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I saw some questions asking about digitigrades and bipeds, but I could not find much when it came to swimming. I know the two are capable of swimming, but I was wondering whether digitigrades or bipeds would be more efficient at swimming.

I would think maybe plantigrade would be better because the legs do not have a bend and can move up and down like a flipper. A digitigrade could still swim, but it is more like how a quadruped would swim in which it is limited to a dog paddling style. However, that is not adding in a tail and fins. If it had fins along the body and a shark like tail, I would think the digitigrade structure would not be as much of a hinderance because the tail and fins help to counter the disadvantage. The opposite is also a factor. If it was a plantigrade with a tail of a dolphin and fins, I would think the outcome would be better because the legs work in a vertical motion along with the tail plus support from the fins.

Just for clarification, I am referring to human leg anatomy against anthropomorphic leg anatomy. Digitigrade is referring more to walking on the toes and not touching the ground with its heels while plantigrade means walking on the soles of the feet. Was unsure if those are the right terms to use but hopefully that helps.

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    $\begingroup$ Humans swim mostly with their arms because of how well hands cup water. Which direction the leg bends is far less important than the surface area of the foot. $\endgroup$
    – Dragongeek
    Sep 24, 2021 at 15:06

4 Answers 4

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I was wondering which type would be more efficient at it

Efficiency in swimming comes from adaptations to swimming. If you're not adapted to the sea, you're not going to be great moving about in it. Get you some swimming fins, or a nice tail.

Humans can swim OK, but imagine if we had skinny plantigrade legs with claws on the end, in a vaguely birdlike fashion. Those kind of legs are obviously pretty poor for swimming with, independently of the walking style. Give those feet some nice webbing like a seagull, though, and you're back in business.

If it was a plantigrade with a tail of a dolphin and fins, I would think the outcome would be better because the legs work in a vertical motion along with the tail plus support from the fins.

If you've got a nice tail suitable for propulsion under water, you want to be keeping your limbs tucked in to minimise drag. Have a look at how crocs swim, for example... when moving around slowly, they'll use their legs a bit, but when moving at speed the libs get tucked in and that big tail provides all the power.

which is better for swimming?

Without adaptation, I don't think either is much better for swimming.

With adaptations though, there do seem to be a larger number of plantigrades, even evolving from quite different starting points... consider penguins and polar bears, for example. Clearly there's something driving that difference.

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Intrinsically, it does not matter

Digitigrade vs plantigrade only describes if an animal prefers to focus its weight on its tarsals or phalanges. But there is no hard rule in nature saying anything about how that affects your ability to swim. Without knowing what exact two animals you are comparing, there is no way to say for sure.

While most amphibious animals get called plantigrade, this is not really a good classification system to use for them since, it is more accurate to say that things like seals and turtles prefer not to walk at all, and what they do for land based locomotion is neither plantigrade nor digitigrade stride since they typically make ground contact with everything elbow/knee down, not just the foot; so, lets throw them out of the pile and look at actual digitigrade vs plantigrade.

There are many plantigrade animals like porcupines and chimpanzees which are terrible swimmers, and some like brown bears are great swimmers. Then there are some digitigrade animals that are poor swimmers like most breeds of domestic cats and dogs, and then there are those which are great swimmers such as tigers and horses. More over, even within a single species, there can be HUGE variances in swimming abilities. After all, some humans would drown if you threw them into a swimming pool while others can cross the English Channel despite having very similar walking preferences.

And sometimes... we just break the digitigrade / plantigrade rule all together:

The difference between digitigrade vs plantigrade

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Plantigrade

Aquatic animals tend to have shorter, wider limbs. This clearly points towards human legs over digitigrade legs

Examples of this tendency can be seen in pinnipeds

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  • $\begingroup$ I see, so seals, walrus, etc. $\endgroup$ Sep 24, 2021 at 11:57
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It depends. What role do the legs play?

Nature is filled with all sorts of different animals which have varied forms despite all coming from the same ancestor. The beauty of this is that even though we can find very different creatures, thanks to convergent evolution we can also find different creatures that look very much alike thanks to being exposed to a similar environment and evolving a similar bodyplan as a "solution" (yes, I am speaking as if evolution was a magical process where everything is premeditated even though it is clearly not). So basically, before looking at the limbs themselves, let's look at what nature selected as the ideal traits for a good swimmer:

  • mostly streamlined, ideally torpedo-like shape.
  • the ability to float.
  • some method for forward propulsion.
  • some method for changing direction.

These basically cover the most important things from a purely mechanical perspective (ignoring things like the ability to breathe underwater or hold your breath for long periods), and that's where leg shape becomes important or not:

Penguins, being part of the bird family, are naturally digitigrade, this however doesn't keep them from being good swimmers, because they made full use of their avian ancestry to turn their wings into paddles which they use to swim through the water, keeping most of their legs essentially inside their bodies, their digitigrage leg anatomy being more important for allowing them to move on land than they are at swimming.

Seals and sea lions took a different approach, descending from plantingrade bears if I'm not mistaken, they completely converted their hands and feet into flippers, with the main differences between the two being that the seal bodyplan is slightly more adapted for an aquatic environment than the sea lion bodyplan, allowing them to normally reach higher speeds at the cost of loosing most of their mobility on land (they can basically only move around by bouncing, meaning they're screwed if they're on land being pursued by something and water is too far away) while sea lions, while very capable of swimming at great speeds in short bursts, are overall a little less adapted for aquatic live, retaining their ability to turn their legs around and actually walk on land, as well as some other "minor" traits that differentiate both bodyplans. In this case the plantigrade posture was very useful, because the entirety of the feet could be converted into a flipper, rather than just the toes.

Dolphins and whales showed yet another design, which is also shared by fish: the absolute majority of the propelling is done via their tail, and unlike sea lions and seals, they had no plans on going back to land ever again. In such a scenario, neither plantigrade nor digitigrade is ideal, because the best course of action to get a torpedo shape if you don't need your hind limbs is to loose them altogether.

Whale evolution also shows us that at least for mammals plantingrade might be best, since while pakicetus (with little to no aquatic adaptations) was digitigrade (with traits common to hooved animals at that), ambulocetus (the mammalian attempt on making a crocodilian) had a more plantigrade bodyplan, with more evident adaptation to a semiaquatic lifestyle.

Crocodilians are the last best example I can thing of: they still needed their limbs due to not giving up on land, but don't often use them when actively swimming at high speeds, meaning their limbs are kept small, but strong in Order to do their job. Crocodilians, like many reptiles, are plantigrade.

What you have to keep in mind is: while digitigrade limbs are normally more associated with animals that need to reach higher speeds on land, plantigrade limbs normally mean slower movement speed, but more stability due to a greater surface area, which is also one of the traits you want to see in a fin used for swimming. If you're still going to go on land but your hind limbs aren't doing any swimming (be it because you mostly use only your forelimbs or a tail), either digitigrade or plantigrade is fine, so long as you have ways to either keep them small or within your body to be more streamlined. However, if you only need to swim with a tail and those hind limbs won't ever do any walking, ditching them is probably a better approach so they don't increase your drag in the water.

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