Tuesday, 23 September 2008

Why there is no 'walking with tentacles'...

Tentacles are cool: they look sleek, effective, a bit mysterious, and also slightly repulsive, particularly when covered with lots of suckers and slime. It is no wonder that science fiction illustrators equipped their creations with tentacles when something like the above-mentioned criteria were called for. And longer ago monsters like the 'Kraken', a giant octopus, were the staple monster of heroic tales.

As an aside, it is interesting to speculate why exactly the boneless movements of an octopus evoke fascination as well as revulsion (not in me, but certainly in many people). The explanation might be that their body scheme is so completely different from ours that it is difficult for us to comprehend what is going on when they move. (there are neurons in the motor areas of our brain that respond when we see body parts moving in other people; I would predict that their response is less as a body scheme departs more from ours). But that is not the point here. Let's take it as a given that tentacles create interest, so it would be nice to design some creatures that walk on tentacles.

Those who really know their cephalopods might say that there is no need to design animals walking on tentacles, as they already exist. Indeed; here they are. But those are 'walking' underwater, and I want animals walking on dry land.

Before anyone thinks of the loophole that land animals could quite well walk on tentacles in a very light gravity, I will accede the point. But I want animals walking on dry land in an Earth-like gravity.

Some among you might now think of ballonts (see earlier entries) that are starting to evolve back to a terrestrial life again, so they can walk with their bodies partially suspended from a balloon. To stop that, and lighter than air parts are now declared officially out of the question. The demands put on this design are to walk on tentacles in an Earth-like gravity on dry land, without any unfair ballooning aids.

I will count the solution of having a very very large number of legs as a possibility, but it doesn't sound energetically efficient. So try to design the biomechanics in such a way that the animals could actually work.

Has this been done? Well, yes. In fact, I designed one many years ago (in 1982...), turning one tentacle in a slug-like foot, leaving the others free. It was the Jellyshell, and here is part of its description:

"Without any apparent reason they will suddenly set out towards the beach. Many specimens crawl ashore more or less simultaneously, move around for a while on the beach, leaving trails of glistening slime behind them. These mysterious maneuvers last for several hours; afterwards they return to the water and take up their normal habit of slithering and algal grazing. They will not be seen on land for weeks on end; then you will suddenly encounter dozens of their trails on the beaches, left during one of their forays. Just why they come on land, and why they glide and slither there in their slow and solemn way, no one knows."

But having one slug-like foot doesn't really count as walking, so I guess that one does not count. Dougal Dixon did one several years later: the 'Coconut Grab' (found in 'The New Dinosaurs' from 1988). Here it is (click on it for a full scale depiction; it is fairly large). I think the book is still to be found, and I definitely recommend it.

That comes much closer to 'walking on tentacles', but not close enough: as the description says, the grab draws itself along the ground with four of its legs, and crawling doesn't count as walking.

So the only ones I know of that really count are the evolved cephalopods in 'The Future is Wild': the megasquid especially. This was a television series that featured some speculative evolution. The megasquid is a gigantic descendent of squid (probably octopuses, come to think of it), and it walks upright on its eight columnar tentacles. The program does have some comments on how the legs actually work, and these make it clear that the tentacles are properly boneless, as they should be. Here is a frame taken from the program.

I have my doubts though; is the proposed bimechanical explanation sufficient, or, to be more precise, would evolution leave it at that?

But first some explanations are needed about what makes a tentacle into a tentacle; but that's for another installment.


Anonymous said...

Fascinating article, I myself am interested in the area of terrestrial locomotion through the use of adapted tentacles. My theory is that as the specific cephalopod species adapted to life on land the tentacles became smaller and sturdier to better support the body. Perhaps some rudimentary cartilaginous structures would form inside the limbs (though I'm at a loss to how much of an asset these structures would be) and in some distant future the animal would stand erect and tall on limbs supported by pseudo-bones. Height would be useful for high browsing and a far carrying of sounds such as mating and possibly territorial calls. I can easily see the octopuses jet organ evolving into some sort of vocal apparatus.

Sigmund Nastrazzurro said...

Dear Anonymous,

Have you read my later posts on the subject?

If not, you may find them of interest, because your reasoning follows the same logic as I worked out in these posts. If animals have to make do with tentacles on dry land as their mode of moving about, evolutionary pressure to make these limbs mechanically better will be large. In effect, given time, I proposed that evolution would develop bone-like structures, so the end result is that the tentacle cannot be called that any more: it will heave turned into a leg!

Dinoboy said...

My mistake, I stumbled upon your blog during a Google search and decided to comment rather prematurely. I am but an amateur in the field of Speculative Biology and am always eager for some new website or book devoted to the subject.
You may note that I have logged into my (rather ancient) google email account. I'm fascinated by dinosaurs too, hence the name.

Pds3.14 said...

One problem would be control. A hip has 3 degrees of freedom. A knee has 1. A foot has 3, plus whatever toes are there, but toes aren't strictly necessary for even bipedal walking. This makes 7 important degrees of freedom which must be managed.

A tentacle, on the other hand, has an enormous number of degrees of freedom. So much so that 2/3rds of the neurons of octupi are in their tentacles, and not in their brain. The result is that, for the brain, controlling the tentacles is more like "filing a request for movement" than actually moving the limb themselves.

So any coordinated movements in a tentacled cephalopod are examples of excellent cooperation and teamwork between the different tentacles and the brain, but the octopus cannot send exact, precise controls to its limbs. It's like army ants. They can do things in a coordinated way, but the queen doesn't directly control her subjects. The hive isn't quite one entity, and not is a cephalopod, with the limbs each having to make highly intelligent decisions of their own accord.