I was thinking about humanoid designs, usually the most prolific version adds a digitigrade posture, looks cool and have some advantages for cursorial creatures providing speed but reducing stability and efficience when walk requiring mor energy than necesary for keep this humanoid creature walking or even just standing, my current conclusion of the causes of this is because the support area is notoriously reduced with just having the fingers contacting with the floor that made me wonder could these humanoids even walk? Won't they continually fall? Is the energy cost of keeping your legs this way worth it?.

Those are the reasons of why I thought a kangaroo-like tail could be very useful for improve the posture providing with third support point and helping with balance.

In fact currently was in develop a robotic tail to help with balance problems and almost sure is possible use this tail to stand, not for much time but maybe for rest short time periods is good.

enter image description here

Even if my assumption about the use of the kangaroo like tail is wrong, the tail presence still being interesting and problematic, because I remember read that the presence of tail in a humanoid shape is harmful or contradictory to the correct posture, gluteus and other muscles used to stand, because the tail required other massive muscles to work.

But even I found some infomation which would indicate this is less problematic as could seems. With sthenurine kangaroos which probably had a semihumanoid erected posture convergent in some aspects with hominidae features, obtaining a digitigrade (or even unguligrade) posture supported by the tail, coupled with the fact that this family of kangaroos would have put jumping aside in favor of walking, being able to move their legs independently. But this were just some visions and reconstructions of these species, as they are not conclusive and something truly functional still needs an analysis.

And as last complemente, kangaroos' tails are special, are very different to other species, these kind of tails have a great versatility, because even looking as rigid supports to stand, can turn flexible and be used with ondulatory moves to swim (as you can see in this video) and other interesting feature is the nail tail from the wallaby, which practically have a little claw with unknown function but could be for defense or be used in drepanosaurus like style.

So my questions are: how to get or develop and anatomically correct humanoid with a strong and versatile kangaroo like tail?, what kind of anatomical changes are necesary?

I emphasize that my problems are with the skeletal and muscular configuration of this fictitious species, since generally it would have to have a torso similar to a human but underneath with the aforementioned characteristics of digitigrade and a versatile and strong tail like a kangaroo.

A last addition, this question can be highly related with this How much I can change the humanoid body plan and still be able to perform the high speed throwing?

  • $\begingroup$ Does this answer your question? worldbuilding.stackexchange.com/q/119542/30492 $\endgroup$
    – L.Dutch
    Mar 29, 2021 at 10:24
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    $\begingroup$ "Could these humanoids even walk?" Most people can walk on tiptoes with no problems. Most people can stand on tiptoes with no problems. When running and accelerating, most people instinctively adopt a digitigrade posture. For unmodified people walking, running and standing on their toes is energetically expensive because our feet are not made to function in that position for long periods of time; but adding a locking mechanism (as for example in horses) would easily solve the problem. $\endgroup$
    – AlexP
    Mar 29, 2021 at 12:27
  • $\begingroup$ Not related to the question, but en.wikipedia.org/wiki/Comma_splice $\endgroup$
    – JDługosz
    Mar 29, 2021 at 14:17
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    $\begingroup$ Ostriches, Emus and rheas all use their toes to support their body, and not only do they walk without problems, but they're also very speedy, with ostriches being able to reach 70km/h. As for your problems with such legs being poor at being stable or energy efficient, it's likely because you are thinking of a creature evolved to have plantigrade gait trying to maintain a digitigrade one. True digitigrade bipeds have some minor changes to how their legs are structured. Great for running and standing, not so much for sitting in chairs. $\endgroup$ Apr 13, 2021 at 14:17
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    $\begingroup$ @Frostfyre for duplicates, gold badge holders (and mods) can change what the duplicate is linked as well as add more duplicates. Since this question was closed by a gold badge holder, I've asked them to add your link as a dupe. It's a good middle ground instead of re-opening and VTCing immediately. Only works with dupes, though - it's not possible to swap the actual reason easily, just the dupe target lists can be edited. $\endgroup$
    – VLAZ
    Apr 13, 2021 at 15:09

3 Answers 3


Tails don't help you be digitigrade

First off: some humans ARE digitigrade. About 5% of healthy children undergo growth spurts where their calf muscles grow slower than their tibia and fibula causing them to walk on their toes. Because the calf mussels are pulled tighter than normal, they stand on their toes using the tension in the muscles rather than needing to exert any extra force to bring them up to a digitigrade posture. So, for these people, it is as comfortable to stand, walk, run, and jump up on the balls of their feet as it is for most people to do those things flat footed. This can cause temporary balance issues as they may transition back and forth between digitigrade and plantigrade, but these balance issue have nothing inherently to do with being digitigrade.

