Tuesday, 30 March 2010

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.

Click to enlarge
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.


video

9 comments:

j. w. bjerk said...

Very Nice!

I don't think you should criticize yourself for making it too gibbon-like. I'd say it's roughly about as un-gibbon like as you could make it without hurting it's plausibility as a brachiator. At least without abandoning bilateral symmetry. The extra segment in the fore-limbs give it a nice alien feel.

I can vividly imagine how the 4 short legs would grapple it's prey at the end of a pendulum swing, and then beak strikes down for a killing blow. I can also imagine something along these lines as an ambush predator.

The rendering and color scheme is quite nice too.

I'm having a hard time seeing how the eyes relate to the brow ridges. Are they on a stalk?

I'm curious about your thinking in giving it a "hand" without fingers. Is that a trait of it's taxonomic group, or something for brachiating?


Also FWI: You list their habitat as "steppe", which i assume you know refers to treeless land.

Evan Black said...

I must say I really enjoy your marblebills. Not only are they a unique addition to the sampling of Furahan life on your website, but your analysis here on the blog taught me some things I did wrong with my own brachiating aliens.

Sigmund Nastrazzurro said...

Hi J.W.,

About the 'steppe': that is obviously wrong. I started with a html file for another animal entirely, and did not notice this incongruency. I will fix it.

The hand design was completely on purpose. Type 'gibbon hand' into Google, and you will see that they have very long hands, with a thumb set very much near the wrist. The first photo I found even showed that the thumb was not involved in grasping a rope. According to the literature gibbons tend to break bones, which is not surprising. If you would approach a branch with a thumb stretched out in the direction of movement, it is more likely to get hurt than if it is not there. That is why the marblebill hand is shaped like a flattened hook. Gibbon hands function like that too, so I decided to take the idea a bit further.

The eyes are indeed fixed on stalks. Compound eyes on stalks are part of the general hexapod body plan, although there is a large amount of variation in the stalks.

Hi Evan; thank you!

Anonymous said...

So all "vertbrates" have compound eyes then? Also you get me very excited every time you mention the encyclopedia of furahan life. I hope its published soon I could always use a good spec bio read.

j. w. bjerk said...

Re: compound eyes

I was thinking, wouldn't compound eyes tend to be significantly inferior at focusing things at a distance than vertebrate-type eyes of equal size? With our eyes you have the whole diameter from front to back of the eye to focus the light, while with a compound eye, you have a small fraction of the diameter.

Arthropods aren't generally know for their long-distance vision, but of course they are generally a lot smaller, with more or less correspondingly smaller eyes, though you'll notice that dragonflies which need long-distance vision (for a bug) have relatively humongous eyes.

Of course there is more to vision than simply the size of the eye. The eagle with a smaller eye has vastly superior distance vision.

Still i wonder if the Marblebill (from what we know about earthly compound eyes) would be able to see a vine before he swings out towards it.

Sigmund Nastrazzurro said...

The problem with compound eyes does not have to do with focusing, which is something they simply do not do. Instead, the problem is one of resolution. The individual 'eyelets' (ommatidia) typically produce one 'pixel' of visual information, and the resolution of the image depends on how many of these structures you can pack. You cannot make them too small because there are physical constraints that affect whether the ommatidium can be excited by incoming light.

How bad are they in principle? Normal human eyes can resolve 0.016 degree of arc, and some dragonflies reach 0.24 degrees. That is 15 times worse, but this acuity still enables them to pluck flying insects out of the air!

But there are some tricks. Firstly, relative eye size may be a problem in arthropods, but it is not in large hexapods, so all animals can easily reach the resolution of dragonflies. The second trick is that the ommatidia do not necessarily have to sample unique portions of the world. Instead, their fields can be made to overlap, particularly when eye size is not a big problem. This opens the way for intricate neural image processing improving the image. Secondly, it means that you can have stereo vision with one eye! So here is an exclusive: many hexapods move their eyes independently, as stereo vision does not require two eyes.

Finally, cats have a visual acuity that is about 5 times lower than than of humans, but they still manage to pluck birds or insects out of the air.

Taken together, I decided that there was enough uncertainty is all these factors to decide that hexapod visual acuity remains worse than human vision, but is good enough to allow an active life style.

Anonymous said...

An interesting idea for a branchiating organisms which is probably not capitalized among vertebrates on Earth is a Furahan organism where the first two organisms have been modified into grasping devices like the marblebill, but the middle pair of arms have become useful grasping appendages for food. Branchiating animals on Earth probably cannot develop specialized grasping appendages due to the liabilities mentioned with gibbons (breaking anything that isn't a part of the grasping "hand") or the legs being needed to help balance the organism, assist in climbing, or help the organism hang from trees (the last can be seen with tree sloths). But Furahan hexapods have an extra set of limbs, ones that could develop into appendages which could provide a specialized purpose of grasping fruits or small organisms from the treetops while not interfering in the total organism's climbing capabilities (other than accounting for the shift in weight caused by the moving limbs).

Anonymous said...

"The problem with compound eyes does not have to do with focusing, which is something they simply do not do. Instead, the problem is one of resolution. The individual 'eyelets' (ommatidia) typically produce one 'pixel' of visual information, and the resolution of the image depends on how many of these structures you can pack. You cannot make them too small because there are physical constraints that affect whether the ommatidium can be excited by incoming light."

The sheer size of most hexapods also means that you can get much more ommatidia on the surface of the eye than in arthropods, because many more photoreceptor cells can be packed into the area of the eye. This may give Furahan hexapod eyes better resolution than arthropod eyes.

Anonymous said...

Here's a scenario where a brachiator may have no legs:

What about on an ocean world where the land is mostly underwater, but a lot of it is quite shallow covering most parts of continents save for mountain areas. There may be vast mangrove forest like areas, where tree like organisms have grown out of water that is shallow enough for large trees, but deep and spread out enough to cover large tracts of land.

This environment is then mostly trees and water. What if the first animals out of water on this planet do not become land dwellers but tree dwellers. On this brachiation planet, with little ground not under at least 5 meters of water, perhaps the brachiators will have forgone evolving hind thins into legs...

At all possible?