Ankles up
January 30, 2020 11:50 AM Subscribe
Why have some animals' ankles evolved up to be where other animals' knees are, halfway between the body and the ground? Is there any pattern to which animals have the halfway joint pointing back and which have it pointing forward?
There are a handful of animal examples in this Quora question, in which - you will note - all the answers seemed to miss the "why" and instead focused on "it's not a knee, it's an ankle". I'm looking for the "why". Does ancestry or ecology give us any clues, or is the upward evolution of the ankle (or downward evolution of the knee, if that's what it was) a mere accident of history?
There are a handful of animal examples in this Quora question, in which - you will note - all the answers seemed to miss the "why" and instead focused on "it's not a knee, it's an ankle". I'm looking for the "why". Does ancestry or ecology give us any clues, or is the upward evolution of the ankle (or downward evolution of the knee, if that's what it was) a mere accident of history?
The explanation I was taught in school is that the configuration of a horse's leg keeps the heavy thigh muscle short and minimizes its motion, while the toe and ankle, which move the most in a horse's stride, are light. The action of running is therefore optimized for efficiency.
posted by SPrintF at 12:18 PM on January 30, 2020 [3 favorites]
posted by SPrintF at 12:18 PM on January 30, 2020 [3 favorites]
I can't speak generally, but tarsiers (named for their extremely long tarsal bone - that extended ankle) and galagoes (bushbabies) use that extended tarsal bone as a lever when leaping.
posted by ChuraChura at 1:02 PM on January 30, 2020 [2 favorites]
posted by ChuraChura at 1:02 PM on January 30, 2020 [2 favorites]
There’s an episode of the Brain Scoop that at least touches on the answer. It talks mainly about different types of locomotion, but that’s determined by leg morphology.
posted by duien at 5:46 PM on January 30, 2020
posted by duien at 5:46 PM on January 30, 2020
Best answer: If I understand correctly it's all about how much jumping the animal does. When the foot is longer there is more spring and the animal can bound higher. If you look at a kangaroo with its incredibly long foot you can see that the thigh, ankle and foot are all relatively the same length enabling the two joints to hinge shut and then open hard, giving it a great spring. When a kangaroo jumps it can get a lot of height and the spring means that the momentum of the previous jump feeds into the next jump without much energy loss.
Animals like horses are surface grazers so they spend a lot of time with their heads down. They need thin legs so they can see predators and keep track of other members of the herd while their heads are down. They are flight animals so they need to have no surplus body mass to ensure they can move fast. They jump and they bound to go faster so they need a lot of spring and thus their hock is much higher up than our human heel. Their leg length keeps them alive.
Cows spend rather less time grazing than horses do as they chew their cud which takes up some of the time. And if there are predators they are more likely to stand their ground because of their mass. Part of that is because they can't run as fast and part of that is that they are less vulnerable to broken legs than horses. Predators like wolves go after the legs. Cows have a similarly high hock to a horse, but it is slightly lower because they need thicker legs to support their greater mass.
Animals that walk on their toes are designed to go fast. Toe walking lengthens the legs so it increases the length of the stride as well as giving it extra spring.
Humans, by contrast walk flat footed because we are totally not jumpers, we are endurance walkers and designed to pace steadily along. When we do jump we don't do it with both feet together like the kangaroo, but throw one leg over first in order to compensate for our relatively weak thrust. That means that we don't use the force of both legs simultaneously to achieve height or distance, we use the lengths of our legs spread instead. When a seriously trained human is trying to do major jumps they add a complete flip as with a vaulter who leads head first but turns around in mid air to bring the feet forward and land foot first. But this is a good way to get injured so it's a trick humans don't do very often.
Look at the different types of work the different breeds of dogs do and compare the difference in their legs. Salukis are designed to run long distances at high speed while terriers have strong short paws designed to rapidly dig their way into rat holes and burrows, and not be so long that they can't move fast in tight quarters once they get into the hole to drive the rats out.
posted by Jane the Brown at 6:35 PM on January 30, 2020 [8 favorites]
Animals like horses are surface grazers so they spend a lot of time with their heads down. They need thin legs so they can see predators and keep track of other members of the herd while their heads are down. They are flight animals so they need to have no surplus body mass to ensure they can move fast. They jump and they bound to go faster so they need a lot of spring and thus their hock is much higher up than our human heel. Their leg length keeps them alive.
Cows spend rather less time grazing than horses do as they chew their cud which takes up some of the time. And if there are predators they are more likely to stand their ground because of their mass. Part of that is because they can't run as fast and part of that is that they are less vulnerable to broken legs than horses. Predators like wolves go after the legs. Cows have a similarly high hock to a horse, but it is slightly lower because they need thicker legs to support their greater mass.
Animals that walk on their toes are designed to go fast. Toe walking lengthens the legs so it increases the length of the stride as well as giving it extra spring.
Humans, by contrast walk flat footed because we are totally not jumpers, we are endurance walkers and designed to pace steadily along. When we do jump we don't do it with both feet together like the kangaroo, but throw one leg over first in order to compensate for our relatively weak thrust. That means that we don't use the force of both legs simultaneously to achieve height or distance, we use the lengths of our legs spread instead. When a seriously trained human is trying to do major jumps they add a complete flip as with a vaulter who leads head first but turns around in mid air to bring the feet forward and land foot first. But this is a good way to get injured so it's a trick humans don't do very often.