While more rare, some perfectly healthy adults are also permanently digitigrade. This is most common among people who are/were athletes who needed MORE balance (boxers, wrestlers, etc.) where they spend/spent so much time training in a digitigrade posture, that the muscles tightened and they assumed this as the default posture. A plantigrade posture is easier on your bones and muscles, but makes you slower, less well balanced (not more as the OP suggests), and only uses less energy if your muscles are not designed to be digitigrade. Despite the the smaller ground contact area, humans in a digitigrade stance have much better balance than plantigrade humans because they can react better to lateral forces without the risk of being rolled over their heels: thus the expression "caught flat-footed".

The only reason most humans are plantigrade is because it causes less health problems as we get older. Because the bones in our feet are not fused together, extended use of the digitigrade posture can stretch out ligaments and cause foot pain. Humans are designed to be nomadic. We can walk for very long distances compared to other animals, but this is not a necessary feature of all humanoid or bipedal beings. Ostridges for example are bipedal and digitigrade and suffer no significant negative side effects from it... but they are built for sprinting, not distance, so they have evolved to be better suited to a digitigrade default stance.

Why so many bipeds have minimal tails

The biggest problem with the bipedal body design is weight. Things that make you heavier are a bigger deal than when you are a quadruped; so, heavy things like tails actually slow you down making them counter-productive. Tails only help bipeds with balance when their body plan is leaned forward (like a kangaroo or t-rex), but when you make your weight distribution more vertical, like a human, you can drop the weight of the tail allowing you to become faster and still maintain good balance by centering your weight over your feet. If you are upright, a tail will just make you want to fall backwards.

Going upright, and loosing the tail frees up a lot of weight which means your bipeds can also support larger stronger arms which is important specifically for humanoid body designs. For a biped to have a forward leaning torso, adding the weight of large arms would have to be offset by an equally heavier tail (which is probably why kangaroos and t-rexes have such small arms)

In summary: if you want an upright biped to be digitigrade just make them digitigrade: it works well, and adding a tail only adds problems, not solutions. BUT if you want to add a tail for the sake of adding a tail, then you will need to make the body lean forward so that the body and tail can counterbalance each other.

  • $\begingroup$ Upright bipedalism has evolved twice therizinosaurs had a tail the other, hominids line lost the tail long before becoming bipedal. there is no reason tp believe bipeds will lose their tail. $\endgroup$
    – John
    Jul 27, 2021 at 19:34
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    $\begingroup$ @John Therizinosaurs were not truly upright bipeds. They were somewhat more upright than many other dinosaurs, and but far from truly upright SEE en.wikipedia.org/wiki/Therizinosaurus. As for why upright bipeds need to loose their tail is because standing is all about keeping your center of mass inside your base. Any mass that you place in front of your feet that leverages you forwards needs a counter-weight on your backside to offset it and vice versa. If you had a tail, sticking out behind you, you would need to lean forward to keep your balance. $\endgroup$
    – Nosajimiki
    Jul 27, 2021 at 20:10
  • $\begingroup$ Also, I did not say "bipeds will lose their tail", I even pointed out a few that exist. I just said that tails are bad for the humanoid body plan. $\endgroup$
    – Nosajimiki
    Jul 27, 2021 at 20:12
  • $\begingroup$ Only one upright biped line by your definition, and it lost its tail long before to became upright. So again no reason to presume it would lose the tail, reduce certainly but no reason to lose it. the only biped to reduce the tail after evolving bipedalism is birds , which obviously has other confounding constraints. $\endgroup$
    – John
    Jul 27, 2021 at 20:16
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    $\begingroup$ @Drakio-X Here is a decent article seattlechildrens.org/pdf/PE1532.pdf The thing about certain athletes developing idiopathic toe walking was something a podiatrist told me once, but I don't know if its something that he read somewhere or if it was just his personal observation since I've never actually seen it mentioned in any articles on the topic. $\endgroup$
    – Nosajimiki
    Jul 30, 2021 at 4:25

Let us suppose that we have a normal human being without a tail. If we focus on the pelvic skeletal area, we can see the vestigial remnants of the tail which has devolved from our distant ancestors, in the form of the coccyx.

