Shadeshifting III: a clear skin

The first post about ‘shade shifting’ described it as an animal’s ability to change colour in at most a few hours. Octopuses are most impressive in this regard: not only can they shift colour very quickly, but the resolution of their skin is very high, and their nervous system allows patterns with much detail. Earth’s shadeshifters are generally small. It would be nice to see large animals on other planets change colour, but that is not easy. Shade shifting, also called metachrosis, is usually brought about with small grains of pigment moving around in large flat living skin cells (chromatophores). Skin typically has a dead outer layer, which, being dead, does not contain chromatophores. Such layers will be thicker in larger animals, which is bad for shade shifting, because the thicker the layer, the less visible the living layers underneath are. Of course, if an animal has fur (dead material) or feathers (dead material), there is no chance at all to see living skin layers. 

There are two possible workarounds: keep the dead layer but make it transparent, or find a way to change the colour of dead skin. The present post discusses the first approach, with various detours. 

Adapting dead skin 

An obvious solution is to make the dead layer very thin, but its materials would then also have to become much, much stronger. That would entail more changes than just a strong skin, as those superior materials would also be used in the rest of the animal. Nice, but we just invented an animal that compares to a normal animal like Superman compares to a normal man: bring Kryptonite. 

No, the other solution is to make that dead skin transparent, or rather, translucent. When I wrote the first post that seemed extremely unlikely, but perhaps it is merely very difficult. There are two things I had not considered: some Earth animals are wholly or partially transparent, and some tough materials can be made transparent, including wood and leather. How does that work? 

 

Transparent wood 

Transparent wood and transparent leather are artificial materials. Transparent wood received much interest, because transparent wood offers is as strong and tough as normal wood, so it can deal with a lot more stress than glass. To understand how wood can be made transparent, we need to understand how light interacts with materials. 

Refraction on the left, combined effects on the right

Light usually reflects back from a surface, which is why we can see even highly transparent materials such as water or glass. When light enters a material, it may be refracted and scattered in any direction. The refraction index is a number stating how much the direction of light changes when entering (or exiting) a material. The difference in indices between two material is important. If you hold a glass (index 1.52) up in the air (index 1.0), you will see the glass easier than if you hold it in water (index 1.33), because the difference in indices is smaller. The background you see through the glass will be distorted more in air than in water, helping you to see the glass.

Finally, the light may be absorbed, and that is what causes colour; absorb every wavelength except the ones we interpret as ‘green’, and what you see is green. 

A perfectly transparent object would let all light through (so no absorption) without changing its direction (no refraction or reflection). In wood, lignin absorbs a lot of light, so to make transparent wood lignin is removed chemically, called ‘delignification’. The process does not sound like you would want to do to a living tissue, as it involves baths of boiling hot sodium hypochlorite and hydrogen peroxide. After that, the wood still scatters a lot of light because there is air inside it. As we saw above, large differences in refractive index cause a lot of scattering. To get rid of the air, it is replaced with fluid epoxy or various polymers with refractive indices close to that of cellulose (1.47). In most pictures I saw, there is still considerable scatter, so I would call the result translucent rather than transparent. 

Translucent leather is produced with similar tricks; it is of interest for shade shifting because the main ingredient of leather is collagen, nice to have in tough skin. 

So far, these artificial materials show that you can have strong and tough translucent materials based on biological products. But can living organisms get there too?  

From Barnett et al. 2025 

This scheme shows almost completely transparent animals (a,b,c), transparent patches (d,g,h), variable  translucency (e,f)
 

Transparent animals 

I found a nice chapter about animal transparency in a fascinating book, called ‘Avoiding attack’, about how animals use their appearance to avoid being eaten. That section showed that transparency evolved many times in many different phyla, which sounds good. But transparent animals have things in common that must be due to limitations of transparency, which are less good. The first constraint is that most transparent animals are small; the second is that they tend to live in the sea, and the third specifies middle regions of open oceans. 

• The preference for small size probably has to do with transparency not being perfect: if a one cm slab of tissue lets though 95% of light, meaning you may not notice it, then a 10 cm slab will let through only 60% (0.95^10), increasing the chance you will see it. 
• Living in the sea works because it reduces the difference between refractive indices. The index of water was 1.33, close to cytoplasm (1.34) and dense proteins (1.55). 
• As for middle layers of open ocean, that is where there’s nothing to see in any direction. Distorting a background image doesn’t matter if the image is completely featureless. Near the sea surface you get contrasts and shadows, and the deep ocean is so dark that being black works better than being transparent. 

