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Edit: This question is very similar to this and related to this one (though the latter focuses on homology instead of scaling laws). However, the answer to this question is far more comprehensive, in particular it offers a plausible explanation why horse legs evolved as they did (vs human or even rhino legs).

Large grazing mammals such as horses, moose, and cows tend to have relatively thin legs despite being up to ~1000kg. For example, this rider's and her horse's legs appear to have about the same cross-sectional area both for below and above the "knee": tight fitting pants

If this horse is 500 kg (a mid-range mass for horses), each leg would have to support 125 kg, compared to only 37.5 kg for a 75 kg adult. Why don't we see a corresponding difference in cross-section?

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    $\begingroup$ Possible duplicate of What is the anatomical term for a two jointed leg? $\endgroup$ – Remi.b Oct 10 '17 at 4:24
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    $\begingroup$ Possible duplicate of Comparative leg sizes $\endgroup$ – canadianer Oct 10 '17 at 6:19
  • $\begingroup$ On that photo, the horse's shoulder is easily twice as broad as the woman's gluteal region whereas the horse's gluteal region is over 4 times as broad. That translates to 4 times and 16 times more cross-sectional area (mostly equal to strength). The calf is admittedly slimmer, but you should note that on the horse it's about 90-95% bone whereas on the human it's more like... 8-10% bone, the rest being muscle (... which a horse doesn't need due to its anatomy...). $\endgroup$ – Damon Oct 13 '17 at 14:00
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    $\begingroup$ Elephants? Just saying. $\endgroup$ – Aravind Suresh Oct 15 '17 at 4:02
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Elephant, rhinoceros, &c all have much thicker legs in proportion. The answer, I think, lies in the fact that the animals you mention all evolved as cursorial animals (that is, they run to escape predators). Less mass in the lower leg means it swings easier, so the animal can run faster.

There are two things you're apparently not noticing in that picture. First, the the horse's lower leg is almost entirely bone (and some tendon), and it's bone that does the supporting. The propulsive power comes from the large muscles of the hip, thighs, and shoulders.

Second, the lower part of the leg (with the white wrappings) is not anatomically equivalent to the human's lower leg, but to the bones of the hand and foot. You can see this if you look closely at the rear leg in that picture. The femur, equivalent to the human's thigh, ends at the knee just above the belly line. Then the tibia extends about halfway down, ending at another joint which you might think is the knee, but which is called the 'hock' in horse-speak. The white-wrapped part is a metatarsal, equivalent to human foot bones, then there pastern bones equivalent to human toe bones, ending in the hoof/toenail.

So consider that you can, if reasonably fit, walk around on tiptoe without crushing your foot and toe bones, then imagine the end result of your ancestors having done this for the last several tens of millions of years :-)

PS: With horses, there is some effect from human selection, too. Racing & show breeds tend to have thin lower legs, draft horses & working breeds have proportionately thicker ones. My first horse, a thorobred/arab mix, had legs about as thick as my wrists (granted, I'm a fairly muscular guy); my current mustang, about the same height & weight, has legs about twice as thick.

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    $\begingroup$ May I suggest you add (Or can I do this?) a picture of the horses leg structure? As you say a lot of people don't know the lower part of a horses' leg is more equivalent to the bones of a human hand or foot. $\endgroup$ – Summer Oct 11 '17 at 11:31
  • $\begingroup$ @JaneDoe1337: It would indeed be a help, but I have no idea how to do it (since SE insists on using its own arcane methods for such things). Feel free to do so yourself. $\endgroup$ – jamesqf Oct 11 '17 at 17:41
  • $\begingroup$ @jamesqf there's a little button that appears when you are writing an answer and that lets you upload images or link to them from other places in the web. See the "Images" section at the bottom of this page. That doesn't seem too arcane, really. $\endgroup$ – terdon Oct 12 '17 at 14:58
  • $\begingroup$ @terdon: Maybe the little image appears in your browser. It doesn't seem to in mine. Also, I'm dysiconic: unless a button has actual words spelling out its function, I am highly unlikely to be able to "intuit" its function. $\endgroup$ – jamesqf Oct 12 '17 at 18:53
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This is a mistake in comparative anatomy which is somewhat common. When looking at four-legged animals, people often mistakenly map the parts of the hind legs.

Here is an image that shows the different morphology of the same bones in horses and humans:

enter image description here

What people often think of as the thigh of a four legged creature is really our calf and shin bone! And what is often mistaken for the calf is actually our foot.

Horses evolved to run on what is actually our toenail. You can see a similar pattern when humans sprint, our fastest running pace — we pull our heels up, and land on and launch off of the balls of our feet and our toes. That foot-strike is actually how animals such as the cat and dog walk habitually.

There's an urban myth that "dog's knees bend backwards", but by looking at this graphic, you can see that what appears to be the dog's knee is actually its ankle. And both the dog's ankle and knee bend the same way ours do.

enter image description here

Habitually landing on our heels in a normal walking pace makes us plantigrades, alongside raccoons and kangaroos. Dogs and cats walking on their toes is called digitigrade, while swift grazers such as deer and horses we classify as unguligrade, which means walking on their toenails.

So, other answers are correct, and flesh out the full information. I just wanted to give a visual indication to correct this common error.

