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...