How about terrestrial transparent animals? In air, they have a harder job than marine animals. They share with marine animals that some body parts are not transparent, such as the retina (if the retina lets through all light, you are blind). Red blood cells also hamper transparency; some ‘glass frogs’ shift their red blood corpuscles to their liver, so the part of the frog that you see is an amorphous blob that doesn’t look like a frog. If food is not transparent, the digestive system will show up. Some glass frogs have nice translucent limbs, but are still coloured green on their backs to hide the otherwise visible gut content. 

From: https://en.wikipedia.org/wiki/Greta_oto

Quite a few insects have wings that are wholly or partially transparent, which may break up the overall shape of the animal, making it difficult to recognise.

Unfortunately, no-one seems to understand how transparent animals achieve functional transparent tissues. However, their tissues become opaque after death, suggesting that maintaining transparency may require continuous energy. 

A final weird twist is that some researchers managed to make abdominal wall tissues of living mice transparent, to see what is going on in their abdomen. That trick was achieved with food dyes, which is important because food dyes are not (generally) poisonous, so these mice also show that living animal tissues can be transparent. 

Towards large shadeshifters 

The above suggests the possibility of transparent dead outer layers of an animal’s skin, or at least translucent ones, letting enough light through to allow deeper layers of the skin to shine through with whatever colour they have. But can such systems evolve? Possibly. Let’s start with something like a small frog with a thin skin that evolves the ability to change colour. That y itself should be an advantage. After all, animals use colours for many purposes colours such as hiding (‘Nothing to see here’), or to send signals to members of other species (‘I am toxic’) or its own species (‘I’m strong’ or ‘I’m sexy) or to control temperature. The ability to change messages must be a substantial improvement over a fixed message. When such animals evolve larger size, heir skin will thicken, and given enough time and a clear advantage, the skin could adapt to form translucent dead outer skin layers. Possible? I think so. Likely? Hard to say… 

 

Reading material

Ruxton, Allen, Sherrratt, Spek. Avoiding attack. Second edition. Oxford university press 2018

Barnett, Yeager, Kjernsmo. Animal transparency: How should we define form and
function? Functional Ecology. 2025; 39: 929–941

Yeager J, Robison A, Wade CD, Barnett JB. High Contrast Markings Can Negate the Benefits of Transparent Camouflage. Ecol Evol. 2026 Apr 10;16(4):e73490

Wang F, Zhou C. 'Transparent mice': deep-tissue live imaging using food dyes. Commun Biol. 2024 Oct 11;7(1):1307.

Sun T, Su J, Zhao Y, Tie X. Enhancing the efficiency of achieving optical transparency in live animals using absorbing molecules. J Biomed Opt. 2026 May;31(5):054702.

Before I sculpt and 3D-print Furahan life forms, here is the 'Mekon'!

I use a variety of tools to prepare paintings of Furahan animals. Often, a few sketches were all that was needed to position an animal in its environment, ready for painting. But sometimes I needed to get a better feeling for the shape of an, or for where light should make way for shadow, or where I should place all six, or all twenty-four, legs of an animal. In such cases, I made a model, most often a digital one. I bought Zbrush back in 2009, based on its fame and use by professionals. However, it was difficult to learn and the user interface seemed designed to scare you away. Still, I did use it, limiting myself to organic scape sculpting.

Blender sculpt and painting of a droodle (copyright Gert van Dijk) 

Here is an example of a Zbrush model as well as the painting that it was made for: the droodle (Lorica segmentosa), a painting later chosen as the cover of the Furaha book (reviews: see here!). If you compare the two images, you will see that the 3D model is not very detailed. There was no need for details, because the model was only meant as a fairly rough aid for a painting. I actually provided more details than necessary, because sculpting was fun (provided ZBrush did what I wanted). I had an urge to do more with such sculpts, but The Book had preference. Once The Book was published, I could do other things: I branched into a different style of painting, played with animation again (here and here), and other things besides. Still, wouldn’t it be nice to have a fairly large model of a rusp, or of a marblebill?

I could of course sculpt non-digitally; my earlier experiment with dinosaur sculpting was done with Furahan animals in mind. One consideration in all this was whether I could make multiple copies of a sculpt. With classical sculpting, that would require moulds and casting, possible but laborious. The alternative might be 3D printing, and, although it was clear that that would not be easy either, I decided to experiment with that. 3D printing starts with good models, meaning good modelling software. I never became good at ZBrush. Its awkward interface was never overhauled, and instead of buying it once you now have to rent it, a business model I dislike. The alternative was Blender, which is totally free and offers much more than just sculpting; you could make a professional animation film using Blender as the only software! 
     
