemag.org/content/298/5592/389.long

And unlike the clamworm (Nereis limbata), whose jaws contain the metal zinc, the copper in the mineral in the jaws of Glycera is actually present in its crystalline form.

http://www.pnas.org/cgi/pmidlookup?view=long&pmid=12886017

It is theorized that this copper is used as a catalyst for its poisonous bite.

(I got this from Wikipedia)

RyanRulingRama
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Looks like some parasitoid wasps have zinc coated barbs on their ovipositors which may function to help them bore through wood and lay their eggs.

Here's the blog entry about it on IFL Science, and the original article:

parasitoid ovipositor specimens had a weight percentage of zinc of 7.19±3.8% (N=42) in the tip regions, which was significantly higher (P<0.05) than that in pollinator and parasitoid remote regions (<1%; N=10).

Kundanati and Gundiah (2014) Biomechanics of substrate boring by fig wasps. J Exp Bio 217: 1946-1954

Oreotrephes
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keith fedak
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https://en.m.wikipedia.org/wiki/Scaly-foot_gastropod

Gastropod that incorporates greigite, pyrite, and graphite on it's shell and foot.

Due to the large quantities of these compounds in dissolved form surround the hydrothermal vents.

Speculation for purpose: the shell is extremely resilient, the metal does improve this greatly. Though whether evolution deemed this adaptation necessary because of an abundance of strong predators, or as a means of detoxification of the injested compounds, is unclear.

The three populations of these snails have varied compositions, one which even being magnetic, due to the different compounds produced by the vents.

Appologies, here is non wiki http://www.esa.org/esablog/research/iron-plated-snail/

Loki'sbane9
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  • Very interesting example! It does seem to be a compound, but it's probably still the iron atoms that make the compound strong? – Cerberus Mar 12 '16 at 00:32
  • Digging a little deeper into the references there is more speculation than answers. – Loki'sbane9 Mar 12 '16 at 21:26
  • Out of the three independent populations, the Solitaire Fields population, does not utilize the granular greigite in scerlites at all. Original hypotheses theorized that the granular pyrite(Dodo field) and granular greigite(Kairei fields) was utilized by the Gastropod to improve defense. Further analysis concluded it is not the compound alone, interfacial geometries, layering, and the incorporation of other materials do aid in penetration resistance, energy dissipation, mitigation of fracture and Crack arrests, and other beneficial traits. Predators include, cones nails and seafaring crabs – Loki'sbane9 Mar 12 '16 at 21:43
  • I belive it is the structure of the greigite molecules that aid more in the scerlites and shell strength. Shell using layering and scerlites arranged in a shingling manner resembling scale mail. Furthermore, Greigite is only a 4-4.5 on Mohs hardness scale, by itself is not all that hard, comparative materials(Flourite, Nickel, Iron, Steel). Arogonite(polymorph of calcium carbonate) makes up the majority of the tertiary layer of the shell(core), hardness 3.5-4. Secondary layer composition periostracum, fleshy material. Primary layer composition pyrite, greigite, or none(solitaire). – Loki'sbane9 Mar 12 '16 at 22:14
  • Hmm so...the effectiveness of the shell may not be the result of the typical tensile (or other) strength of metal after all? P.S. I'll believe that crabs might crack shells, but I don't imagine a cone snail could penetrate or crack any half-decent shell anyway, don't you agree? Or does it have special, boring radulae that can do so? – Cerberus Mar 13 '16 at 02:10
  • The metal does help greatly for the 2 groups that take advantage of it. My comment on how the 2 minerals are not that hard was addressing that a hardness of 4-4.5 is just under average. Steel has the same hardness. Apatite has a hardness of 5, whivh is harder than steel, and hydroxylapatite is the main component of tooth enamel and bone. For the snails though the greigite and pyrite are harder materials than the shell materials alone. So combination of the metals and Arogonite, and the way they are structured, aid in the tensile strength, heat dissipation, and reflective abilities of the shell – Loki'sbane9 Mar 13 '16 at 18:44
  • But in other animals the bone and tooth are a harder material than either of these 2 iron compounds. – Loki'sbane9 Mar 13 '16 at 18:54
  • OK that's clear. I wonder whether hardness or tensile strength would be more interesting for shells: against drilling predators, you want hardness, but against crackers (like crabs), you want tensile strength, don't you? – Cerberus Mar 13 '16 at 23:29
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Though not in metallic (0) stage; an Iron ore called "Bog-Iron" is formed via microbial process.