Now, the OP proposes that we add a substantial tail to this basic body plan, a tail that is as capable and useful to its owner as a kangaroo's is to its owner, capable of supporting the being's full weight and providing propulsion in both terrestrial and aquatic locomotion. The OP is correct in believing that adding such a tail would not be as simple as replacing the coccyx with the far more substantial appendage desired.

Why wouldn't we be able to just 'whack on' a big tail and call it a day? What would we have to do to make it work?

Consider the dimensions of this new tail. As any tail, it would consist of an extension of the vertebral column beyond the pelvis. In order to be able to support its owner's full weight and contribute to locomotion, it would have to have vertebrae that are robust enough to support the load, with muscle attachment processes big enough to anchor strong muscles. It would have to be at least as long as the leg plus the length of the foot - it would not support its owner's weight directly on its tip, but on at least twenty to forty centimeters of the end of the tail, almost like an extra foot, much as kangaroos use their tails.

With all this muscle and bone, we are looking at a substantial, massive appendage, with vertebrae roughly the same thickness as the lumbar vertebrae, surrounded by muscle that might make it as much as ten centimeters in diameter where it emerges from the pelvis. Unlike the picture of the artificial tail, it would emerge from the pelvis in line with the spine, not at right-angles to it.

This extra mass of muscle and bone would not come without consequences. The main consequence would be the alteration of the pelvic opening. Naively, this tail would 'bulk up' the coccyx on both sides, but this would result in it projecting into the pelvic girdle. For kangaroos, which give birth to offspring that weigh a few grams, this is not an issue, but for a human, which gives birth to offspring weighing five kilograms or more, with heads that have a diameter of ten centimeters, obstructing the pelvic opening could be fatal for both mother and child. This could be solved by moving the pelvic girdle ventrally so that its opening was unobstructed. This would result in the lower back appearing not almost flat from side to side as in modern humans, but with a rather more projecting bump on the lower back. At the very least, this would make lying supine more difficult, as the pelvis would tend to roll to one side or the other.

The next problem is that of locomotion. Unless the tail is even more bulky and muscular than I have described, it would not be used as a means of propulsion when walking or running. Fully tripodal locomotion involving the tail would be awkward enough that it is unlikely to evolve at this point. However, the presence of this weighty, bulky appendage would not be without its effects.

When walking and running, if the pelvic spine was vertical as it is in modern humans, a tail of this bulk would collide with the legs and the ground unless it was held up and to the rear. However, in that case, the weight of the tail behind its owner would shift the centre of mass backwards unless the torso was moved forward to compensate, resulting in a forward-leaning posture when walking or running. In fact, this would likely lead to the natural position of the legs being not directly in line with the torso, but being at a slight angle. This change would be reflected in the shape of the pelvis, with a slightly different shape to the pelvic bones to allow for the altered stance.

Having such a tail would mean that when stationary, the being could lean back on their tail, holding their torso fully erect in a tripodal stance, almost as if they were carrying a tall, narrow unipod seat around with them. As humans' upright stance likely evolved in order to see over tall grass in a plains environment, this tail would assist in such a posture, and allow a lookout to maintain a more comfortable stance while standing watch.

Given the strength of the tail, it would be capable of pushing its owner forward from a standing position, making the transition from standing to walking or running easier and more rapid, and would also allow a powerful braced two-footed kick as practised by modern kangaroos.

Given sufficient lateral musculature in the tail, it could contribute to aquatic locomotion to a small degree, given that in humans, the bulk of the power when swimming is provided by the upper body. This musculature would also enable the tail to be used as a weapon, to be swung about to bludgeon an enemy, as well as its use as a brace to strengthen a kick.

The tail is unlikely to be prehensile, given that African monkeys do not use their tails for grasping, while only South American monkeys use their tails in that fashion. Tailed Humans, probably being evolved from African monkeys and apes, would be unlikely to have prehensile tails due to that ancestry. However, the OP may be considering an alternative evolutionary path that allows for a prehensile, grasping tail.

Should this tail be prehensile, its strength would allow it to be used for crude grasping, and to help anchor an object being worked upon by the more dexterous hands.