I started by modifying the old droodle sculpt, but also decided to start a sculpt from scratch in Blender. I did not choose a Furahan animal, because, knowing myself, I would set my targets too high, so it would be easier to start with something allowed to go wrong (yes, that is probably neurotic). This time it wasn’t a dinosaur, but a character from a comic I grew up with: the Mekon! The Mekon is an alien humanoid, and the arch enemy from the very English space hero ‘Dan Dare’. We are talking the 50s and 60s now; here is a link to an extensive website. I have written a very short post on the Dan Dare comic before, drawing attention to the quality of the artwork in a depiction of an alien forest. 

 

The Mekon!

Here is a view of the Mekon: the character was bred to be a superintelligent ruler. In spite of his enormous brain, he acted as silly as any James Bond supervillain: instead of killing Dan Dare, his archenemy, on sight, The Mekon usually first explains all his plans and then leaves the hero, who promptly escapes. Great fun!

Blender sculpts of the Mekon 

Here are some screen shots of the progression of my Mekon Blender sculpt. I found that my knowledge of human musculoskeletal anatomy came in handy when sculpting a humanoid. In the comics, the Mekon has extremely thin limbs, suggesting feeble bones and almost no muscles. Without thinking about it much, my version evolved towards thin but well-defined muscles. I could not help giving the Mekon a sizeable chest, thinking that that massive brain would need a lot of oxygen. He also has strong muscles in his neck to balance that gigantic skull.

Mekon model

Here is that model imported into Vue. I gave the model an overall green colour but otherwise did not bother with colour.  

Right; so how about the 3D prints? I bought a Creality K1C 3D printer, for those of you who know about such things. My first few weeks of 3D printing led me to conclude that enjoying 3D printing requires a considerable ability to tolerate frustration. Within the first week I had to dismantle the print head because the filament was stuck. I had a recurring problem with the print head kicking over parts of the print, first because the print head knocked over the supports -nneded under parts that stick out sideways-, and after I solved that, the print head knocked the parts off their supports. I think I solved that too.

This type of 3D printer lays down molten material layer by layer; the bad news is that you can usually see those layers. The solutions include filling the gaps between layers or sanding them down, but that is a lot of work and risks sanding away all your nice details!

Here are examples of droodle 3D prints. The white material is slightly translucent, which means you hardly see layer lines, but you do not see other details well either. The orange / brown material is less translucent, which makes it easier see details (and flaws). 

Mekon 3D prints

The larger of these two Mekons lost one arm because of the ‘knocking off’ problem. Its head is partially white because I am experimenting with plastic putty to fill in the gaps between layer lines before sanding the rest away. The small Mekon was printed with thicker layers that are therefore well visible. I will work on getting a consitant good quality. 

 

This image was done wholly within Blender, showing it can do that. Right; does anyone need a Mekon model?   
 

Back from DinoCon 2026

Dinocon 2026 is already a few weeks in the past, so I am a bit late in writing about it. The reason is simple; other things got in the way, such as writing an important review, travelling, etc, etc.

So, how was it? It was truly massive, with a much larger audience than before; I heard that the number of visitors was twice that of last year’s event!. For me, the experience was completely different from last year’s DinoCon and from earlier TetZooCons, because I was a vendor this time. I had been a panel member, art exhibitor or just a guest in previous events, but being a vendor turned out to be a new experience. As a vendor, I could hardly leave our stall to listen to a talk, because there were always people (customers!) walking around in the vendors’ hall. I think I listened to only about 10 minutes of talks in all, so I cannot report on the talks at all. Then again, as a vendor you talk to many people, both customers and other vendors, which was also interesting and great fun, just in another way! 

My wife at our half of the stall; the other half belonged to CM Kosemen (not present in the image). Copyright Gert van Dijk

My wife and I shared a stall with CM Kosemen (Snaiad, All Tomorrows) to increase our percentage of dinosaur-related goods to the required level. I only had Furaha-related merchandise, but Mehmet sold speculative biology material as well as dinosaur art. He had brought 20 copied of ‘All Tomorrows’ that were all sold on the first day. I had brought 40 copies of the Furaha book, and those were all sold by early afternoon of the second day, so I probably could have sold 50 copies in all. Other vendors told me that those numbers represent very good sales. I hope the organisers take good notice of this, so speculative biology can become a fully accepted part of the conferences again, just as it was in TetZooCon days.

From left to right: me (Gert van Dijk), Dougal Dixon and CM Kosemen. Borrowed from CM Kosemen. 

