Furahan biology and one of its allied matters: origami. Yes, origami!

My series of rusp posts have created something I never expected: an origami rusp. For more on rusps, simply start at the previous post in this blog. Regular commenter Petr (Petr Stuchlý) has sculpted four models of Furahan animals in what for me is a novel medium: origami. He posted photos of one of them, the brontorusp, on DeviantArt under the title of Xenorigami, which I rather like. Petr was kind enough to send me all four models in a neat package, which I appreciate very much.

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Let me show you the models. Here are all four of them along with a prototypical matchbox  for size. Let me close in on all of them.

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 First, of course, the origami model of the brontorusp (Brontocrambis brucus). I have never published a complete image of the brontorusp, so Petr could not know that the front and hind heads are not identical. If you take a look at the image of the origami brontorusp on his DeviantArt page, you will see that the front and hind heads are identical. Once we got to talking about that he has made some changes to the model, and now the hind head no longer has a rostrum (snout). Before you ask about two-headedness (the Janus effect), here is an explanation. The basic ancestral rusp design involves developed segments with eyes on each one. When the lineage increased in complexity, 'cephalisation' set in, so nerve clusters, eyes, whips etc., concentrated in one spot leaving the animal with a head... Actually, no!. That may be what happens usually, but in rusp evolution there was not just an increase in complexity, but cephalisation also involved a loss of some functions, such as visions in middle segments. That, probably coupled with a need for defence on both ends of the rusp, stopped the loss of vision there, so there are eyes there, with a fully functional whip capable of giving any predator a good wallop. There is also a circular secondary brain, the neurannulinus, communicating -we assume!- with the neurannulus in front. There is no hind mouth though, so no rostrum there, and rusps definitely have a front and a hind part.

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Let's look at the details. I think that Petr caught the shape of the brontorusp's head very well. I have only a very vague idea of how you would go about to create such an intricate design from a square piece of paper of 50x50 cm. From his own comments ("hell on earth to fold!!! XD") it was not exactly the easiest thing to do.

 

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The many folds at the rusp's bottom are interesting. Petr explained a particular featre as the result of his folding technique, but regardless of the cause the design parallels my rusp design in a surprising way.  Rusps, as large animals, have vertically-orientated legs that swing forwards and backwards under the body, and not sideways, so there is a risk of them kicking one another. That problem was sidestepped (sorry for that one) by having the legs alternately offset to the centre or the side of the animal. The origami rusp has the same feature!

What you see here is the 'crease pattern' of the rusp laid out on a piece of paper. The 'CP' has lines indicating valleys and ridges, and helps the designer shape a flat piece of paper into a remarkably solid and three-dimensional sculpture.

Click to enlarge; bottom : copyright Gert van Dijk

Petr has also made a hexapod neocarnicore, with its typical 'raptorial appendages'. The model captures the form well, which must be difficult as it is a very small model: he wanted to keep the models within a certain scale range. They are not fully in the same range, but I do not think I ever published enough data on exact rusp size for him or anyone else to judge the size accurately. Above is a new scale diagram for The Book, so you can see how large brontorusps are.

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A marshwallow! It is complete with three horns, and even seems to have the continually irritated expression that humans project on the animal's cranial features...

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 And finally, two sides of the marblebill, here suspended from a Japanese eating stick, to stay  in style. Origami paper can have one colour on one side and another on the other, and here that principle was obviously expanded by having a two-layered sheet of paper with metal foil on one side. Somehow Petr managed to have the dark paper end up on the dorsal side of the upper limbs and the metal side on the palmar side, doing justice to the pattern of the marblebill.

It is obvious that Petr is rather good at origami. If you wish to learn more about his art visit his Flickr page,  DeviantArt page or a page at an English origami site. If you like palaeontology, and the chances of that are high with a blog like this one, you should visit his page on origami versions of extinct animals.They're very good.

Evolving another aggie

Click to enlarge; copyright Gert van Dijk

What you see here is a sketch of the 'aggie', the favourite prey of the marblebill. The pages detailing the marblebill in The Book have this to say regarding the aggie:

"The marblebill’s favourite prey is the ‘Aggie’ (Agitator augur), a tree-dwelling fructivore. Once caught, the victim’s feeble attempts at defence have little chance of success against the marblebill’s armoured chest and abdomen. There is little time for resistance anyway, as marblebills usually disarm their victims quickly by snapping its cervical medullae."  

