You may have noticed that the topics for these blog posts
are often inspired by misconceptions I had when I was younger. This entry is no different.
Cheetahs came somewhere at the top of my “favourites” list when I was little. I often tried to imitate them in my games, but found that I was clumsy and slow on all fours. Cheetahs could bend their hind paws forward underneath their chins, kicking their front paws back to meet them between each enormous, graceful bound. I tended to fall off the bed when I tried this.
In fact, cheetah’s legs seemed to be entirely backwards compared to my own, with their hind legs appearing to angle forwards and their arms bending backwards. I concluded that their knees and elbows were on the wrong way round, and that this was the secret to their immense speed. I was a little bit right, and a little bit wrong, as it turns out. Let me explain.
Here is a human.
Cheetahs came somewhere at the top of my “favourites” list when I was little. I often tried to imitate them in my games, but found that I was clumsy and slow on all fours. Cheetahs could bend their hind paws forward underneath their chins, kicking their front paws back to meet them between each enormous, graceful bound. I tended to fall off the bed when I tried this.
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| Go on, give it a go. |
In fact, cheetah’s legs seemed to be entirely backwards compared to my own, with their hind legs appearing to angle forwards and their arms bending backwards. I concluded that their knees and elbows were on the wrong way round, and that this was the secret to their immense speed. I was a little bit right, and a little bit wrong, as it turns out. Let me explain.
Here is a human.
Her thigh extends about halfway down her leg, and she stands on the soles of her feet, with her heel touching the floor. It works very well for her unusual upright style of walking. This should sound familiar- if not, I hope you've checked it out with your doctor. This leg design makes her a plantigrade. All primitive mammals were originally plantigrades, but we're in somewhat mixed company these days: bears, other primates, most rodents, raccoons and the weasel family. Big palms and a flat sole are great for gripping things, including trees and food. However, a lot of mammals have switched their adorable little grippy paws for running shoes, and changed the design of their legs in the process...
This is where my friend the cheetah comes in, but to demonstrate, I've hired a friendly dog who has similar leg anatomy.
Dogs, cats and a few other groups of mammals are digitigrades. They walk on their toes with their heels lifted off the ground (as if they're wearing massive high heeled shoes), and it was this heel that I mistook for a backwards knee. The knee is actually close to the body, at the end of a relatively short thigh. If you have a pet willing to be prodded, gently massage the section of the leg above the paw, and you'll feel clusters of bones and tendons similar to the ones in your hands and feet. So why the redesign?
Dogs and cats are predators, so running fast is a useful skill. Having small points of contact with the ground makes movement much more efficient, so they can expend less energy when chasing prey. It also lengthens the legs without any one bone becoming excessively long and potentially weak. This is such a good design that predatory dinosaurs evolved similar legs, and passed them onto their descendants: the birds. Most birds don't make use of this generous gift for its original speed-boosting purpose, but almost all have retained it. Here is a flamingo to demonstrate. Flamingoes are often falsely thought to have backwards knees, but as you can see, the bend is simply its ankle.
Unlike mammalian digitigrades, the ankle and foot bones are fused in the lower section of birds' legs, forming a single, unique bone called the tarsometatarsus. This might have made their dinosaur ancestors' legs stronger, reducing the risk of injury as they chased prey at high speed.
Dogs and cats are predators, so running fast is a useful skill. Having small points of contact with the ground makes movement much more efficient, so they can expend less energy when chasing prey. It also lengthens the legs without any one bone becoming excessively long and potentially weak. This is such a good design that predatory dinosaurs evolved similar legs, and passed them onto their descendants: the birds. Most birds don't make use of this generous gift for its original speed-boosting purpose, but almost all have retained it. Here is a flamingo to demonstrate. Flamingoes are often falsely thought to have backwards knees, but as you can see, the bend is simply its ankle.
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| Twins! |
Unlike mammalian digitigrades, the ankle and foot bones are fused in the lower section of birds' legs, forming a single, unique bone called the tarsometatarsus. This might have made their dinosaur ancestors' legs stronger, reducing the risk of injury as they chased prey at high speed.
With the festive season fast approaching, you can also become an expert at bird anatomy by examining the remains of a turkey carcass (or bucket of fried chicken, if you prefer). The thigh, which is usually hidden under the living bird's feathers, is short and muscular. Attached to it is the cut commonly known as the drumstick, which is equivalent to your calves and contains the tibula and fibia bones. The tarsometatarsus and toes, whilst interesting, aren't usually present on the bird at the point of sale, unless your butcher was drinking Christmas cocktails on the job.
Of course, predators aren't the only ones modifying their legs for speed, or Earth's great meat buffet would have been licked clean by now. Hoofed mammals, also known as ungulates, have gone one step further with the invention of the unguligrade foot design. Here, the animal walks on its toenails, which are modified into what we usually call a hoof. Many groups have a reduced number of weight-bearing toes, too. This is because running on two strong toes, for example, is more efficient and safe than running on four weaker ones. Come to think of it, ballet would be a lot easier if humans had hooves, but you'd have to sacrifice standing on two legs, which defeats the purpose somewhat.