Bookmarks from the 'Syndicate'. Copyright Gert van Dijk

Dougal Dixon showed up too, so the three of represented speculative biology. We had even made bookmarks to celebrate the ‘Syndicate of Planetary Biology Societies’, in the form of a set of three bookmarks with one bookmark for each of the planets Greenworld, Snaiad and Furaha.

An important part of selling a book is having good reviews, so I showed a printout with phrases from reviews of ‘Wildlife on the Planet Furaha’. I thought I might as well show those excerpts here, so they follow below, with their links.

What’s next? Well, more ‘serious’ posts, of course. I also finally bought a 3D printer. I made many rough 3d sculpts over the years to help me work out lighting and perspective of various animals. I thought that I could edit them and liven them up in Blender, and then print them. The first attempts look better than expected... 

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Reviews of 'Wildlife on the planet Furaha'

Dougal Dixon
This is a must-read book, which I heartily recommend.

Darren Naish  Wildlife on the Planet Furaha is a real tour-de-force that does its subject very, very proud. I adore this book and recommend it extremely highly.
https://tetzoo.com/blog/2026/3/30/gert-van-dijk-wildlife-on-the-planet-furaha

Biblaridion  Furaha is a fantastic example of how scientific rigour can be used to make a fictional world more cohesive and immersive, and a great addition to the pantheon of SpecBio classics.
https://www.youtube.com/watch?v=Q0lqBgO2xs4

Adrian Tchaikovsky  For those who, like me, love speculative and alien evolution, there's a remarkable new book out, "Wildlife on the Planet Furaha" by Gert van Dijk. I met Gert at TetZooCon a few years back and some of the Furaha art was on display there. It's an incredible book, both for the images and the science on display. 
https://bsky.app/profile/aptshadow.bsky.social/post/3m74jyqxxts2y

Davide Gioia This level of rigour and research sets Furaha apart from the majority of similar projects, making it among the most, if not the most, believable in this regard, a gold standard of hard SpecEvo if you will.
https://weirdanatomy.blogspot.com/2026/03/book-review-or-possibly-most-plausible.html

Matt Wedel  The book embodies so much creativity and imagination, intellectual rigour and artistic virtuosity ... that to ask for more seems greedy in the extreme. If you’re coming to DinoCon this weekend, you can buy a copy from Gert in the vendor hall, at least until he sells out, which I think is quite likely.  Go do the right thing.
https://svpow.com/2026/07/23/wildlife-on-the-planet-furaha-by-gert-van-dijk/    

Start of direct USA sales of the Furaha book, a new very nice review, and off to DinoCon!

No science this time, just a quick update.

My book ’Wildlife on the planet Furaha’ appeared in the UK on 3 December, 2025. People in the USA could order it from the UK, but they won’t have to any more: The Book will be available directly in the USA as of the 28th of July, or next Tuesday. If you are interested, go pester your local bookshop to get it for you. It’s worth it, I promise!

That opinion is shared by Matt Wedel, one of the authors of the famous blog ‘Sauropod Vertebra Picture of the Week’. Matt just published a very positive review of my book, right here.

Finally, I will be at DinoCon in the UK with a limited number of copies of the book (books are heavy!). If you want one, or a nice print, visit stand 66. I am sharing that stand with CM Kosemen, known for ‘All tomorrows’ and ‘Snaiad’. If you think that two Speculative Biology planets aren’t enough, come find out about the ‘Syndicate of Planetary Biology Societies’ (also starring Dougal Dixon…).  

How many legs should animals in speculative biology projects have?

By Sigmund Nastrarruzzo and Biblaridion

We recently published the first of two posts in response to comments on Biblaridion’s YouTube review of Sigmund/Gert’s book ‘Wildlife on the planet Furaha’. The number of legs of Furahan creatures evoked many comments, in particular Furahan scalates. Scalates are animals with some similarities to Earth vertebrates, such as a comparable size range and bilateral symmetry. But the six legs of Scalates urged some people to argue that larger animals should always have at most four legs. The previous post dealt with limits posed by a Bauplan: the genetic building plan that defines an animal’s main anatomical features. In this post, we start with discussing Bauplan effects some more and then try to work out what effects the number of legs may have on animals. 

Bauplan limitations 

The odd thing about a Bauplan is that its contents are largely locked and cannot be modified through evolution, and yet that same Bauplan can only have come into being because its contents were at one point highly malleable! If the number of legs is locked in a Bauplan, that number stays fixed. The four limbs of Earth vertebrates are locked, so vertebrates will not have offspring in which the basic pattern jumps to two, six, or eight legs. There are three major lessons here. 

