To cernuate or not to cernuate... (cernuation III)

It would appear that my previous explanation of cernuation left something to be desired, so I will try again. When I coined the word, I was thinking about a mode of locomotion invented by the people responsible for 'The Future is Wild' (TFIW). The squibbons, descendants of cephalopods, have taken to the trees and swing from branch to branch from their tentacles. That could be just what a tentacle might be good at, as it mostly involves tensile forces; tentacles are not well fit to withstand compressive forces needed for walking.

Brachiation is what comes to mind when gibbons or monkeys swing from branch to branch: they hang from one arm while the other swings to grab another branch. While they do so the body stays largely upright, meaning the head is always above the feet.

The way squibbons do this, cernuation, has similarities: the animal swings suspended from one or more arms while other arms move forward to grab a new branch. But the movement is like an inverted somersault, so, while one swing sees the head upwards, the next has it downwards. I presented a fragment of 'TFIW' showing a squibbon moving that way, but the lighting was not very even so the movement may not have been well visible. This post rectifies that.

Click to enlarge; copyright Gert van Dijk

Above you see a rough model of a cernuator. It is only a simple ZBrush model, not meant as a proper animal (I hope my ZBrush friends will forgive me). The animal has very long eye stalks, long arms and long legs, and the head is kept downwards so the eyes stay near the centre of the body; the body is even bent to make room. Having the eyes near the centre was borrowed from the squibbon; this particular concept is stressed in the book 'TFIW'. The idea is probably that this eye position minimises the vertical distance the eyes travel over during cernuation. We'll get back to that.

Click to enlarge; copyright Gert van Dijk

This image shows part of the movement cycle, starting when the body hangs vertically from the arms, and ending where the animal can grasp another branch with its feet.

Click to enlarge; copyright Gert van Dijk

The next diagram starts at the same stage, and ends at the point where the hands are ready to grasp a branch. Note that the animal is upside down during this part of the movement, and moves with its back towards the direction of swing.

Click to enlarge; copyright Gert van Dijk

And finally the animal swings from its hands right up to the point where the cycle began. Notice that the body is now right side up again, and the back faces, well, backwards. That's cernuation.

As I wrote before, I admire the ingenuity that went into its design. The more I think about it though, the more I start doubting how well it would work. There is no mechanical problem at all in swinging like this. Instead, the problem is one of motor control and of visual perception. It is amazing that animals like gibbons actually manage to travel through woods at high speeds. Just imagine how much more difficult their task would be if their image of the world not just zoomed forwards as well as bobbing up and down, as it does in brachiation, but also rotated constantly as it must in cernuation. Have a look at the animal's head above: it is actually turned upside down at one point. How do cernuating animals manage to pick out the next branch?

Click to enlarge; copyright Gert van Dijk

That must have been the reason why the squibbon's designers situated its eyes near the centre of mass. The picture above shows all stages in the movement, with a black line indicating the position of the eyes. Compare that to the position of the feet or the hands (blue line), and you will see that the centre of the body moves less vertically than any part far away from the centre. You may now also understand why the squibbon's eyes stick out sideways: if not, it would not be able to look forwards at times because its body would be in the way!

Click to enlarge; copyright Gert van Dijk

While this eye position minimises vertical displacement, it does nothing to solve the problem of the eye rotating, and with it the animal's view of the world. Perhaps that can be rectified. Let's suppose the head can be rotated by about 180 degrees. Have a look at animal A shown here. Hanging from its hands like this it will view the world as being the right side up. If it were to change the position of its head as in B, it would see the world upside down, right? That is not at all useful, unless of course the body itself would be upside down. Obvioulsy that is the case while cernuating, so if we take animal B, freeze it and rotate it, head and all, you get animal C. The trick would be for the animal to rotate its head from one position to the other quickly when the point of contact changes. During the swing, the animal could then always keep the head in the same vertical position so it could see what it is doing! I do not know whether squibbons were supposed to this by rotation of the eye stalks, but why not...

Mind you, I think brachiation is by far the easier solution, and I do no intend to fill Furahan forests with screeching cernuators. Still, it is interesting to think about, isn't it?

This is probably the point where the rift between those who say 'yes' and the vast majority of mankind is revealed...