In deer, cows, goats, sheep and antelope, the third and fourth toes of the foot are strengthened to form the classic cloven hoof, with the second and fifth toes showing up as non-weight-bearing dewclaws further up the leg.
Horses have gone even further, running on just a single, extremely strong third toe! The other toe bones are barely there, hugging against their massive neighbour.
Interestingly, horses can suffer from a genetic condition known as polydactyly, meaning they're born with extra digits. On humans, this manifests itself as six-plus digits on a hand or foot- not always that noticeable unless you're counting- but it gets a little odd when you've only got one functioning toe to begin with. Check out this case of a Belgian foal with the condition.
So if ungulates have managed to evolve even longer, more efficient legs with hooves, why haven't predators done the same? There did once exist a group of hoofed predators called mesonychids, related to today's ungulates. They didn't have the extreme hooves of deer or horses, but a foot design more like that of a rhinoceros, with a flattened, blunt, toenail at the end of each toe. This makes a nice, sturdy surface for running on, but doesn't hold the animal's entire weight like other hoof designs. In rhinos, the toes don't touch the floor at all, and the rear of the "foot" is made of a fleshy pad of fat. It's anyone's guess what sort of footprint was left by a mesonychid on a muddy path, but maybe they had paw pads like a dog.
They had a superficial resemblance to wolves with freakishly large heads, shortish legs and a fairly inflexible spine, Their hooves might have been great for running, but they probably lacked the versatility of clawed paws; they wouldn't be good for hunting like cats' claws, for example, nor would they have been much use for digging. The reason for their eventual extinction ~30 million years ago is unclear, though they may have competed poorly with more modern predators. I like to think that in some alternative universe, the mesonychids survived and were domesticated into an adorable race of pseudo-puppies, clip-clopping behind their owners with oversized muzzles on their toothsome faces. Adorable.
So there you have it. The rest of the animal kingdom's legs and feet might appear pretty alien to us, but when you look closer, it turns out we're the unusual ones. Take off your socks, take a long hard look at your feet, and tell me they don't look weird to you now. You massive flat-footed freak.
Of course, predators aren't the only ones modifying their legs for speed, or Earth's great meat buffet would have been licked clean by now. Hoofed mammals, also known as ungulates, have gone one step further with the invention of the unguligrade foot design. Here, the animal walks on its toenails, which are modified into what we usually call a hoof. Many groups have a reduced number of weight-bearing toes, too. This is because running on two strong toes, for example, is more efficient and safe than running on four weaker ones. Come to think of it, ballet would be a lot easier if humans had hooves, but you'd have to sacrifice standing on two legs, which defeats the purpose somewhat.
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| I would like to apologise to all horse-kind for this picture. |
In deer, cows, goats, sheep and antelope, the third and fourth toes of the foot are strengthened to form the classic cloven hoof, with the second and fifth toes showing up as non-weight-bearing dewclaws further up the leg.
Horses have gone even further, running on just a single, extremely strong third toe! The other toe bones are barely there, hugging against their massive neighbour.
 |
| Yes. That tiny little toothpick of a bone. |
Interestingly, horses can suffer from a genetic condition known as polydactyly, meaning they're born with extra digits. On humans, this manifests itself as six-plus digits on a hand or foot- not always that noticeable unless you're counting- but it gets a little odd when you've only got one functioning toe to begin with. Check out this case of a Belgian foal with the condition.
So if ungulates have managed to evolve even longer, more efficient legs with hooves, why haven't predators done the same? There did once exist a group of hoofed predators called mesonychids, related to today's ungulates. They didn't have the extreme hooves of deer or horses, but a foot design more like that of a rhinoceros, with a flattened, blunt, toenail at the end of each toe. This makes a nice, sturdy surface for running on, but doesn't hold the animal's entire weight like other hoof designs. In rhinos, the toes don't touch the floor at all, and the rear of the "foot" is made of a fleshy pad of fat. It's anyone's guess what sort of footprint was left by a mesonychid on a muddy path, but maybe they had paw pads like a dog.
 |
| Sidenote- if somebody could please discover some fossil mesonychid footprints, that would be really cool |
They had a superficial resemblance to wolves with freakishly large heads, shortish legs and a fairly inflexible spine, Their hooves might have been great for running, but they probably lacked the versatility of clawed paws; they wouldn't be good for hunting like cats' claws, for example, nor would they have been much use for digging. The reason for their eventual extinction ~30 million years ago is unclear, though they may have competed poorly with more modern predators. I like to think that in some alternative universe, the mesonychids survived and were domesticated into an adorable race of pseudo-puppies, clip-clopping behind their owners with oversized muzzles on their toothsome faces. Adorable.
So there you have it. The rest of the animal kingdom's legs and feet might appear pretty alien to us, but when you look closer, it turns out we're the unusual ones. Take off your socks, take a long hard look at your feet, and tell me they don't look weird to you now. You massive flat-footed freak.
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