Why quadrupeds?

[Mike]

[Elle]

There is no real limit to the amount of words I would like to put in this space, so I'm just going to go and and ask why we don't have land animals that are like salp chains and translate your post into the only real meaning it has for me: there exists a facebook page where a person posts shit I would probably NOT cut off my left pinky to be able to read, but the fact that I'm reaching for that analogy gives you an idea, and the mere fact of thinking about that fact is like being pierced in the guts by a greater slaying arrow and I will in fact make my fucking saving throw successfully because I have that one option.

[Mike]

Too disjointed; didn't read

[Elle]

Ha ha ha, no joints! Thank you for at least giving me a moment of levity. Joints are expensive!

[Elle]

Sorry for the earlier reply above; I forget that my private anguish is not transparently understood by everyone who might read these threads. The issue is simply that the original questioner would doubtless be extremely displeased if I were reading his words, much less responding to them in any fashion, even via an intermediate set of questions. I don't want to trample upon his wishes in any way, so now that I have made my successful saving throw to avoid the surprise death-arrow, let's try to have some amnesia about where Mike got these ideas and instead respond to Mike's interesting questions.

Among the questions are why quadripeds tend to grow the largest and why creatures with exoskeletons don't have fewer legs, but those questions involve other questions. For instance, what pressures make any creature grow more or less legs? The advantage of having very many is that you can specialize and duplicate. Human arms might be a way that tree-dwelling quadripeds responded to the same sort of evolutionary pressures when moving out of the trees. Lacking some other awesome solution like a built-in pouch (and how awesome would that be?), being able to carry things around while running overland (like babies, or lots of food over long distances, or tools) provides a whole host of advantages. Limbs are often expensive and vulnerable, especially at our sizes, so you're better off having the fewest you need to perform your specialized maneuvers, and some evidence suggests energy efficiency favors bipedality (e.g. humans requiring less energy to walk around on their two legs than chimps on four).

For creatures who depend on lots of legs to perform those maneuvers, the exoskeleton can contribute to or even be required for specialized abilities. Problem is, when you have an exoskeleton, it's hard to grow the necessary legs to support it at a larger size, and if you could have legs like that they'd be more vulnerable than being bigger is worth. A worse problem is illustrated if you think about why dinosaurs COULD get as big as they did. One reason is that they had astonishingly good lungs, like birds. How is a giant lungless creature with an exoskeleton going to grow, regardless of number of legs, without a change in the way respiration occurs?

Anyway, what excites me about this issue is not so much giant sea sponges, though that is pretty darn exciting, but the very notion of what we consider an organism or its parts. What do we make of swarms of bees, for instance, or salp chains, or collections of corals? There are different ways of getting bigger, in other words, or more to the point, gaining advantages from growing bigger. Maybe what we need are detachable limbs. I feel like I would be far more successful in many ways if I had an extra pair of arms, but I'd like to be able to remove them at my whim.