Fig-1: Bog iron
Bog Iron
(Wikimedia)

It is formed inside bogs and swamps, classically in Sphagnum-moss-bogs. It is also found in peat.

Fig-2: a bog
A bog
(Wikipedia) , (Wikimedia)

Fig-3: Sphagnum sp, common bog moss of temperate and cold regions. *Sphagnum* sp
(Wikimedia)

When Fe(2) or ferrous ion, the more soluble form, obtained in the groundwater of bog region from some mineral-source such as spring, the anaerobic iron oxidizing bacteria, such as Gallionella and Leptothrix etc, oxidized it into Fe(3) or ferric form; which very easily get precipitated as insoluble compounds.

Fig 4: Spring acts as iron source.
spring works as source of iron
(Wikipedia) , (Wikimedia) , (USGS) , (USGS url).

Fig. 5: Leptothrix sp. , found in ferruginous environment.
*Leptothrix* sp.
(Wikimedia)

The iron components found in bog-iron, is commonly iron(III) oxyhydroxides (FeO)OH of varying compositions; geologically Goethite and Limonite.

Fig. 6: Samples of "bog ore" from Nassawango Creek show vugs lined with goethite around massive "ochre".
Samples of "bog ore" from Nassawango Creek show vugs lined with goethite around massive "ochre".
(USGS) , (URL)

Sources: >

  1. Wikipedia.

  2. Iron Production in the Viking Age, at http://www.hurstwic.org http://www.hurstwic.org/history/articles/manufacturing/text/bog_iron.htm

  3. Google books: Topics in Ecological and environmental microbiology/ Edited by Schmid and Schaechter/ AP; Chapter-37 ---> metal precipitation

  4. Google Books: Environmental Microbiology: Fundamentals and Applications: Microbial Ecology/ Jean-Claude Bertrand/ Springer. Chapter 14 (Biogeochemical cycles)

  5. Google Books: Bryophyte Biology / Edited by Shaw and Goffinet / Cambridge; Chapter 9: Mineral nutrition, substratum ecology and pollution/ J. W. Bates

  6. Metal depositing bacteria and the distribution of Manganese and Iron in Swamp- waters/ Ghiorse and Chapnick/ jstor.org

  7. Bog iron formation in the Nassawango Creek watershed, Maryland, USA/USGS (photos)

Always Confused
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im no biologist, but while not commonly cosidered, calcium IS a metal, so technically skeletons count. additionally, while not technically a metal, limpet teeth are quite impressive. http://www.bbc.co.uk/news/science-environment-31500883

Rubric
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    Quite interesting, thanks for the link! As to calcium, while bones are strong, do they actually contain calcium in the form of a metal grid/lattice? I think not? – Cerberus Jun 06 '17 at 01:29
  • bones are mineral composition of a Calcium Phosphate called hydroxyapatite – shigeta Apr 22 '23 at 22:36
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Have you ever looked up the scaly foot gastropod? It uses iron as a form of body armor. Literally scale armor on It's foot. enter image description here

user29013
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    Oops sorry I just realized someone posted this information already. My apologies. – user29013 Jan 13 '17 at 05:51
  • Now just imagine if the snail had weaponised love darts lol. Metal tipped spears to fight back with instead of reproduction. Yes I know about the conch and it's venomous attack. More proof that nature is just as alien as anything we can imagine. – user29013 Jan 13 '17 at 06:00
  • Your answer provided this great picture, and you explain more clearly how it actually uses pieces of (partly) metal as scale armour on its foot; the other answer was mostly about its shell. A bit more of an explanatory text would improve your answer, though! Incidentally, I wonder why it has a greenish hue, while iron is usually reddish? – Cerberus Jan 13 '17 at 14:20
  • yeah i like it too! – shigeta Jan 14 '17 at 02:56
  • @Navyguymarko What is the scientific name (identity) of this snail? and which other answer already mentions it? (i could not found that, sorry). Could you please provide the source of this image, and some other links, so the information could be verified? – Always Confused Feb 10 '17 at 18:44
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Barden et al (2017) have discovered an extinct species of ant (hell ant) that was alive 95 million years ago that had naturally occurring metal mandibles. Mandibles on ants are essentially the same as fangs on spiders or teeth on humans.

WYSIWYG
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Terrell Taylor
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