In article <1sti5kINNoq4@gap.caltech.edu> palmer@cco.caltech.edu (David M. Palmer) writes: >gary@ke4zv.uucp (Gary Coffman) writes: >>Lack of a skeleton means that muscles have to actively resist >>gravity at all times on land rather than supplying only balancing >>forces. That means that much more energy would be required for the >>creature to function. The bones also supply leverage points for >>pushing and lateral movement. That's why you don't find large  >>active boneless creatures on land. > >Hydraulics can make rigid tubes.  No energy required just for >support.  Hydraulics also allow a creature to produce large >forces with weak muscles, using the principle of a hydraulic jack >rather than a lever.  This is one of those "yes, but" things. It's true that a hydraulically pressurized tube can be somewhat more rigid than an unpressurized tube, but even at 2000 PSI levels a hydraulic hose will bend rather easily, though it's straight-on compressive strength is high, and it's torsional  resistance increase is practically nil. On the other grasping member,  there's no doubt that hydraulic "leverage" exists in nature. Tree roots  are an example. Given time they can shatter concrete as osmotic pressure increases.  >>It's interesting to note that, on >>land, creatures are either two legged or 4 legged, with tiny insects >>having 6 or 8 legs, but never 3 legged, though that would be a  >>stable configuration. It can be argued that 2 legged creatures  > >Kangaroos are 3-legged.  The specific number of limbs a creature has is >an accident of evolution, it is hard to make changes in some >structures.  The panda has a thumb plus as many fingers as its >ancestors (five?) on each hand, but the thumb is a modified wristbone >rather than a modified finger, and extra fingers are much more common >than extra limbs, especially fully-functional ones.  Kangaroos 3-limbed? I don't think so. If you take the view that the tail is a limb, then monkeys and kangaroos are 5-limbed. I think the tail is a different kind of structure, grossly enlarged in the case of the kangaroo, but primarily still an instrument of balance rather than locomotion. I don't know much about panda "thumbs", so I'll ask is it opposable?  >>Thermodynamic considerations of surface/volume relationships would >>seem to dictate that active complex creatures  stay in a size range >>similar to what we see about us. 6 inch tall intelligent aliens >>seem unlikely, as do those much larger than the elephant. > >Why can't a lemur or a brontosaurus* be intelligent? > >[*Yes, I know that the brontosaurus is a mythical beast produced by >putting the head of another dinosaur on an apatosaurus, but so far >space aliens are also mythical.]  Well I won't say flat out that they can't be intelligent, but I'll bring a couple of lines of argument to bear to try to show why I don't think it's likely. First let me say that when I say "intelligent" I mean complex behaviors in response to novel situations on a level with, or greater than, human tool use and tool building. IE assuming suitable manipulators are present on the creature to allow it to alter  it's enviroment in a planned way, it will do so. That's certainly not a universal or complete definition of intelligence, but it will suffice for a putative technological alien.  Now no one knows exactly what makes a brain capable of thought, but it's generally accepted that one of the criteria is a certain level of complexity. This is generally determined by the number of neuron cells, and their interconnections. So a creature the size of a lemur wouldn't have enough neurons to support complex thought. This argument is considerably less clear in the case of the dinosaur. There's room for a large brain, though no indication that one ever developed. One reason this may be true is neuronic speed. The electrochemical messages  that trigger neurons require time to propagate. This makes it difficult for a highly complex central brain to coordinate the movements of very large creatures. So there's little selection pressure for such brains. Instead, a simpler distributed network evolves. This doesn't rule out intelligent dinosaurs, but it points in that direction.  Then there are the thermodynamic arguments. A tiny creature like the lemur needs to eat frequently because it's internal heat is rapidly lost due to it's high surface to volume ratio. I contend that a creature that must spend most of it's time and energy feeding won't have the time to develop and exercise intelligence. That argument may be somewhat weak. The dinosaur's problem is the reverse, it must moderate it's heat production  because it's high volume to surface ratio makes it tend to retain waste heat.  I'm assuming that a certain temperature range is optimal for chemical reactivity reasons for productive neuron function. So creatures would tend to need to maintain a regulated temperature in a range near that of humans if they are carbon based. That tends to rule out cold blooded creatures as potential homes of intelligence. Some people contend that some of the dinosaurs may have been warm blooded. But for a creature the size of a brontosaur, it's activity levels would have to be restrained or it would be prone to generate an internal steam explosion from the waste heat. Whales are similar size, but they can reject heat to the ocean, a much more efficient sink than air. I suspect that for intelligence to manifest itself, a certain degree of activity in interacting with the environment is necessary. IE monkey curiousity. I doubt a large dinosaur would be capable of that much activity.  Gary