 "A troop of Aggies, admittedly not the brightest of beasts, may suddenly see a branch swaying and a baignac falling. Only when they hear the marblebill’s triumphant howl does it dawn upon them that one of their comrades had just now been sitting on that branch and been munching that baignac."

That's all that is known in the entire universe regarding the aggie. I have started sketching them several times, but was never too happy with the result. The sketch you see here is not the definitive aggie either. This particular one is a brachiator, just like the marblebill. The degree of adaptation of the marblebill to its arboreal brachiating life style suggests that its environment has been around for quite a while. If so, other species could be equally well adapted to an arboreal way of life. That does not necessarily imply brachiation (see here and here); the animal could be a jumper, a climber, or even a glider. But this one is a hexapod brachiator.

Click to enlarge; copyright Gert van Dijk

In previous versions I toyed with the idea of using the second pair of legs as the main. In fact, here is an old very quick and dirty sketch showing that approach (Brynn Metheny also did one once, the 'pygmy esorifleu', which I discussed previously). In the 'Mark I' the body is suspended from the middle limbs, and the front and aft ends hang down. It must have evolved from basic hexapod stock, and it is hard to imagine an ancestral species with six more or less equally-sized legs preferring to grasp branches with its second rather than its first pair of limbs. You can see the 'Mark II' next to it. That sketch was ancestral to the marblebill's design, and they still swing from their front limbs.

Click to enlarge; copyright Gert van Dijk

Still, there may be a way to evolve a brachiator with 'middle limb suspension'; take a typical Furahan neocarnivore, one of those animals exhibiting centaurism. As you may remember their first pair of legs are not used for locomotion but to catch prey. If such an animal started climbing trees, it might keep its weapons intact, and adapt its second and third pair of legs for locomotion among the branches. Its offspring could become either become jumpers of climbers, using four more or less equal limbs, but they could also turn into brachiators. If so, they would swing from their middle limbs and use their front legs as weapons. In my mind, I  see the hexapod evolutionary tree sprouting a new branch even while I am writing this...
Then again, a neocarnivore taking to the trees might use its spears or clubs to hook a branch. Being at the front of the body they are well placed to do so. If these limbs then become brachiating arms they would resume a locomotor function again; I see another evolutionary branch exploding into view with an almost audible 'whoomph'. By the way, that latter branch is also the first official example of 'decentaurism', or the reversal of nonlocomotor limb use to a secondary locomotor purpose.

Anyway, back to the aggie. Have a look at some of its features.

• It sits upright, which may make sense for a brachiator: its body is held vertical while brachiating, and it might easily keep doing so at rest.
• Its limbs are attached to the body with joints that allow three axes of rotation. The brachiating arms are attached to the body through a short bone that ends at the 'shoulder'. Unlike Earth primates, the shoulder girdle is attached through bones to the axial body skeleton rather than through muscles only, but the animal still needs thick muscles to control the position of the body with regard to the arm. The unfortunate result is that the attachment looks much like a primate shoulder girdle; parallel evolution or a limit of my imagination?
• You might just make out the ancestral hexapod toe branching pattern (more about that here). 
• This particular aggie version has a pot belly. While sketching it I had forgotten about it being a fructivore with a preference for baignacs (remind me to show you a baignac one of these days). Fruits usually offer high quality food, so animals does not need many of them. While sketching I had low-grade food in mind, say fibrous leaves, and such food requires a lot of processing and a sizable gut. Specialising on low-grade food has the advantage that there will not be much competition, but the end point might be a slow animal that is not at all energetic: something like an Earth sloth. While sloths are preyed upon by harpy eagles, the dense parts of the forests are probably closed to eagles. But introduce the marblebill, and anything as slow as a sloth has a problem. So, the aggie cannot be too slow. It should probably lose its potbelly and resume a high-energy fruit diet. Of course, it should perhaps be better able to defend itself, or use its social skills, or perhaps...

...never mind; thinking about the aggie has once more led to interesting predators rather than their prey. One of these days I will design the definitive aggie; this is not yet it.

The Devonian feet of the Furahan marblebill

I really should stop having fun with incomprehensible blog titles. Anyway, too much work again left me no time to write a solid scientific essay. So I will just relate the story of where the 'branching toe theme' originated. The previous post was about redesigning the marblebill, and Spugpow had observed a novel branching pattern of the marblebill's toes.

