Explain in terms of breaking stress, why elephant has thicker legs as compared to humans?
Answers
Can anyone explain in terms of breaking stress, why an elephant has thicker legs than a human?
Sure. This has to do with the science of allometry.
But rather than compare a human to an elephant, let’s start off with a small, but close, relative of an elephant.
This is a hyrax:
Hyraxes are members of the clade Paenungulata, along with proboscideans (elephants and their extinct cousins, like mammoths and mastodons) and sirenians (manatees, dugongs). At their largest, they don’t get much above 10 lbs.
Notice the legs, in particular. You’ll see that they tend to be relatively long and thin bones of the limbs.
Even with the images at more or less the same size, you can see a dramatic difference in the relative dimensions of the bones. (Also note that the hyrax is positioned with its arms and legs flexed, where the elephant’s limbs are more like vertical pillars underneath. That’s interesting for a similar reason.)
Why would bones be relatively wider in the larger animal?
Well, it turns out you hinted at the answer in your question: breaking stress. If you don’t want your bones to snap under stress, the most important element is cross-sectional area.
But when we scale things up from small to large, everything can’t stay in proportion because of…allometry. There’s that word again. Let’s define it. The allo- prefix literally means “other”. The “-metry” part is the same root as metric, and just has to do with measures. In short, when things change their size, they often change their shape too.
So, what happens when we take something like a hyrax femur and blow it up to the size of an elephant femur?
Well, if we increase the length, width, and height of the bone proportionally, our volume is going to increase at a cubed rate. This should be fairly intuitive if you’ve ever taken a geometry class.
Our small cube here has a volume of 1 cubic unit. The larger cube has only double the length, width, and height, but its volume is now 8 cubic units (two to the third power). All good?
So, what happens to cross sectional area? Well, it increases as a square of the linear dimension increase. If we were to saw our larger cube in half, we’d see that the cross-sectional area is 4 square units.
Thus, volume is increasing faster than the cross-sectional area.
This presents a dilemma. Volume and weight are going to be correlated (all other things held equal). If our volume is increasing as a cube of the linear dimension, and the cross sectional area is only increasing as a square of the linear dimension, the larger bones won’t be as strong at resisting breaks as the smaller versions were.
Solution: As we increase the linear dimensions, we want to increase the cross-sectional area relatively faster. And that’s precisely what Mother Nature has done in the limb bones of elephants. An elephant’s bones are much wider, giving them a thicker cross-sectional area, making them more resistant to the type of stresses that can break the bones.
Oh, and remember when I mentioned the pillar-like limbs of the elephant skeleton above? By positioning their legs more or less directly beneath them with no significant flexing of the joints, the elephant can use the stiffness of their bones to support their weight, rather than relying upon muscular effort pushing against the force of gravity. This too is the result of allometry. Guess what dictates the strength of a muscle? It’s not so much the total volume of a muscle; it’s the cross-sectional area. A large animal has to grow muscles with much greater cross-sectional area to get the same performance as the smaller animal. Rather than making big giant muscles to support their weight on flexed limbs, they have just evolved limbs that bear their weight directly underneath them.
Because elephants are generally larger and heavier than humans. There's also the entire issue that surface area increases through linear growth while volume increases through exponential growth (or something like that), meaning that the weight of an elephant will always be a lot more stress on it's “proportial” legs than our weight will be on ours. If an elephant had the proportions of a human, it likely wouldn't be able to get around due to the inmense stress put upon its legs. So even if their legs are proportional to their surface area, they aren't proportional to their weight, so they need larger, thicker legs to make up for it.
I hope that made sense.