1. The fact that ‘four legs’ is part of the vertebrate Bauplan does not mean that this number works significantly better or worse than another number; all we can conclude from the long history of vertebrates is that four legs work well enough. 
2. The number of legs does not have to be locked in a Bauplan in each and every animal clade! The number is in fact variable in Earth millipedes, and speculative biology creators can use that fact to decide whether the number of legs in their creations is fixed or malleable (it is variable in Furahan rusps). 
3. The basic instruction ‘make four limbs’ still allows anatomical modification of those four limbs. Evolution can find other purposes than walking for front legs, a principle baptised ‘centaurism’ in this blog; examples are the front legs of mantises, wings in birds, the hands of primates and theropods, and mouth parts in just about any arthropod. The result is that the animal gains new functionality at the cost of a pair of walking legs. Such centauric evolutionary changes probably starts with animals using their front legs both for walking and for a new purpose, and over time the new function takes over completely, so the walking role is lost. Some commenters on Biblaridion’s video felt that many-legged animals would lose legs until they arrived at four legs, without a new function. But in an evolutionary process each step has to bring an advantage. To make their scenario work, animals would have to start using only four of their six legs even though all six were fully functional; we see no clear advantage in that, and evolution is not driven by future goals, but by advantages here and now. 

Building costs 

If a leg is only a weight-bearing cylinder, then each leg can be more slender the more legs there are. But what is the total mass of many slender legs, compared to that of a few stout ones? The calculations on that question (here and here) showed less total weight with fewer legs, with the least mass for just one leg! But you only have to think about how fatiguing it is to hop on one leg to realise that building costs cannot be the only consideration. In evolution, many factors usually play a role, and the best solution then is a compromise between conflicting demands. For example, factors such as stability and surviving harm may outweigh saving weight. 

Surviving harm

If a one-legged or two-legged animal breaks a leg, it will be unable to eat or drink, and will quickly become someone’s dinner. Four-legged animals are probably also doomed (dogs and cats can adapt in amazing ways, probably because humans help them through the critical phase). A six-legged animal can probably limp away after the loss of one leg, and the consequences of injury become less severe with a larger number of legs. Do millipedes even walk slower after losing a leg? In short, multiple legs allow better chances of surviving injury than four or fewer legs. 

Speed 

Animals that walk fast use four principles to speed up. The first three are simple: steps become longer, step frequency increases, and the fraction of time a leg is in the air increases, while time on the ground decreases. The fourth factor is gait, meaning phase differences between legs. During a slow walk, the body has to be supported at all times, which is best done by having all legs move at different times, so no two legs move in synchrony. For slow walks, this works best with four or more legs. However, fast running works best if the gait contains jumps, meaning periods in which all legs are off the ground at the same time. To achieve that, leg movements have to be synchronised to a degree, which can -in principle- be done regardless of the number of legs. For instance, take a Furahan rusp with 24 legs. When such animals run, all right-sided legs move in unison, and so do the left-sided legs, but exactly 50% out of phase. 

Bipedal running cockroach; click to enlarge. From Full and Tu 1991

 

However, that is not how many-legged animals run on Earth. Cockroaches, crabs and lizards may use just two legs when running fast. Why do they not use all their legs and run like rusps? Well, rusps have their feet close to the midline, which makes it easier to support the body with legs on one side only; cockroaches, crabs and lizards have splayed legs, which makes one-sided support difficult. Another part of the answer appears to be that the preferred legs for bipedal running in cockroaches and lizards are longer than other legs, so the other legs are less useful for speed. Note that, even though crabs may need only two legs for maximum speed, this hasn’t resulted in crabs with only two legs! The other legs are useful at other speeds. For instance, crabs need good stability when walking slowly, helped by having more legs, and their life may depend on their ability to cling to a rock using all legs in heavy surf. 

For large animals, it is less likely that one pair of legs provides enough power for maximum speed, so they need more legs to achieve that. Perhaps another factor weighs in too, such as the animal not achieving stability with only two legs while running. 

Stability and size 

We will use ‘stability’ essentially as the likelihood of not falling, while standing or walking. Animal size is extremely important here, because the effects of gravity are relatively much more important for large than for small animals. In contrast, sideways forces such as wind and water currents are more important for small animals.  

Gravity always presses large animals firmly to the ground, so their feet do not have to cling to the ground or grasp it. Stability primarily means keeping the centre of gravity over the area defined by where the feet touch the ground (a support diagram), and secondarily being able to keep the body orientation the same. 

Small animals, say insect size, do need to cling to or grasp whichever surface they reach, regardless of whether that is vertical (as in the illustrations below), horizontal or even upside-down. For them, stability is keeping the body position and orientation stable regardless of where the footholds are. 

