In my last post I promised to report back on any interesting alien animals in the recent Discovery Channel series 'Into the Universe with Stephen Hawking'. Well, the Discovery Channel has the relevant footage on its site, right here. Some of the images of the program on aliens are rather nice, particularly where the computer generated imagery concerns space hardware. The underwater creatures and the ballonts weren't bad either. One life form really caught my attention however, but I cannot say I am positively impressed. Have a look at the following video.
I was interested right up to the point where camera rotates to reveal that the animal is in fact clinging to a vertical rock surface. Until then I thought that the mouth design was kind of interesting, scraping algae or their equivalents off a flat surface. You could debate on how well it would work, and that would be fun. But the rather large problem is how the animal manages to cling to a vertical rock surface.
It pays to think about the forces needed to keep an object fixed to a vertical surface. The centre of gravity of the object will be some distance away from the wall, producing a lever effect on the contact area. Think of a painting hanging from a nail: the top pulls away from the wall while the bottom presses against it. If you had to fix a heavy object to a wall with only one screw, you would put it in where you have to counteract a pulling force: at the top. The bottom of the contact area will stay against the wall anyway. In short, the top end of the contact area pulls away from the surface and the bottom end presses against it.
This also holds for rock-climbing humans. Feet press against the wall, but fingers pull on it. This is much easier if there are gaps the fingers can be hooked into, as otherwise friction is the only force to stop the body tumbling down. Keeping the body close to the rock minimises how hard the fingers have to pull, so the less secure the grip is, the closer you will want to stay near the surface. The same principles are at work on a woodpecker on a tree trunk: woodpeckers rest their stiff tails against the the bottom contact point, and their claws, the top contact area, dig into the bark and pull on it.
Back to Discovery's 'rocksucking cliffhanger' (as it appears to be nameless I made that name up). The image above was taken from the video, but was rotated to show the animal in its vertical position. It has two legs at the bottom, and at the top the mouth is the obvious device to pull on the rock surface. I must say that this is in itself a rather nice idea. You may argue that such a large mouth would need improbably large flat areas, or that dust, snow or rain water could really make life difficult, but such matters might be worked out.
In the sketch above you see my take on the situation (marked A). The body is held fairly far away from the cliff wall, meaning that there are serious lever effects. A line connects the centre of gravity with the lowest point of contact, where rotating forces will push the feet against the rock. The mouth has to suck rather hard to prevent a fall (blue arrow). But what happens if the animal lets go with its mouth, as in sketch B? Ah, right... No cliffhanger here: it will fall, and I can think of only two ways to stop it doing so. The first is that the new top contact area, the toes, must exert an unbelievably strong sucking ability; that possibility is shown here, but I see no evidence of that, and it is very impractical. The other way would be for the cliffhanger to shift its mouth to a new position extremely quickly, before the body has moved downwards so far that there is no stopping it anymore. The video shows that repositioning the mouth takes about one second. On earth an object will in principle fall 4.9 meters in the first second of a fall! This distance would be less on a low-gravity world, and I do not know how many body-lengths that is, as there is no size indication. Still, it sounds as if the fall would then be unstoppable. You would want the loss of sucking power to last as short as possible: it it would only last for 0.1 second, the animal's centre of gravity would only move downwards by 4.9 cm. The process had better be failsafe though; no second chances here. A much better solution would be to having two or more sucking devices, as their use could then be alternated.
As shown, I do not see the animal working as designed. It's a pity, really. As with Avatar, it would not have been that difficult to get someone to have a critical look and iron out the problems. It's not rocket science, after all. Why do companies as big as Discovery let such matters slip? The only answers seem to be that they take science a lot less seriously than they pretend to do, or that they do not take their audience seriously. I am very much in favour of science popularisation, but fumbling it gives the wrong idea.