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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.

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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 2:58
    
@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 3:10
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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. –  Paul A. Clayton Jul 25 '13 at 4:23
    
Homing pigeons? –  Oreotrephes Jul 25 '13 at 6:21
    

3 Answers 3

up vote 22 down vote accepted

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

Its not that there is no metal available, but I can think of several reasons you don't see iron exoskeletons on animals all the time. Firstly, fully reduced (oxidation state 0) metal has a high energetic cost to create in reduced form.

Take iron the second most common metal after aluminum on the earth's crust. Most biological iron functions in the +2/+3 oxidation state, which is the same as rust. Cytochromes and haemoglobin being examples of how iron is more valuable as a chemically active biological agent than a structural agent. Aluminium, the most common metal on earth has relatively little biological activity one might assume because its redox costs are even higher than iron.

If there are some reasons why reduced biometal doesn't show up very often, inability of biological systems to deposit reduced metal is not one of them. Bone and shell are examples of biomineralization where the proteins depositing the calcium carbonate or other oxides in the material are structured by the proteins to be stronger than they would be as a simple crystal. 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.

I would say that 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.

The big issue is that biological materials actually perform as well as or better than metal does when they need to. spider silk has a greater tensile strength than steel. mollusc shells are models for tank armor - they are remarkably resistant to puncture and breakage. the time it would take for metallized structures to evolve biologically might be too long - by the time the metallized version got started, the bones, shells and fibers we know probably have a big advantage over them.

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Very nice answer! –  Brandon Invergo Jul 25 '13 at 9:31
    
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 0:45
    
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 3:20
    
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 5:40

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.

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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 3:27
    
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 5:41

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.

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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 3:31
    
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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