What happens to stability and position for large and small animals when the number or legs increases?

Click to enlarge; copyright Gert van Dijk

• One leg allows a large animal to stand and to hop, with gravity ensuring it always returns to the ground. One-legged standing and hopping amounts to a continuous balancing act, which in turn needs very sophisticated nervous and muscular systems. Animals that are just learning to walk on dry land will not yet have evolved such sophisticated systems, so it is doubtful that one-legged animals could even begin to evolve the skill of walking. For a one-legged small animal, hanging from its one leg, life is simple: if it relinquishes its grip, it falls away from its support surface; not a good idea. 

Click to enlarge; copyright Gert van Dijk

• Two legs means fewer directions to fall in, which helps. Birds, some other dinosaurs and people all walk well with two legs. It is not easy though: faster walking requires dynamic stability, meaning the centre of gravity needs to kept over a continuously changing support diagram. To do that, you will again need exquisite control, so we advise bipedal animals that aim to become terrestrial walkers, but cannot do so yet, to not bother. Very small two-legged animals could theoretically hang from one leg while moving the other, but it would be very difficult to prevent the body from just hanging down from one leg: not a good idea. 

 

Click to enlarge; copyright Gert van Dijk

• Three legs, for larger animals, allow a neat support triangle; that is all you need to stand still, and you do not need complex control for that either. But walking with three legs will require excellent control. Very small animals can hang from two legs, but it will still be difficult to control body position. 

 

Click to enlarge; copyright Gert van Dijk

• Four legs, in larger animals, do not make standing easier than with three legs, except when there are strong sideways forces such as water currents or strong winds. If there are, it helps to have a leg sticking out in the direction of that force. Walking with four legs offers a fundamental advantage over walking with three legs: you can lift one leg and still remain stably supported by the other three. This is still not exactly easy: tortoises, moving slowly, have stability problems, as they tend to fall in the direction of the lifted leg. It takes a specific order of moving the legs to prevent that, so here’s that neural control again. Small animals with four legs can maintain their body position using three legs, while one limb moves to a new foothold. 

 

Click to enlarge; copyright Gert van Dijk

Click to enlarge; copyright Gert van Dijk

• Five and more legs will not improve stability much, for large standing or walking animals. This probably holds for small animals too: with many legs, they can choose which ones grab a surface and which ones move to the next foothold, all while nicely keeping the body in position. However, many legs may help to grasp objects better: the eight legs of crabs may act as fingers, making a crab a sort of hand to grab rocks with; insects walking uphill may keep more feet on the ground than they do on a horizontal surface; the many legs of millipedes may help them bulldoze through dead leaves or soil. 

Stability becomes easier with more legs, at least up to a point; we guess that stability will not increase much beyond, say, six or eight legs. We suspect that ‘beginner walkers’, animals that are just learning the art of moving on dry land, will find the task easier if they have more legs, say six or more. If you are just learning to walk and have only four legs, don’t expect to be able to lift the body at all times at first! Start by crawling on your belly, and only try to lift the body when your control system can guarantee stability. However, if you start with six or more legs, you’ll learn to walk in no time! 

Planetary effects 

Many of these effects will depend on local gravity. On a planet with low gravity all legs can be spindly, and falling won’t hurt as easily. With a very strong gravity, legs have to be very strong and as many legs should be on the ground for as long as possible, which is easier the more legs there are. With a high gravity a lifestyle clinging to branches and twigs is asking for trouble, unless there are always lots of legs keeping a secure grip. 

Conclusion 

The number of legs plays a role in stability, building costs, surviving harm, locomotor efficacy and probably factors we didn’t consider. The number is restrained by evolutionary aspects and may be locked genetically. Whenever there are many factors determining an outcome in biology, it is usually impossible to identify one factor as the sole determinant of success or failure. We think that this holds for the number of legs too. The optimum number of legs is like most biological optima: a summation of many functions with lots of compromises. 

However, this can only be true if the number of legs can in fact vary freely, in which case evolution can modify the number as the outcome of such a summation. But if the number is locked, which seems more likely, then evolution has to work with what it was given. The number may then constrain evolution: for instance, an animal with just two legs would find it difficult to combine flying with walking. 

With all these factors weighing in, we do not think it likely that all large animals in the universe must have four legs (or six, for that matter). There are too many variables at play, so we expect the usual evolutionary mixture of Bauplan constraints and many factors weighing in. That seems to be how biology works, and for that reason speculative biology should work that way too. Are we certain of that? No, we aren't; it’s all speculation, remember? 

