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.