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From: Gould SJ, Ed. The book of Life. Ebury Hutchinson London 1993.

To see where that came from, we have to go back, either to the Devonian or, more appropriately, to books on palaeontology up to about 10 years ago. The image above is from such a book, still available from Amazon in the UK and in the USA. No doubt you have seen similar images before, illustrating how Earth's tetrapod legs evolved from the fins of fish. In the middle right, you will see such a prototypical tetrapod leg: one bone in the upper limb, two parallel bones in the lower limb (the radius and the ulna), a bunch of small bones as a sort of shock absorbers, and five (well, seven) parallel digits with several bones in series. That is basically our arm or our leg. The middle image on the left shows the bones in the ancestral fish limb. Do you see the resemblance? If you do, good for you. I never did, as to me the branching patterns seemed completely different. In effect, the two boned destined to become the radius and ulna may start parallel where they touch the upper limb bone, but their lengths are quite different, and while there are no other bones connected to the radius, some do connect to the ulna. In fact, if you start at the upper arm, there is just one other bone at the front of the limb before you reach the 'lepidotrichia' (the stiffening rods in the fin), but there are three at the hind end of the fin.

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Sébastien Steyer. La Terre avant les dinosaures. Copyright Belin 2009

This branching pattern was never explained in such books. In a newer book by Sébastien Steyer, (who by the way is not just a palaeontologist but one of the driving forces behind the future book on future animals I discussed here earlier) you will find the images above. Mind you, besides the French and Dutch versions it is also available in English. The left part shows the fin/limb skeleton of Ichthyostega, and to the right Sébastien has shown a series of limbs showing intermediate forms. Very well, I believe the transition happened, but have never stopped wondering about the branching patterns.

Click to enlarge. Copyright Gert van Dijk

So here are some old sketches. The one on the left in the middle row is more or less our current branching pattern: all fingers have equal numbers of segments (phalanges) and in the middle one of that row I experimented with different patterns: you could first split a limb into two parts and have these two each split into two more, etc. That would be equally 'consistent' in the sense that the number of steps from wrist to tip of a toe is the same. But in the right one on the middle row I played with another way of branching, in which some bones both give rise to a new bone and also continue downwards themselves. In the bottom row I developed the latter pattern some more, finally arriving at the right one, which looks like Ichthyostega's skeleton.

Click to enlarge. Copyright Gert van Dijk

And finally some sketches that show the design taken to extreme consequences. The left one is designed for a marshland creature that needs to spread its weight over a large surface. Mind you, I would never equip Furahan animals with legs that split that far up. I do not think that that makes much sense, as it will double the weight without appreciable advantages. Toes are good for many things: one good thing about them is that they extend stride length, and another is that they help spread weight, act as shock absorbers and help direct forces. Are more toes and longer toes therefore better than few short ones? Well, for these purposes, yes. But there is always a need to conserve mass and energy, so fewer and thinner bones have advantages too. As always, there is an optimum.
The foot on the right is suitable for a very heavy animal of a not too athletic build, rather like an elephant. Its toes are mostly there as shock absorbers and a means to transfer ground forces. They are good for standing and walking sedately. This particular design is not at all suited for athletic animals though, as the toes do not help extend stride length at all: they are much too short for that and hardly bend at all. If you want toes for an athletic animal, think of the feet of a chicken or a tyrannosaur. Well, now you known why I not like the feet of many of Barlowe's animals in 'Expedition' that much: he equipped active athletic animals with unsuitable elephantine feet.

But I digress. In the current Furahan redesign, I decided to build the anatomy of hexapod feet on a pattern that I thought I saw in the fin/feet of Devonian amphibians, that never seemed to be explained. So that is where the marblebill got its feet.

A marblebill in progress (also known as the becdacier)

No time this fortnight to write anything elaborate, unfortunately. Some posts take much more time than others. The ones that take most time are those that require checking the physical aspect of some matters, not just because finding sources and digesting the content takes time, but usually also because I then need to do some programming of my own or I have to make some specific illustrations. In short, the heavy science bits take a lot of time. Over the years I have written quite a few words on such subjects, and I started wondering whether I should perhaps bundle them, work them over, write some new chapters, and produce a book on the biomechanics of alien life. Something like 'Darwinian creativity in a Newtonian Universe'. The title is probably much too enigmatic for the book to sell, but perhaps it could be a subtitle. Mind you, it would be completely separate from the Furaha book. But would anyone buy it? Let me hear what you think.