 

Short reading list

• Büschges, Ache. Motor control on the move: from insights in insects to general mechanisms. Physiol Rev 2025; 105: 975–1031
• Full RJ, Tu MS. Mechanics of a rapid running insect: two-, four-and six-legged locomotion. Journal of Experimental Biology. 1991; 156: 215-231
  
• R McNeill Alexander. Optima for animals, Revised edition. Princeton 1996 
• R McNeill Alexander. Principles of animal locomotion, Princeton, 2003
• Riiska CA, Harrison JS, Thompson RD, Nina JQ, Gallice GR, Rieser JM, Bhamla S. Katydids shift to higher-stability gaits when climbing inclined substrates. Integrative and Comparative Biology. 2025 Dec;65(6):1667-77. 
• Weihmann T. The smooth transition from many-legged to bipedal locomotion—Gradual leg force reduction and its impact on total ground reaction forces, body dynamics and gait transitions. Frontiers in Bioengineering and Biotechnology. 2022; 9: 769684  

What -on Earth- is this?

A week ago Jean-Sébastien Steyer, a French palaeontologiost and author of various books on speculative biology (here, here and here), alerted me to an Instagram video. The video showed a ghostly white arthropod against a black background, swimming vertically in a most peculiar way. I was immediately intrigued and at first wondered whether the video was AI trickery, but a commenter had already checked that and said it was real. I checked for myself (that's what scientists do) and found a key paper on Bathyopsurus nybelini, as well as a paper on another Bathyopsurus species.

 

Here is one of the videos you can find on the internet. Others can be found here. The one above was clear enough to show how the animal moved. Bathyopsurus nybelini is an isopod, the clade that also contains woodlice. The species is not very large, with body lengths of two measured specimens at 34 and 35 mm. 

The paper by Peoples et al., from 2024, makes it clear that the animal swims vertically or parallel to the sea floor, holding a bit of sargassum weed with its mouth parts. The swimming legs are at the rear end of the animal, which is at the top of the video. The animal is swimming backwards, but for a blind animal that may not matter much. The paper focused on the importance of bits of sargassum sinking and fuelling deep sea ecology. The authors had this to say about locomotion: Isopods swam paddle stroking above the seafloor with their two natatory legs (pereopods V VI) . Elsewhere they wrote: The large dual paddles support the isopod s characteristic swimming gait, which involves alternating power strokes of adjacent appendages. In addition to the broadening and separating of the two paddles on each appendage during the power stroke, setae provide additional surface area for propulsion power . (Setae are stiff hairs that in this case apparently increase the effective surface of the paddles.) 

I watched them several times and the following seems clear. There are two pairs of legs that provide propulsion, and those four legs move in pairs of two, so there are almost continuously paddles moving down. The legs have flat surfaces that are held perpendicular to the movement direction on the downstroke and parallel to it when moving up, which no doubt reduces resistance on the upstroke. In other words, the animal is rowing, or paddling, which are both characterised by flat surfaces pushing against water (note that animals such as turtles, penguins and seals do NOT row; their wings/flippers are held at a shallow angle against the water while providing propulsion, meaning they are basically flying through water). 

But what wasn't clear to me was how the limbs open and close; how the paddles are turned to reduce drag on the upstroke; finally, I couldn t really see which legs do what. The camera is jumping about a bit, making it difficult to see what was going on. To have a better look, I selected a part of the video that seemed to have a good quality, and used Matlab to split it into separate frames. I marked the point where the big round tail segment is connected to the rest of the body on some key frames, and defined those points as the centre of new frames. All that remained was to do let the program solve that for 481 new frames, increase the frames in size and make a slow-motion video (10 frames per second instead of the original 29.9). 

 

So here it is! You still have to look at it a few times to understand the movement. I think I see the following:

•  There are two paddle surfaces on each leg. The last segment carries an oval paddle, and the penultimate segment carries an asymmetrical flange. 
• When the folded leg arrives at its highest point, the entire leg is extended and then rotates down in the joint very close to the body. This movement brings the paddle surfaces perpendicular to the downward movement. For the upwards stroke the leg is folded which aligns the paddle surfaces to the upward movement. 
• The two legs that are attached highest up in the image always move in front of the other pair. As the animal is upside down, the legs moving in front of the others are the hindmost pair of swimming legs. We are looking at the belly side of the animal, so these hind legs move more ventrally than the front paddles. In contrast, the front paddles move in a volume that lies more towards the backside of the animal. The volumes in which the four paddle legs move reminded me of Luke Skywalker s X-Wing spacecraft (Bathyopsurus will be slower, though). 
• I was really surprised to see that the front left leg moves in unison with the rear right one. The remaining legs also move together, but at an opposite phase in the movement cycle. I thought at first that the two front legs would move together and that the two hind legs would form the contrasting pair. But no, the legs move in diagonal pairs; if this was a mammal, we would immediately recognise this gait with diagonal pairs as a trot! I can only guess as to why a backwards swimming four-legged paddling isopod use a trot; perhaps this represents a bit of programming, carried over from its past as a walking animal? 