The last significant difference in anatomy would be in the brain. In contrast to modern humans, these tailed humans would have a slightly different brain structure in order to provide the motor output from and sensory input to the tail. It wouldn't be a particularly major difference, but would likely be enough that an expert could tell the difference between a modern human and a tailed human from their gross brain anatomy, especially if the tail was prehensile.

The energy for this tail could easily be provided for by a modern human's digestive system, so there would be no need for any significant change in dentition, gastrointestinal or cardiopulmonary anatomy.

There would be other consequences to this change in anatomy, including likely alterations in the manner of copulation, preferred sleeping positions, furniture and transport design, and other sociological implications, but these go beyond the scope of the OP's question.

  • $\begingroup$ About the prehensile tail, if is not the most important requirement this question and could be less related, I wonder if a creature with tail but not evolved in an enviroment for prehensile, could other enviroment or the use of tool permit to adapt that tail to be prehensile? $\endgroup$
    – Drakio-X
    Jul 29, 2021 at 6:57
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    $\begingroup$ Given the strength of the tail, and the intelligence of its owner, even if it didn't evolve to be prehensile (the vertebrae may be a bit long) it may still be able to be used in a semi-prehensile manner. It is strong enough to support the whole body weight, after all. $\endgroup$
    – Monty Wild
    Jul 29, 2021 at 14:44

Nature provides a model of an upright digitigrade biped.

Therizinosaurs are a group of dinosaurs that are basically upright digitigrade bipeds, they are not as upright as humans, but that is because they have a tail which means they need to sling the body slightly forward for balance, the bigger you make the tail the more forward the body needs to be. so your best bet is to estimate how much the tail of your creature weights, that will tell you how much you need to change the posture. Your problem is if you want a tail long enough, thick enough, and strong enough to support the weight of human well, it ends up weighing so much your person is closer to horizontal than upright.

enter image description here

Note the pelvis of this group has a very sharp bend in it to keep the tail pointed backwards. The tail supports the muscles that move the legs so it needs to remain roughly horizontal. Your tail does not need to be that horizontal.

there is already a question and answer about tails and balance so you can look for more info there. Lizardfolk and the problem of balance and anatomically sound tails

You are correct that putting a tail on a human frame causes all kinds of problem, but you don't have to worry about that. Mammals evolved butt muscles to move the hind leg because they could not anchor them to the tail the way lizards and dinosaurs do, because they had greatly reduced the tail. If your creature evolved with a tail there is no reason to believe it would have this kind of musculature, Kangaroos do not. Note the kangaroo musculature is also different than the dinosaur system.

Digitigrade animals have much larger toes than humans have, so as long as you make the toes bigger you can put a biped on a digitigrade stance no problem. The energy cost if being digitigrade is not significant, you need muscle tension to stand no matter what you are, being digitigrade does not make it significantly worse. It is harder for humans to walk digitigrade but humans evolved plantigrade, it is harder for digitigrades wo walk plantigrade as well. Just like any posture it comes down to the arrangement and size of muscles. There is a saving to being plantigrade but it is so minimal that you can safely ignore it when designing a fictional organism unless you absolutely need them to be endurance walkers like humans.

As a side note at least two groups of dinosaurs use their tails for support while standing on their hind legs, diplodocid sauropods and stegosaurs, however they are not bipeds, only standing to feed. these tails are very thick and strong (just like kangaroo) also both groups use them as weapons, so you do have some comparisons to look at.

So what changes do you need to make

You need bigger toes you also need to decide what kind of foot to give them, specifically how many toes and in what arrangement, kangaroo use four toes to get a pseudo-three-toed foot, wallaby foot dissection but you could use a dog like foot, dinosaur like foot or a selection of other digitigrades.

enter image description here

You need a bent pelvis, your pelvis needs to have a bend in it just like therizinosaurs to get the tail pointed is a reasonable direction, although it does not need to be bent as much. You will likely want to reinforce the sacrum as well, since you want it to support lateral loading.

You will need to adjust the musculature, for one thing your creature will not have a butt shaped like a human one, the musculature from the tail will blend together with it.

you will need to tilt the body forward, your creature will not be as upright as a human, they will be leaned forward, likely close to 45 degrees maybe a little less, to balance the weight of the tail.


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