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It is stated on this website that the density of a Grizzly bear bone is ten times our own. And since I am not into biology at all, I am confused as I don't understand the meaning of bone density here.

I know what the "bone mineral density" term means, and I know it is measured in $g/cm^2$, but there is also the "physical" density term which is the mass by volume and it's measured in $g/cm^3$.

Now, what does it mean when we say that an animal has a bone that is 10 times more dense than ours ? Does it mean that the bone weighs 10 times more for the same size (or volume) ? Or does it mean that it has 10 times the amount of minerals and so it is somehow stronger ?

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The word density in the medical term bone density is used to quantify only the mineral content of the bone (bone mineral). It's not intended to describe the total mass density of the bone like we are used to from physics.

This is all on wikipedia: http://en.wikipedia.org/wiki/Bone_density

Thus, when one say that a Grizzly bear have 10 times the bone density, it implies that its bones have 10 times the mineral content compared to humans, and this is indeed seen as a proxy indicator for bone strength. However, this claim does seem unsupported and unrealistic as kmm points out.

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  • $\begingroup$ I have to think they are using density incorrectly, and since they provide no reference, we really can't check. There is no way that bone density, in the scientific sense, can be 10 times higher in bears. It says "for a given size," which indicates to me that it must mean some kind of skeleton as percentage of body mass. Or it's just made up, with no basis in observation. $\endgroup$ – kmm Apr 27 '15 at 15:15
  • $\begingroup$ @kmm I found some data on the bears here: asbweb.org/conferences/2006/pdfs/20.pdf, now we just need the same numbers for humans to compare. $\endgroup$ – Alex Apr 27 '15 at 15:33
  • $\begingroup$ And I found a story about a man with a LRP5 mutation that is said to have had eight times the normal bone density, haven't been able to verify though. $\endgroup$ – Alex Apr 27 '15 at 15:37
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    $\begingroup$ The "ash fraction" value is the key here. Bear bone is 66% mineral. Typical human cortical bone is 65-70% mineral. So, no, bears don't have denser bones than humans. $\endgroup$ – kmm Apr 27 '15 at 18:09
  • $\begingroup$ @kmm, may I ask where you got these numbers ? Also, do different spices have similar "physical" bone densities ? That is, in terms of mass over volume, can an animal have bones that are double or triple the density of ours ? $\endgroup$ – Abanob Ebrahim Apr 28 '15 at 1:20

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