
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.


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?


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