Anyone, I have been working on an update of the marblebill. I showed you another such update once before, and the marblebill is on the Furaha website but featured previously on this blog as well. You may recognise some general update principles. The eyes on stalks are now less prominent, but certainly still occur in various species. There is also eye specialisation. The images below are taken from the sculpting program Sculptris, a free programme I recommend unreservedly.

Click to enlarge; copyright Gert van Dijk

The marblebill is an arboreal brachiating predator, and has two forward facing eyes to help it judge distances and fixate its prey. As is the case for dragonflies, the size of the ommatidia (the individual eyelets in a compound eye) depends on where you are in the eye: they are smaller in the part of the eye facing directly forward. This increases visual resolution at the price of sensitivity to light. The marblebill is a diurnal creature, which makes sense I think: you would need impossibly good vision to allow an animal to hurl itself from branch to branch at high speed at night.

Click to enlarge; copyright Gert van Dijk

There is another pair of eyes, the 'oculi posteriores'. Note that these were not posteriorly placed in ancestral hexapods, whose four eyes were placed around the head. What became the anterior eyes were once the bottom ones, and the posterior ones are the former upper ones. Anyway, in the marblebill lineage the upper ones, alresdy in the posterior position, over time migrated outwards, providing an all around vision, not just in the horizontal but in the vertical plane as well. For an animal living in three dimensions this is more important than for a grazing animal. One result is that it would not be easy to sneak up on a marblebill. Not that there is any other predator up there in the trees that would perform such sneakish acts anyway: it would be too dangerous. The marblebill also does not need much vertical vision for its territorial disputes, as these involve no sneaking whatsoever, but are announced frighteningly loudly. But detecting prey is another matter, and for that these eyes are superb.

Click to enlarge; copyright Gert van Dijk

Here is the painting in progress. I used to work out perspective and draw everything completely without any aid except for the occasional ruler, but I now make use of what the digital age has to offer. So I exported the sculpted head into Vue infinite, made certain the lighting came from the correct direction, adjusted the perspective angle and produced two renders. Cut out the head, place them on a separate layer in Painter 12 (to be deleted later), and everything is in place to start painting. Now all I need is the time to do so...

Between the Morae River and the Red Valley

I discussed Brynn Metheny's 'Morae River' project once before in this blog. The project dealt with life forms in a certain geographical area called 'Solturna'. The animals there at first glance looked like Earth's mammals, reptiles, or fish, so you might think that the Morae valley might be somewhere on Earth. But a closer look revealed small but telling differences in anatomy, so you might think that the creatures perhaps stemmed from a not too alternate time-line and were mammaloid, reptiloid and ichthyoid. But a few animals departed so much from regular Earth stock to make you think that they required an extremely early divergence or an unearthly origin. Brynn gave no clue as how to place her creations, and preferred a fluid interpretation. Earlier this year she unfortunately decided to stop working on the Morae River. But that should those who never visited the site from doing so, as it is a great project even if it is no longer updated.

Luckily she did not stop producing odd animals. Far from it! I gather that she is making 'creature design' her career, which hopefully means that there is much more to come. There are several places where you can admire her work: she has a site on Deviant Art where she goes under the name of LenoreKitty. She has a site under her own name, brynnart.com, as well as one going by the name of Fishhook studio. I selected a few paintings for you to see here, and expect that they will make you hungry for more.

Click to enlarge; copyright Brynn Metheney

The pygmy esorifleu
As you can see, this is an arboreal creature with a strong bill, like a parrot's, and six limbs. The middle pair are placed at the top of the animal and are directed upwards, whereas the front and aft pairs are directed downward. There is also a tail, which looks like it is prehensile. This animal can only be a brachiating carnivore. It looks somewhat like my marblebill, to be found on the Furaha site but also in this blog (here and here). A fairly large design difference is that the esorifleu using its middle pair of limbs to swing from, whereas I chose the front pair. Brynn wrote me that the pygmy was designed for a creature design contest. I think the marblebill and the esorifleu are nice examples of convergent speculation.

