View Full Version : Re: Summer science quizzes

unknown user
05-25-2001, 12:36 AM
Here is a continuation of our discussion - all messages which
I received during the last week. Enjoy!

Krystyna Gielo-Perczak, Co-moderator Biomch-L
From: "Greiner, Thomas M. Ph.D."

Ho boy, another classical problem.

Yes, the fact the "joint" is the ankle in birds and the knee in humans does
answer the question. In fact, the knee and ankle both bend in the same
direction in both animals. Again, we need to re-phrase the question. Why
does the bird have such a long foot, and why does it walk with such a small
portion of the foot in contact with the ground? (I should point out that
from, a comparative perspective, the human plantigrade posture is the more
unusual adaptation. I should also point out that since both birds and
mammals (humans) evolved independently from a reptilian ancestor the
orientation of their knees and ankles is an example of convergent evolution
-- ie., both evolutionary lineages came up with the same biomechanical

My solution: the foot (the bony link between the ankle and the toes) is the
primary locomotor lever. Animals with long feet, relative to the rest of the
limb, are adapted more for speed than for power. Thus, a bird runs or hops
(a hop may be useful in providing the initial rapid airborne thrust,
although I am just guessing here since I don't know a lot about birds) while
a human strides slowly, but with greater bipedal stability. Humans with
"short" feet are evolutionary hold overs from the necessity of powerful
locomotion in their arboreal (tree climbing) ancestors.

From: brad mcfadyen

A thought in passing about lower limbs of birds vs humans...No one has
wondered about the advantage of such a joint configuration in the bird for
bending over to eat at ground level and not planting their beak in the dirt?
From: Anna Pike
Subject: Summer science quiz #2... more

Dear all,

A late additional point in this interesting discussion.....

....starfish have odd numbers of appendages ('arms'), but these are not
directly reponsible for their locomotion. Echinnoderms have a unique
water-vascular system and move by extending associated tube feet or podia.
Briefly, the water-vasuclar system consits of a radial canal extending into
each arm with a series of lateral canals extend perpendicular from it,
alternating from side to side. There is a tube foot associated with each
lateral canal. The result is that there are 2 rows of tube feet responsible
for the locomotion within each arm. The apperance of 4 rows is achieved in
some species by alternating the actual length of each lateral canal.

Not true bilateral symmetry I know, but the odd number of appendages is
really an even number of rows of podia

Best wishes
From: "David R. Collins"
Subject: Re: Bi-lateral symmetry vs. asymmetry ...more

Dear colleagues,

It seems that an interesting class of movement systems has been missed in
symmetry and movement discussion, that of single-limbed (or un-limbed)
creatures such as snakes, worms and eels, as well as single-celled
flagella-like organisms (my apologies if someone did add these previously).
Such movement systems might be considered both odd-limbed (although
technically one is not an odd number because it is not divisible by itself
one) and bi-laterally symmetric. I believe a more exotic single-limb case
be found with the multi-cellular stage of slime mold colonies in which a
stalk and head are formed. Another odd class would include alligators and
other reptile/amphibians which use four limbs to augment a snake-like
style. Still, these, like the kangaroo and monkey, are further examples of
bilateral symmetry for which the odd limb is in the plane of movement. It
would seem that symmetry is a more satisfying answer than dumb luck for
successful movement systems. Thanks to Jon Dingwell for the references to
excellent work on symmetry and movement.

Dave Collins

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