Hypothetically there is a large-ish planet, which is extremely dense due to its high metal content. This planet has relatively high gravity as a result. The fauna and flora have adapted to the high gravity by incorporating various suitable abundant metals into their structures and ligatures. I was imagining plants would extract the metals to have strengthened stems, trunks, and leaves. The animals of the planet would then take in the metals in a useful form from the plants.

I figured arthropods would be the most common land animal type and could grow large in the rich high density atmosphere. But there are evolved extremophile humanoids and some other small warm blooded squeaky things.

If the metals appear naturally in a correct form, would it be possible that carbon based life would use it in its skeletons and structurally in tendons, connective tissue? Or is calcium, strotium, etc. just in too much of a sweet spot to be competed with?

  • $\begingroup$ Probably not an exact duplicate but related. $\endgroup$ Sep 16, 2019 at 11:29
  • $\begingroup$ (1) Bones are not made of calcium. Calcium is a highly reactive metal, it would not behave well in the warm and humid insides of an animals body. (2) There is no relationship whatsoever between the physical properties of hydroxyapatite (the cristalline substance in bones) and the physical properties of metallic calcium. (3) To give you a simple example, consider iron. Iron is a grey, tough, ductile, fairly reactive metal. Iron oxide, also known as rust, is a reddish, very soft, brittle, flaky, unreactive material. $\endgroup$
    – AlexP
    Sep 16, 2019 at 11:42
  • $\begingroup$ Um, isn't the planet that we live on quite dense due to its high metal content? There's a distinct lack of iron-hulled arthropods out there, or much in the way of iron-based structural compounds at all. Have a think about that. $\endgroup$ Sep 16, 2019 at 13:16

3 Answers 3


Since elements in the periodic table in the same families have similar property, you can have a look at elements just above and below Calcium. This would include Magnesium and Strontium.

Googling for usage of Magnesium in animals brings shows that much of this magnesium is binded in skeleton side by side with Calcium.

So, as long as it is minerals from same group, you can extrapolate them to have same effect as Calcium, that is use them in formation of skeleton etc.

is extremely dense due to its high metal content. has relatively high gravity as a result.

If the animal kingdom is made of high density metals/materials, you can expect there to not be much movement, as increased gravity would increase the energy requirement etc for them to move.

So have them evolve/come to use the lighter elements - like magnesium, berillyium etc in their interior skeleton

While Metal by itself is durable, many compounds can have similar physical properties to overcome effect of higher air pressure, so you can have the organisms evolve a hard exterior, with inner core made of the lighter ones.

I was imagining plants would extract the metals to have strengthened stems, trunks, and leaves.

Increased gravity would mean that the capillary effect would need to be much more pronounced for the plants to get such minerals. So if you want to go this way, make your plants small/shrubs.


Production of pure metals is very energy-intensive, as most of them are in nature chemically bound in ores. Living body would have to expend energy to release the pure metal, by some low-temperature "smelting" process. For the large mass needed to grow the skeleton, there are many suitable and less energy-intensive materials available.

Another disadvantage is that metals aren't stable in living bodies, because the environment is somewhat acidic and they would tend to dissolve. To keep that from happening would incur yet another burden to the animal.

So, metallic skeletons would need to be justified in-story somehow. There might be free (not chemically bound) metals in the soil, but that is incompatible with atmospheric oxygen, and all life would have to function based on some non-oxygen process.

Of course, there are precious metals like silver and gold that are pure in nature and don't dissolve in the body, but they are too heavy and not very hard.


The Scaly-foot snail already does so. It uses iron sulfide in it's shell, showing that this is very definitely possible.


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