Click to enlarge; copyright Brynn Metheney

Elegant hunters
And indeed they are. Mind you, the prey look rather dashing as well. The beaks of the hunters remind me of the mouth of a deinichthys. At first glance their body design seems to say 'bony fish from Earth', with its vertical tail, fin rays, gills and dorsal fin. But then you notice that instead of one pair of pectoral fins there are two, giving a jolt to the idea of what exactly they are. What I do not know is whether you have to know that two pairs of pectoral fins are impossible for that jolt to occur. Anyway, four gill slits is an unusual number as well.
I would like to see more of their prey, whose 'Bauplan' seems much more unearthly. I like the bumps on the front of the flippers. Not many swimming animals have those, but humpback whales have very knobbly leading edges on their enormous flippers, and in their case the turbulence they cause actually seems to help. Is that the end of a siphon I see on their sides? Are those expiratory outlets?

Click to enlarge; copyright Brynn Metheney

'Mamma'
No classification problems here: that's a perissodactyl unguloid, or a hoofed animal with an odd number of toes. But are those concentric structures external ears? Perhaps the animal is not Terran after all...
What I like a great deal about this one is how the animals are not simply shown in side view, but are much more three-dimensional. The calf's head cannot be seen, and the mother's head is turned away a bit as well. This is where the trained artist shows herself, I think.

Click to enlarge; copyright Brynn Metheney

A work in progress
Lacking a name, let's call it a 'wip'. If I see correctly there is just one pair of eyes, and the other markings on its head are nostrils and ears. Even so, this animal is more alien than the previous one, with the sail on its neck and particularly the spikes at the base of its tail. Now what are these doing there? They are not placed well for attack or defence, so perhaps they are for display purposes, and display to members of the same species always boils down to sex. Do the spikes serve to impress other wips, or do they provide tactile stimulation during procreation? I had better reign in my imagination here...
It's a beauty though.

Click to enlarge; copyright Brynn Metheney

Another work in progress
If it is, we might as well label it a 'wiptoo'. It is interesting how this head and neck study immediately evokes the notion that we are looking at a very large animal. One reason must be the relatively thick neck. Large animals need proportionately thicker limbs, and that goes for necks too (see here and here for the reasons). Apparently we are so used to seeing the results of these laws of nature that we immediately draw conclusions from seeing their results. Alternatively, of course, this could be a moderately sized animal from a heavy-gravity world, but I do not think so: its eyes are also small in relation to its body. They seem to be camera eys such as vertebrates have on Earth. While bigger animals generally have bigger eyes, eye size does not increase directly with body size, so large animals have relatively small eyes. As with neck thickness, the observer takes these cues and judges the size of the animal, consciously or unconsciously.
Its skin glistens. You can tell from the linear nature of the reflections that its skin is smooth, and I wonder whether it is wet because it just emerged from a swamp or something similar or because the skin itself is wet or oily. A large animal with a permanently wet skin would need a permanently moist and saturated environment. Perhaps it lives as brontosaurs were once thought to do: in humid steaming swamps.

Click to enlarge; copyright Brynn Metheney

There are no animals on this painting, but the image contains a promise. The website of the Red Valley project is already up, but there's not much to see yet. We are promised that animals will appear there in the Fall, so hopefully Brynn won't keep us waiting too long. She wrote me that she is not going to reveal all about the planet: "I might know details about the whole of the planet and such but as far as my viewers are concerned, I'd like it to just be about this valley." I agree with that sentiment: always leave the viewer or the audience hungry for a bit more, and a hint that there is in fact more does wonders to whet the appetite. She added: "I want the flora and fauna to feel alien enough but I want viewers to relate to them as well."
From what Brynn has done in the past, I think she will succeed. The text on the Red River site also states: "No regions, no classification, just this place as it is." Oh very well, I get the message: I should stop trying to classify these animals to see where they belong and what makes them work.

Hmmm; as if I could...

Brachiation versus cernuation, as well as mono- and tribrachial brachiation

I have tried to make the title of this post as obtuse as I could; I hope everyone appreciates that...

In my recent post on the marblebill, a Furahan brachiating predator, I discussed several restrictions that a brachiating lifestyle puts on an animal's body plan. Among others, those were a need for other limbs beside the swinging ones, so the animals would also be able to walk as well as climb vertical surfaces. There is of course no need to completely separate such purposes among limbs. After all, gibbons use their arms for climbing and walking as well.

