Are our bones well designed? Insects and crabs have a leg up on us

Sep 12, 2012
Image: Stephen Friedt/Wikipedia.

Researchers from Trinity College Dublin have recently shown that the legs of grasshoppers and crabs have the ideal shape to resist bending and compression. If human leg bones were built the same way, they could be twice as strong.

"Like all Arthropods, grasshoppers and crabs have so called exoskeletons made from a very special material called cuticle," said Professor David Taylor, of the Trinity Centre for Bioengineering at Trinity College Dublin, Ireland. "This protects the animal like a knight's suit of armour. Recently we have shown that this cuticle is in fact one of the toughest natural materials."

"In terms of evolution, having your bones on the outside has been a pretty good concept," said his colleague Dr Jan-Henning Dirks. "Since millions of years animals with exoskeletons such as insects, spiders and crustaceans can be found basically in every ecosystem in the world."

So what makes this exoskeleton so successful? Together the interdisciplinary team used the latest principles of engineering mechanics, materials science and biomechanics to address this question. In particular, Taylor and Dirks were interested in the diameter and thickness of the bones. They used a special machine to generate X-ray images of insect legs with a resolution of only a few thousands of a millimetre and collected and compared data from crabs and .

Their results, published in the article 'Shape Optimisation in Exoskeletons and Endoskeletons: a Biomechanics Analysis' in the Journal of the Royal Society, Interface, showed that, whilst human are relatively thick-walled tubes, the legs of insects and crabs have a much thinner wall in relation to their radius.

"This relation of wall-thickness to radius can tell us a lot about the of the structure," said Taylor. "Imagine the bones as simple tubes. Now, if you had a limited amount of material, what would you do? Would you make a thin solid rod or a hollow, thin walled tube? When compressed, the rod might easily bend like a straw, the hollow tube however might buckle like a beer-can."
However, for a given weight there is a mechanical optimal wall-thickness. And interestingly, the researchers found that the leg-shape of represents an ideal compromise to resist both the bending and compression forces the crab experiences when walking under water.

The locust leg on the other hand is optimised to withstand the huge bending forces which occur when it jumps.
In comparison, the human thighbone didn't do so well. Using the same amount of material and taking into account its mechanical properties, the human thighbone could be "redesigned" as an exoskeleton to be twice as strong as it is now.

"Of course there are numerous other factors determining the evolutionary advantages of endo- and exoskeletons," said Taylor. "However, we think that by taking a design engineer's view on the problem we've been able to shed some light on the evolutionary development of skeletal forms."

Explore further: Deep sea fish eyesight similar to human vision

More information: 'Shape Optimisation in Exoskeletons and Endoskeletons: a Biomechanics Analysis', Taylor and Dirks, in the Journal of the Royal Society, Interface (in press)

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User comments : 11

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barakn
4 / 5 (4) Sep 12, 2012
And every year or so until about the age of 20, our skin/skeleton would split open and dump out a wet, defenseless blob of gelatin incapable of sitting or eating, let alone defending itself until the new skin/skeleton hardened.
mikero
3 / 5 (4) Sep 12, 2012
@barakn, I agree this article and the researcher's statements are simply on the verge of plain stupidity. Basic anatomy, growth, ability to find sources of calcium, and mobility of joints seem to have gone out of the window. Compare a crab's range of motion in its joints to a human some day.
John_He
5 / 5 (2) Sep 13, 2012
@mikero Last paragraph: "Of course there are numerous other factors determining the evolutionary advantages of endo- and exoskeletons," said Taylor. "However, we think that by taking a design engineer's view on the problem we've been able to shed some light on the evolutionary development of skeletal forms."
Husky
1 / 5 (1) Sep 13, 2012
carab people?
chromosome2
5 / 5 (5) Sep 13, 2012
kevinrtrs
1 / 5 (12) Sep 13, 2012
we've been able to shed some light on the evolutionary development of skeletal forms.

They've done no such thing.
All they have accomplished is show just how well-designed the exo-skeleton is. Nothing more. They have certainly not in any way shape or form come close to revealing ANYTHING about how the exoskeleton developed from a single cell organism into a grasshopper - or any other organism whatsoever. The evolutionary story is purely imaginary since there is no way to go into the past and confirm that it happened. Or vice versa - there is no documented observational evidence that shows it in progress. It's just a make believe story.
Besides, here they show just how well-designed it is becuase for all intents and purposes it has all the appearance of design.
Yet in the same breath they also want people to believe it simply developed spontaneously via random accidental processes. Get real.
alfie_null
4.4 / 5 (5) Sep 13, 2012
... how well-designed the exo-skeleton is

Is it? Then why don't we humans use it?

It's just one of those evolutionary experiments. It only needs to work well enough in the species that use exoskeletons to ensure their success. Other species evolved other techniques to serve similar purposes.

If you examine any species closely you'll find non-optimal solutions that nevertheless work well enough to allow success. For example, the spine we upright walking humans are saddled with.
Torbjorn_Larsson_OM
5 / 5 (4) Sep 13, 2012
Our skeleton has no need to be selected for optimum strength, while it serves many other uses. In other fishes like sharks it is used to store nutrients, and they can *shrink* if they have a bad feeding season. In our case it serves as store for minerals and fats, are used to produce blood components et cetera.

Crationists shouldn't comment on science, it is hilarious to see.

Have you noticed how they attack evolution and reject the presented science results - and therefore gods - instead of say gravity despite evolution being the much more well observed process and well tested theory? We depend on evolution every day as we fight evolving viruses and cancers that tries to overpower our immune system, as much as we depend on gravity for retaining our atmosphere.

"Designers" is a cave man idea of early tool makers, before we had science in the 17th century discovering that nature is all process and not result of "tool use". And now we know this - except cave men creationists.
evolution3
4.7 / 5 (6) Sep 13, 2012
we've been able to shed some light on the evolutionary development of skeletal forms.

They've done no such thing.Get real.

Kevin...shut up.Are you searching all day long for an article with evolution in so you can spew your nonsense over it?This is a website for SCIENCE.Not for your beloved delusion.Get real.

Topic: I think the article has its weaknesses, as it doesn't become clear, that exosceletons are good...but only up to a certain size.You Get problems with thick exoskeletons and breathing when getting too big.So its a good evolutionary solution for small animals(or for bigger in water) but for environmental niches for bigger animals its not the optimal solution(only if oxygen rates in the atmosphere were much higher)
chas_c
2 / 5 (2) Sep 13, 2012
Wow, they've re-discovered the moment of inertia!
PJS
5 / 5 (3) Sep 13, 2012
would you all just stfu. the article is not claiming that humans should have an exoskeleton. theyre also not claiming to have unraveled the entire evolution of the exoskeleton. theyre ALSO not claiming everything should have an exoskeleton. theyve simply analyzed the structure and discovered some properties which are just another piece of the puzzle that we can add to our collective knowlege-base. just because they use the terms "design" and "engineer" in the article does not mean anything about "intelligent design".. yes, they have shed "some light" on the evolution. not a lot, but definitely some... you can't form an whole picture of the evolution without looking at the resulting structure..

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