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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 
men...

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?)

Don



----- Original Message ----
From: MICHAEL HABIB <habib@jhmi.edu>
To: d_ohmes@yahoo.com
Cc: dinosaur@usc.edu
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.

Cheers,

--Mike H.