I also wrote that I knew of only one other brachiating type of animal in speculative evolution, and that this was the squibbon in 'The Future is Wild'. It turns out that that statement was wrong on at least two counts. Firstly, in Dougal Dixon's 'After Man' there is a striger, a tree dwelling feline carnivore. Although its description does not stipulate brachiation, the accompanying picture certainly suggests it. There are bound to be other brachiators too somewhere in the growing field of speculative evolution. Secondly, the squibbon does not brachiate at all! It somersaults, meaning its body is upside-down at some stages in its locomotion, which is a fundamental difference with brachiation, in which the body stays upright. I should have checked that before I wrote it.

Clip from 'The Future is Wild' DVD

To illustrate the difference I have cut a short clip from 'The Future is wild' to show the squibbon's way of propulsion through the trees (easily available through Amazon etc). I know of no Earth animal that does this, so this is really a very ingenious design. Often when you try to think of a novel animal locomotion, evolution has been there and done it already. It is a pity that the designers did not give this locomotion mode a nice name. I now propose 'cernuation', derived from the verb cernuare, meaning to 'fall headfirst / dive / turn a somersault'. Cernuation does pose a problem than brachiation does not: the visual field rotates 360 degrees in each jump, which must make the job of working out where to jump at high speeds even more difficult than it already is. The placement of the squibbon's eyes near the horizontal axis around which it cernuates proves that a lot of thought has gone into this animal. Being on the axis the visual field will still rotate 360 degrees with each movement, but will not shift as much as when it would be somewhere else (there may be room for creativity here though).

The remainder of this post will deal with the number of limbs involved in brachiation. The marblebill uses just two arms for brachiation, just like gibbons and other brachiating primates. This being a blog about speculative biology, the question rises whether it can be done with other numbers of arms.

How about just one? Theoretically this is possible: the animal has to leap from one handhold to the next. There would be no way to go slowly though, as you can with two arms. Slow-moving brachiators can afford to let one arm go while the other has a firm grip. In effect, one-armed brachiation is very much the same as hopping on one leg: each hop is a jump and requires lots of energy. It would be dangerous as well.

Two arms has been dealt with, so three is next. Actually, there are three-limbed brachiators on Earth: there are brachiating monkeys using their tail as well as their hands. When I discussed walking with an odd number of limbs, I could not find any animal that did so with three legs. Tripod walking poses the problem of phase: do two legs move together while the third moves on its own, or is the cycle divided in three equal parts? In the monkey case, the starting point is bilateral symmetry: two limbs are paired and the tail is not, which suggests that an equal division of the cycle is not feasible. The other possible solution is not the case either: that would be that the two arms swing together while the tail holds a branch and vice versa. So how does it work? Essentially the monkeys alternate their arms in the usual brachiating way. The tail helps along by being placed in time with the hand, and right next to it, in fact. Moving the tail in this way may act as a safety mechanism, but has an effect on body sway as well.

Click to enlarge;

Turnquist et al; Pendular motion in the brachiation of captive Lagothrix and Ateles.

Am J Primatol 1999; 48: 263-281

The image above requires some study. If the tail is placed next to the hand, does it do so for both hands? That is indeed possible, and the tail then moves twice as often as each hand does. The authors of the paper describe the movement as 'choppy'. I suppose that this may be only known locomotion in which one limb moves at twice the frequency as other limbs do. I know of one speculative animal that does this, but was not aware of anything of the sort occurring on Earth! This is not the only solution though: some monkeys use their tail in the same frequency as their hands, to the effect that the tail only helps one hand, either the left or the right one. Odd, isn't it?

Is brachiation with more arms possible? Theoretically you could do it with four arms, and the pattern then becomes an upside-down tetrapod gait. Nothing new there.

Can you brachiate with radial symmetry? Yes; an intriguing way would be to let each successive arm take the weight. Envisage a spoked wheel and roll it: the successive spokes point to the ground one after another. Of course, this causes the body the rotate once more, and gain the body's axis of rotation is horizontal, at right angles to the direction of movement, so this is cernuation once more.

There are no cernuators on Furaha, or at least not yet. I do not think that spidrid anatomy lends itself well to moving into the trees. One or two species sometimes roll downhill on their sides to make a getaway, but that is as close as they get. I wonder about other places...

Predatory brachiation: an arms race

I suppose that the title of this particular blog entry is not exactly self-explanatory. Never mind, the explanation follows. I added a new animal description to the Furaha main site a few days ago. I also deleted another one a few minutes later, as I do not wish to give all designs for my eventual book away. That would be the 'Encyclopedia of Furahan Life' (and no, it does not exist yet).

