evolution - Why are there no organisms with metal body parts, like weapons, bones, and armour? (Or are there?)

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...

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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.

Answer

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.

Iron is the second most common metal after aluminum on the earth's crust but its almost entirely present in oxidized states - that's to say: as rust. Most biological iron functions in the +2/+3 oxidation state, which is rust or closer to it 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.

If there are some reasons why reduced biometal doesn't show up very often, inability of biological systems to deposit reduced (metallic) metals 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.

What's more, it is a fact that 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). Mollusc shells are models for tank armor - they are remarkably resistant to puncture and breakage. The time it would take for metalized structures to evolve biologically might be 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.