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  • $\begingroup$ Can you source the first image, I want to see if I can use that. $\endgroup$ – John Oct 11 '17 at 15:02
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    $\begingroup$ That image of the skeletal dog standing upright is both amusing and alarming, and I will never be able to look at a dog again without imagining it wandering around like that while people aren't looking. $\endgroup$ – Wayne Werner Oct 11 '17 at 19:57
  • $\begingroup$ @WayneWerner youtube.com/watch?v=7f1ovurzU2s $\endgroup$ – user151841 Oct 11 '17 at 19:58
  • $\begingroup$ @John sorry, I grabbed it off of Google image search. If you do a search for "human horse comparative anatomy" you can find many copies of it, but I don't recall which one I originally got it from. $\endgroup$ – user151841 Oct 12 '17 at 13:41
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How come large herbivores have such thin legs?

They don't. The following book does an extensive comparison of the bones of humans with other animals:

Adams BJ, Crabtree PJ. 2008. Human vs Horse. In: Comparative Skeletal Anatomy. Humana Press.

Here are some images of human bones (left) next to horse bones (right):

Radius/Ulna (fused in horses)

enter image description here

Humerus

enter image description here

Femur

enter image description here

From these images, the bones of horses are clearly larger than their homologous counterparts in humans. Another answer gives a good picture of where these bones lie. Even if you were to consider the horse hock as its knee/elbow and compare, you would find that:

  • the horse radius/ulna is ~1.9 times larger than the human humerus at the smallest diameter
  • the horse tibia is ~2.3 times larger then the human femur at the smallest diameter
  • the horse metacarpal is ~1.3 times larger than the sum of the human radius and ulna at the smallest diameter
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    $\begingroup$ This is a homological comparison, but we should compare the part of the leg that is visible for both animals and "forget" that the horses knee is really it's ankle; it's more relevant for scaling laws. In that case horses do have human-sized but bonier legs. Though the femur difference is still illustrative of the more robust skeleton in horses. $\endgroup$ – Kevin Kostlan Oct 11 '17 at 13:46
  • $\begingroup$ @KevinKostlan I've updated the answer to address this. I understand your point though, and I think even with the rough measurements I made the horse bones are still supporting more weight. I just wanted to provide some data to show that the horse bones are not that small. $\endgroup$ – canadianer Oct 11 '17 at 17:52
  • $\begingroup$ @Kevin Kostlan: The important part of the like-part comparison is, I think, that the muscles remain much the same. The hip and thigh have the largest muscles, the fingers/toes have little, even though the corresponding bones are greatly elongated in the horse. So when you grasp something with your hand, the power is coming from muscles in the forearm - you can see the tendons moving in the wrist. $\endgroup$ – jamesqf Oct 12 '17 at 5:46
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You're assuming that horses and cows are still subject to Darwinian evolution. In fact breeding of domestic animals has mostly been controlled rather carefully by humans, to breed for characteristics which we feel are favourable, even if those characteristics are detrimental to the animal's survival in the wild.

This became most obvious in the Agricultural Revolution, with selective breeding work pioneered by Robert Bakewell and others, where animal size increased dramatically. African cows today give you an idea of how big cows would have been before this breeding effort, but even they are the product of thousands of years of fairly selective breeding.

Horses are a similar case. Genghis Khan's Mongol warriors used ponies of around 12hh, and these would have been relatively wild animals.

Fragility of limbs is just one way modern animals are rendered less fit in the wild, of course. Many breeds of domesticated animals have problems giving birth without assistance, or have genetic defects which can cause the animal distress or reduce its lifespan. These issues are usually tolerated or managed somehow, because humans prioritise the desirable features of the breed.

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    $\begingroup$ Your answer is evidently overstating the importance of artificial selection if you consider that wild animals, such as zebras and deer, also display this characteristic. $\endgroup$ – vkehayas Oct 10 '17 at 12:41
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    $\begingroup$ Wild equines are not really morphologically distinct from domesticated ones. This isn't a phenotype that's emerged in the last 6,000 years since horses were domesticated. Humans have absolutely nothing to do with why horses have thin legs. $\endgroup$ – Nuclear Wang Oct 10 '17 at 12:49
  • $\begingroup$ @NuclearWang Look at different horse breeds, though. Draught horses (speed isn't important) have a lot thicker legs than racing horses (speed is of prime importance). The differences between various domesticated breeds and wild horses are many (and don't confuse wild horses with domesticated-horses-gone-wild). But there's no need to reiterate James' answer, which is a lot closer to the mark. $\endgroup$ – Luaan Oct 10 '17 at 13:44
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    $\begingroup$ "You're assuming that horses and cows are still subject to Darwinian evolution." They still are. It's just that most of their evolutionary pressures are human driven, and humans are a part of nature. This is just another case of one species exerting evolutionary pressure on another. $\endgroup$ – Shufflepants Oct 10 '17 at 14:37
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    $\begingroup$ Evidence suggests that horses may have been domesticated as early as 3500 BCE, strong evidence exists at 2000 BCE. Khan's horses would have been bred for at least 3000 years. They were hardly "relatively wild". en.wikipedia.org/wiki/Domestication_of_the_horse $\endgroup$ – user151841 Oct 11 '17 at 19:42

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