Re: Bipedalism

[rjmeyer@ix.netcom.com (Rob Meyerson)]

John Jackson writes;

>< I wonder if bipedalism would be a detriment, and possibly fatal, for an
>arboreal animal >
>
>Bipedalism arises not from arborealism as such but from leaping - to other
>branches, as opposed to what early birds probably did - to which an arboreal
>lifestyle lends itself.  Interestingly this often leads to hopping, as in
>kangaroos (whose ancestors arose from the largely forested Australia of
>earlier times) and certain lemurs who can only progress on the ground by
>hopping.

Actually, 2/3 of my last posting on this subject was slashed in-transit and did 
not appear (darn you Netcom).  Since an incomplete thought is the most 
vulnerable, I'm not suprised that people spotted holes.

Since I wasn't able to complete my thoughts, here is what the rest of my letter 
was to have said (abridged version):

I went with the idea that bipedalism has it's advantages for cursorial critters 
and can be seen as a cursorial adaptation; I then went on to suggest that 
theropod bipedalism evolved on the ground, before they moved into the trees.

To take a nod to BCF, I then suggested the following model: Bipedalism evolves 
on the ground; these "prototheropods" discover an advantage to life in the 
trees, and begin to adapt to that lifestyle; a part of that adaptation is 
longer arms, to help grasp the branches; these longer arms are now preadapted 
to become wings.  These guys, then, are the ones that evolve into true 
theropods.

I acknowledge that bipeds can adequately function in trees, with some 
modification of their form.  Having claws or wings certianly helps things 
along.  I merely suggest that the bipedalism *trait* is most likely to develop 
on the ground.

>Then why do monkeys and apes retain a normal walking ability?  Well I
>suppose hopping/leaping will only become obligatory if the animal returns to
>the ground only very rarely, and then it may not happen (only some of
>lorris's, potto's etc leap/hop, and many apes are too big to risk it).

Firstly, I would point out that most of the above are "temporarily bipedal."  
When chimps or gorillas travel in a casual walk, they are quadrepeds, using all 
four limbs for motion.  These critters stand only occasionally, and only for 
brief periods of time.  Basically, their skeleton is not designed for constant 
bipedalism.


Shalom,

Rob Meyerson

***
"Listen to the song, not the words."
                -Kosh