Wednesday, June 29, 2005

Is Four One Leg Too Many?

Roger Ebert, in his review of War of the Worlds today, complains about the aliens' use of a three-legged contraption:
And, for that matter, why balance these towering machines on ill-designed supports? If evolution has taught us anything, it is that limbs of living things, from men to dinosaurs to spiders to centipedes, tend to come in numbers divisible by four. Three legs are inherently not stable, as Ray demonstrates when he damages one leg of a giant tripod, and it falls helplessly to the ground.
I think I'll have to take issue with the above argument. Three legs, I think, are a better idea than four when it comes to building a stable structure, which is why chemists and cameramen both use tripods. The reason is pretty simple: three points always form a plane, which means the three tips of a tripod are guaranteed to sit stably on the ground. On the other hand, with four legs, you need them all to be exactly the same length to achieve stability on all surfaces. True, a tripod will collapse if one of its legs is hacked out from underneath but, in all likelihood, four legs won't help that much more.

Having said that, I started wondering about why so few animals are three-legged. I could conjecture two alternative explanations:
  1. Quadrangular body shapes are more desirable than triangular shapes, leading naturally to four-legged animals in preference to tripeds.
  2. When walking, animals would like to pull up a foot and still balance on the remaining three. Being a triped would complicate the situation somewhat.
Does anyone have better guesses?


Anonymous Cpr said...

On the tripeds, or lack thereof:
I did immediately think of walking/running issues as I read your question. However, I think it's a little different.
Most quadrupeds have 2 and not 3 legs on the ground when running, somewhat similar to the way we bipeds have only one at a time. I was thinking of how a triped would run. I came up with two possibilities. One would need an assymetric balance of power where there's one strong front leg and 2 back legs. The front leg moves forward and grips the surface while the two rear legs then lift off and land ahead of the front leg, repeat. This places a very strong dependance on the front leg.
A more symmetric arrangement would involve a sort of rotation to move forward, with each leg stretching out a littlle forward when it's the front leg and then twisting the entire body about that leg about 120 degrees till then next leg becomes the front leg. I wouldn't want to be that guy! Running that way would probably leave the poor creature rather disoriented especially if it used anything like cochlear fluids to govern balance!
I guess in evolving from four-legged to two-legged animals, if it were sequentially reasonable, there must've been an evolutionary trace of one that tried a hand and 3 legs. I guess that didn't work out.
I guess an animal can have an odd number of non-walking limbs (or limb equivalents) - case in point being the Elephant.

6/30/2005 1:35 PM  
Blogger grenade said...

That rotating triped cpr describes is exactly how the reconnaissance bots move in Clarke's "Rendezvous with Rama." In fact, Clarke also mentions the same tripedal locomotion problem when discussing them.

I'd go either with the load balance explanation or blame it on evolution doing things as it has always done them. This also left me wondering if our bodies have any parts that occur in odd numbers other than one.

Some side notes:
I haven't seen the movie and how they've done the alien machines, but to me, they'll always be the awesome striders

Also, the first thing that I would have taken issue with in the original argument is that insects are six legged. Besides, why would Ebert want to pick on something that's explicitly stated in the original work? (Yes, I read the review where he says they would work in 1898, but that made less sense.) And as for "[balancing] towering machines on ill designed supports", I guess we should all stop walking and get back to crawling. I've always hated Ebert's reviews anyway :D.

7/03/2005 7:43 PM  
Blogger grenade said...

This comment has been removed by a blog administrator.

7/04/2005 12:40 PM  
Blogger grenade said...

After some more thinking, I believe that the answer lies with work. The work we put into walking on a flat surface is pretty minimal, second, perhaps, only compared to the energy needed with wheels. This is managed by keeping our center of gravity at a pretty much constant height (since we need to do work to raise it and don't gain much when it falls). With 3 legs, I can't think of a minimal work walk where the third leg isn't redundant (in a bad way).

And maybe you could try it out using Spore sometime. If you haven't heard of this, it's worth watching the video at: (use bugmenot if it asks for registration).

7/04/2005 12:46 PM  
Blogger thedragonsrose said...

I haven't seen the movie, but it could be a simple matter of "gee that looks really cool, let's do that." Which is why many of the movies out there have gotten so many bad reviews. Our society is rapidly forming a "coolness" mentality.

If it "looks cool" go with it. It doesn't matter if it's believable or not.

7/09/2005 2:37 PM  

Post a Comment

<< Home