Click to enlarge; copyright Gert van Dijk

The species chosen is a brachiator, meaning an animal that moves around in forests by swinging from its arms. To read more about this species, the marblebill, you will have to visit the site: simply go to the land page and select the first entry in the menu. The marblebill's resemblance with Earth gibbons should be obvious, and that is not because I have not enough fantasy. Actually that might play a part too, but there are probably not that many ways to design a working brachiator, certainly not if you start from a walking ancestor.

Click to enlarge; source here

The long arms, the relatively small body and short legs all play a role in its mode of locomotion. If you take a look at a brachiating gibbon you will see that it like a pendulum from one handhold to the next. If they move slowly, they grab a new handhold before they let go of the previous one, which seems a wise thing to do. In a hurry they simply jump the distance in-between two handholds. Starting with the safer mode, it's not hard to work out that longer arms will carry you further while brachiating. Long and heavy loosely dangling legs are not going to help setting up a pendular motion at all, so suuch limbs should not dampen the pendulum. Keeping them small and light is the easiest way of doing so. If you wish to see how poorly humans, apes with enormously oversized hind legs, do at brachiating, take a minute to visit a site from the animal simulation laboratory in Manchester here. You will see how awkward humans are when it comes to brachiation. There is quite a bit of knowledge to be found regarding gibbons and brachiation, including some nice mathematical models. Here is a result of one such model, in which the body and the hind legs are simply modelled as a single ovoid blob. The figure has an inherent beauty, I think, but the lack of hind legs in the model made me think.

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Gomes & Ruina. Journal of Theoretical Biology 237 (2005) 265–278

One way to obtain an intriguing brachiating speculative animal would be to do away with hind legs altogether, which would mean the animal perpetually hngs from its arms. In itself that is not a problem, as hanging in this way need not require much muscle activity; none at all, in fact. Perhaps it might then be useful to have more than two brachiating limbs, in order to ensure a good grip. I am only aware of one other brachiating type of animal in speculative biological fiction, and that is the squibbon in 'The Future is Wild'. As I noticed before, tentacles are not good to walk on, but they should be good for tensile forces, and those certainly occur in brachiation. I showed a similar brachiating neocephalopod in last week's blog entry, and added another design point: ideally, brachiating limbs should be attached at the upper end of the body, or else there are novel balance problems.

So why does the marblebill still have 'regular' legs, small as they might be? In truth, when I did the painting I never considered chopping them off, but in hindsight I think I was right not to. Earth's brachiators do not use brachiation as their sole way of propulsion. Such animals also need to climb vertical tree stems, and for that brachiating arms are unsuited. As it may not be possible to depend on brachiation alone, climbing limbs are needed too. If need be, such canopy dwellers may even have to descend to the forest floor and walk. Imagine a legless gibbon on the ground; poor beast... Instead, imagine a marblebill on the forest floor. I admit doing so takes a well-developed sense of imagination; the marblebill is not fast, but it is definitely not helpless, and is in fact still dangerous. Returning to brachiation, it should not be thought that hind legs must be useless. If the legs (or body) are pulled up during the swing, this will aid forward movement. The mechanism is the same as the one allowing a child to make a swing go higher and higher by changing body position on the seat of the swing. Finally, of course, the marblebill uses its four free legs as an aid to capture and kill its main prey. Taken together, it may be best for brachiators to keep some other legs; pure brachiation may be taking the idea too far.

The marblebill also differs from Earth animals in some respects: it is a brachiating predator, which principle seems to be unique, so far. It is also quite large in comparison to gibbons and the like. Still, it much more agile than Earth's orang-utans, which are as large or larger. Its prey, usually an 'Aggie', is a brachiator as well. Most often a pair of marblebills take an aggie by surprise, but if the first charge fails, they will pursue it at full speed. Two adult marblebills moving at full speed is something never seen on Earth; if only I had a video to show you...

What I do have is a video found on YouTube of a gibbon teasing two tiger cubs. Part of the excitement in the video may be due to clever editing of the footage, but there are enough uncut scenes to illustrate how remarkably agile a brachiator can be. On seeeing the video you get the feeling that it is just as well that the cubs are still young enough to be clumsy, or else things might end poorly for the gibbon.

Of course, if the gibbon would be replaced by a marblebill, the cubs would probably be plucked from the ground and eaten.