Look at the different types of work the different breeds of dogs do and compare the difference in their legs. Salukis are designed to run long distances at high speed while terriers have strong short paws designed to rapidly dig their way into rat holes and burrows, and not be so long that they can't move fast in tight quarters once they get into the hole to drive the rats out.
posted by Jane the Brown at 6:35 PM on January 30, 2020 [8 favorites]
I am not sure about the why either, but some search terms that might help are:
digitigrade, which means walking on toes (like cats and dogs)
unguligrade, which means walking on hooves which correspond to the tips of the toes (like goats and horses)
plantigrade, which means walking on the whole plantar (bottom) surface of the foot (like humans and apparently kangaroos)
Wikipedia has this to say:
The primary advantages of a plantigrade foot are stability and weight-bearing ability; plantigrade feet have the largest surface area. The primary disadvantage of a plantigrade foot is speed. With more bones and joints in the foot, the leg is both shorter and heavier at the far end, which makes it difficult to move rapidly.
Among extinct animals, most early mammals such as pantodonts were plantigrade. A plantigrade foot is the primitive condition for mammals; digitigrade and unguligrade locomotion evolved later. Among archosaurs, the pterosaurs were partially plantigrade, walking on the whole of the hind foot and the fingers of the hand-wing.
Non-plantigrade animals tend to have the high ankle/“backwards” knee that you’re talking about, but kangaroos are an interesting counterexample.
posted by danceswithlight at 1:27 PM on January 31, 2020 [2 favorites]
digitigrade, which means walking on toes (like cats and dogs)
unguligrade, which means walking on hooves which correspond to the tips of the toes (like goats and horses)
plantigrade, which means walking on the whole plantar (bottom) surface of the foot (like humans and apparently kangaroos)
Wikipedia has this to say:
The primary advantages of a plantigrade foot are stability and weight-bearing ability; plantigrade feet have the largest surface area. The primary disadvantage of a plantigrade foot is speed. With more bones and joints in the foot, the leg is both shorter and heavier at the far end, which makes it difficult to move rapidly.
Among extinct animals, most early mammals such as pantodonts were plantigrade. A plantigrade foot is the primitive condition for mammals; digitigrade and unguligrade locomotion evolved later. Among archosaurs, the pterosaurs were partially plantigrade, walking on the whole of the hind foot and the fingers of the hand-wing.
Non-plantigrade animals tend to have the high ankle/“backwards” knee that you’re talking about, but kangaroos are an interesting counterexample.
posted by danceswithlight at 1:27 PM on January 31, 2020 [2 favorites]
Best answer: If you look at a lizard as a "basic" quadruped - it's designed to grasp and haul itself around the ground and up and down the vegetation. When you look at horses, I think it make more sense if you imagine the limbs being more or less the same as a lizard relative to the torso but the feet and toes are extending greatly down to the ground. When you're not hauling your body weight up and down trees and only using your limbs for support, it's more efficient to lighten the lower parts of limbs and operate them with skinny tendons.
Now, if you look at our immediate primate ancestors, their limbs are a lot more lizard-like because of the need to grasp and lift themselves through the vegetation. We're a REALLY recent descendants, so the way we adapted for savannah locomotion is kind of ad hoc -we preserved the basic proportions but quickly trimmed down the area below the knee to a mostly tendinous structure and it's served us well enough for what we needed to do.
We still have those climbing monkey/lizard legs with our knees about half way down while most other walking animals have compacted legs with the extended feet and toe design.
posted by bonobothegreat at 5:31 PM on February 1, 2020 [2 favorites]
Now, if you look at our immediate primate ancestors, their limbs are a lot more lizard-like because of the need to grasp and lift themselves through the vegetation. We're a REALLY recent descendants, so the way we adapted for savannah locomotion is kind of ad hoc -we preserved the basic proportions but quickly trimmed down the area below the knee to a mostly tendinous structure and it's served us well enough for what we needed to do.
We still have those climbing monkey/lizard legs with our knees about half way down while most other walking animals have compacted legs with the extended feet and toe design.
posted by bonobothegreat at 5:31 PM on February 1, 2020 [2 favorites]
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One thing to note is that human knees are weird, so what you think of as "normal", maybe isn't.
I'm not an expert on the evolution of mammalian limb morphology, and a lot of the papers I see are paywalled, but the gist of it seems to be: joint location and limb-segment length are largely understood to be selected for functional performance, which is of course related to behavior and a lot of other complicated stuff (hunting/evading predation, foraging, migration, who your enemies are, etc)
Here's a selection of semi-relevant and freely accessible literature:
Evolutionary Approaches for Studying Functional Morphology: Examples from Studies of Performance Capacity
Limbs in Mammalian Evolution (crappy link will break soon. Search title and (Polly, 2007) in google scholar to get the pdf from academia.edu)
Evolutionary Morphology of the Tenrecoidea(Mammalia) Hindlimb Skeleton
For more research on your own, /evolution limb joint morphology [taxa of choice]/ gets pretty good hits on google scholar. E.g. here for reptiles. If you really want a specific paywalled article, let me know :)
posted by SaltySalticid at 12:07 PM on January 30, 2020 [9 favorites]