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When I was a child, my father showed me the classic essay "On Being the Right Size", J. B. S. Haldane. It talks (among other things) about how large animals need stockier legs to support their weight. I weigh about the same per leg as a 350 lb deer, but my legs are closer in diameter to those of a 1500 lb horse. Why?

Ross Millikan
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  • Nice question. :-) I guess he was wrong or was talking about averages and not individual species... – inf3rno Nov 11 '14 at 03:03
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    @inf3rno: he was clearly talking about averages, based on the body mass increasing with the cube of linear dimensions and the weigh bearing capability of legs increasing with the square. I suspect the answer is humans stress their legs in ways that deer and horses do not, so it is advantageous to have stronger legs, but that is why I asked. – Ross Millikan Nov 11 '14 at 03:18
  • There is no way your legs are close in size to a horse. – canadianer Nov 11 '14 at 04:36
  • @canadianer: they are not as long, but in diameter they seem close. I haven't been that close to horses, so I may be off. When I asked a farrier this question he seemed to think it was close. – Ross Millikan Nov 11 '14 at 04:38
  • If you can get access, check this out: http://link.springer.com/chapter/10.1007/978-1-59745-132-1_2 And then there's the musculature… – canadianer Nov 11 '14 at 04:45
  • @canadianer: No, all I get is a horse skull. Not too useful for this. – Ross Millikan Nov 11 '14 at 04:54
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    You need to view the chapter, not just the look inside. Qualitatively, the horse femur appears ~2.5 times the diameter of the human femur. Same for the tibia. – canadianer Nov 11 '14 at 05:01
  • @canadianer: That makes the area ratio close to the weight per leg ratio. I'm surprised. Thanks. That would be a reasonable answer. – Ross Millikan Nov 11 '14 at 05:03
  • I suppose it answers your specific example of horses and humans, but there might be merit in someone actually discussing the biology of weight to leg size ratios. Seems interesting and would be more in the spirit of your question. – canadianer Nov 11 '14 at 05:17
  • @RossMillikan Maybe by a pony it is so long, but an average horse has about 2 fold longer leg than a human... – inf3rno Nov 11 '14 at 05:29
  • @inf3rno: yes, I was more interested in diameter than length, because that sets how much weight a leg can support. More length makes it more prone to break in buckling, but that would argue for more diameter for the longer legs. – Ross Millikan Nov 11 '14 at 05:32

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There is an interesting relationship between the length of an animals femur, the thickness and the mass of the thickness. A point that should be taken into account is that bones can different densities and so might be able to support varying amounts. The muscle in you leg also does nothing for the support of you weight it is (basically) all in the bone. Really looking purely at the leg won't tell you much you need to strip back the tissue and get you measurements straight from that. Given the different gate strategies of various animals this can lead to differing levels of muscle surrounding the leg and could be confounding your answer.

Walter Lewin a professor of physics at MIT did a lecture series where in the first lecture he went off on one about the relative sizes and thickness of animal femurs and it is really interesting (http://www.youtube.com/watch?v=PmJV8CHIqFc - between 11 minutes and 22 minutes). The short answer was that the ratio between the thickness and the length stays similar relative to the length.

Samuel Barnett
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