 

Click to enlarge; copyright Gert van Dijk

Click to enlarge; copyright Gert van dijk

Here is a simple scheme of how I think the animal arranges its movement. It is complex! Are there other animals on Earth that move like this? I cannot think of any that swim vertically, and of all animals that swim horizontally, not many use four limbs to propel them. Plesiosaurs and pliosaurs may have used all four limbs, but probably not as simple oars, so the resemblance is weak. On Furaha, kwals come closest, as they also swim vertically and use their limbs as vertical oars. But the three limbs of kwals move together; they do not have Bathyopsurus unique two sets of limbs with alternating strokes. Maybe this means that Furaha needs diploid kwals that do have two sets of three limbs... 

Anyway, Earth has a deep-sea isopod with four rowing legs, arranged like an X-wing, and moving in a trot! I think the three or four hours spent on Matlab programming for this little project were worth it. Why, some may ask? Well, my curiosity was satisfied and I learned something new about animal locomotion. Was this speculative biology? Of course not, it is just regular scientific curiosity fed by lots of speculation.

The 'spietskip', or 'impaler poultry' (Tabulae mortuae VIII, Archives XVIII)

 The ‘spietskip’ is mentioned in the Furaha book; the book contains a short section about the prehistory of the Furaha project, showing old paintings. The word ‘spietskip’ is a made-up Dutch word (the Dutch language allows everyone to make new words by stringing together existing ones). ’Spiets’ means to skewer or impale, and ‘kip’ means ‘chicken’: a ‘spietskip’ is therefore literally a ‘skewer chicken’, although ‘impaler poultry’ sounds more dramatic. 

Click to enlarge; copyright Gert van Dijk

The painting shown above is really old. Over time the animal, with it bird-like plumage and overly reptilian head, fitted less and less within the evolving hexapod scheme. One solution in such cases was extermination, and the other was a redesign. The idea behind the spietskip appealed to me: a small carnivore, hunting from a hiding place among reeds, and catching prey with front legs that are fishing spears. I also thought at the time that The Book needed more scalates. There is ample room for adaptive radiation within every scalate group. Carnivores definitely needed more radiation, to show their range from mouse-sized squigglers to slow massive carrion eaters specialising on dead rusps.

So I had wondered whether redesigning the spietskip might work. An animal clinging to reeds need to have legs with a large movement range, and feet that can grab a reed from about any angle. That should not pose to much of a problem for the general scalate design.

 

Click to enlarge; copyright Gert van Dijk

Here is a simple Zbrush sculpt showing a spietskip following the scalate redesign, including the distal and proximal neck with the neurocranium in between. I now think it is too bulky. It is often difficult to get a good idea of the mass of small mammals and birds, because their fur and feathers make them look much bulkier than they really are. If you see a wet cat, you can see how little mass it really has. Let’s just pretend that the Zbrush sculpt already includes its covering. As you can see, the limbs are very simple; that is because such sculpts were only made to help with perspective and composition, and details can simply be drawn later.

Click to enlarge; copyright Gert van Dijk

 

After importing the body into Vue, I played with limb positions, here simple cylinders, and the overall viewing angle, which was similar to that of the original painting. 

Click to enlarge; copyright Gert van Dijk

 

I could easily have take this image as a rough background and could have started drawing over it, before finally sitting down to paint.

Click to enlarge; copyright Gert van Dijk

However, sometimes drawing without any such 3D aids resulted in livelier images, perhaps at the cost of some perspective mistakes. That’s why I also did a very quick sketch right over the adapted old oil painting. This sketch could also have been the starting place for a new drawing and painting. 

But around that time I realised that I could keep on designing new radiations forever, and that there already enough material for a book already. And that is why the evolution of the spietskip stopped. 

Perhaps I will pick it up again, some day. But first, I am preparing for DinoCon, where I will be selling The Book, images and some other stuff. There are also posts to write on the ever-intriguing theme of how many legs animals should have in speculative biology projects, and on colour changing. 

And for people in the USA, don’t forget that The Book will finally be sold directly in the USA through Simon and Schuster, starting on 28 July, 2026.