[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Fw: Dinosaurs and birds
I grew up on a farm and we did a lot of stuff every day that would make you
nervous, some of the safest of which involved running behind trailers and
pickups, and hitching rides on same. When I was 9 or 10, I could and did bounce
along behind the ass-end of a flatbed 4 wheel farm trailer for extended periods
of time. It was fun, even a high point of the morning, except you couldn't see
the roots in the road until you were right up on them. When I got taller, it
wasn't fun anymore. Bonus points if you can guess why.
Daily life, especially work, involved imminent risk of death and horrendous
injury (ever seen a guy that's been caught in a corn combine or kissed the
business end of a pulpwood saw?). There was no assurance of profit or
protection from loss, and it wasn't if you got hurt, it was just a matter of
how bad. I personally loved it, when people would let me. City folks _really_
don't understand, having built their own planet, one that allows a theoretical
perspective on natural selection.
On the other hand, I don't really get guilds, gangs, conspiracies, and various
other resource acquisition/social hierarchy phenomena wherein loyalty to the
group overrules larger cultural norms, such as respect for the truth, or
property, including intellectual property. So I definitely lack evolutionary
fitness on the new planet. But then I am not a scientist, nor am I dialed in...
I think I understand the "they-can't-run-any-faster" point/analogy quite well.
The analogy isn't apt, nor are your assumptions. You assume that an animal that
can generate forward thrust directs it exactly forward/parallel to the ground,
and further, cannot quickly re-direct that thrust. You also assume that the
thrust generated is constant in force, another major flaw of the car analogy.
You further assume that for a speed increase from fore-limb assistance to be
advantageous, it must occur at full running speed. None of the above
assumptions, although necessary for mathematical analysis, are correct in an
_evolutionary_ context. Thrust can be, and is, used to increase stride length
(including, but not necessarily at, maximum stride frequency), decrease stride
length, overcome obstacles, increase/reduce velocity and seek refuge (or more
generally, improve tactical position). These exploits convey advantage, and
there is no such thing as an insignificant advantage, or
"narrow margin" in the evolutionary context.
Analogies drawn from adult modern birds are not very useful in constructing
quadruped-to-neornithine evolutionary scenarios, in my opinion. Juvenile quail
and turkeys are another story, and observations of wing-assisted ('fore-limb
assisted' in the evolutionary context) locomotion on flat ground are easily
reproduced, and include all of the exploits listed above. There is an
'assistance gradient' as the wings develop that ranges from zero contribution
from the fore-limbs to full flight, and a narrow time window (in the early
stages) involves very high wing loading and (I assume) forward impetus
("thrust") only. The period from 'zero contribution' to 'thrust only' is
relevant to evolutionary scenarios. I have observed chicks in the wild, and the
ones that flap get further down the road than the ones that don't, especially
quail. That is anecdotal, but I still have some money, if y'all are betting
You may think 'fore-limb-assisted' scenarios for evolving flapping flight that
include inclines are more convincing, more probable, or more efficient, and I
would agree. For the reasons given in the 4th paragraph, I say inclines are not
necessary. And that most definitely includes scenarios in "which forward
progress is directly enhanced by wing oscillation". (If, and only if, "wing
oscillation" is what we called 'flapping' down on the farm. If not, what the
heck does it mean?)
----- Original Message ----
From: MICHAEL HABIB <firstname.lastname@example.org>
Sent: Thursday, April 5, 2007 2:53:50 PM
Subject: Re: Fw: Dinosaurs and birds
> No, I don't. As the car accelerates (w/in reason), I give a little
> jump, and make some progress, effectively increasing stride length.
Sure, but only within a pretty narrow margin. If you are sprinting at your
fastest speed, the extra pull of the car shouldn't help much. If you start
having to sacrifice stride frequency to increase the stride length, then you
won't really be making any progress. Keep in mind, as well, that the analogy
isn't perfect. If you're very agile, you might be able to keep up with the car
by essentially bouncing briefly off the ground every so often and letting the
car pull you through the air. That's not really running with extra thrust,
though, that's just being dragged with style. I'm impressed that you've tried
though, it would make me a tad nervous.
In any case, the point Jim was trying to make is that the speed of a running
(bipedal) animal is largely limited by mechanics of the hind limbs and their
interaction with the ground. Adding thrust from the forelimbs will only add
speed if the hind limbs can keep up. They generally cannot if the animal is
already at a sprinting gait, so forelimb-assisted speed increases don't tend to
be very feasible for birds. This is one reason why running birds don't deploy
the wings unless they are launching, or engaging in a ground maneuver for which
an additional force vector might be helpful in maintaining balance.
>> BTW-- did you notice that Mike H. actually admitted I was (at least
> in theory) right in that inclines are not essential to wing-assisted
> evo-scenario's? It was buried pretty deep, but it was there. Ha!
> Although 'fore-limb assisted' is a better term, 'wing' being the end
> result of the process...
It was never meant to be "buried". You are most definitely correct that
inclines are not theoretically required for cursorial-based wing evolution.
They *are* required for a WAIR type mechanism; ie. a cursorial mechanism by
which forward progress is directly enhanced by wing oscillation, which is all
my original post was meant to imply. That does not rule out maneuverability
and balance being enhanced by forelimb dynamics over level terrain, and these
will obviously bolster forward progression indirectly.