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Re: Fw: Dinosaurs and birds



On Thursday, April 5, 2007, at 07:12  PM, don ohmes wrote:

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.

I don't actually make those assumptions. Those are features for the model in which thrust from the wings is used to increase maximum speed, and I included them to show that such a limited model is not feasible. By contrast, I quite agree that generating lift forces in other directions, or in short pulses, might have advantages for agility or maneuverability.


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.

If it doesn't occur at full running speed, then it still isn't helping from a total velocity standpoint. If the animal wants to run faster, it is more efficient to speed up the hind limbs than to add thrust. Thus, regardless of the gait, aerodynamic thrust is not helpful for faster speed. It can be useful for increasing _acceleration_, and thus getting to a given speed more rapidly. Though, the efficiency of this technique is going to depend on the speed regime, planform, and a few other factors. Some ground birds do use wings to burst accelerate occasionally, especially if startled. Whether or not the force is produced in pulses or is constant is not particular important to the model. For maneuvers (including fast starts), a pulse of near-constant force is usually going to be most efficient.


None of the above assumptions, although necessary for mathematical analysis, are correct in an _evolutionary_ context.

They are not required for mathematical analysis, actually, and I did not mean to imply such assumptions. Is there a reason that you separate mathematical/mechanical analysis from evolutionary analysis? I am generally used to melding the two together.


Thrust can be, and is, used to increase stride length (including, but not necessarily at, maximum stride frequency), decrease stride length,

Using aerodynamic thrust to increase stride length is probably not particularly helpful, especially at the maximum for the hind limbs, unless you mean to imply that the animal is actually leaping.


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.

Overcoming obstacles is a very reasonable use of incipient flight abilities, as is seeking refuge. Changing velocity (ie. acceleration) is also quite helpful, though increasing velocity past what the hind limbs can produce is less mechanically feasible.


Analogies drawn from adult modern birds are not very useful in constructing quadruped-to-neornithine evolutionary scenarios, in my opinion.

I agree.

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.

We must be careful, however, because even juvenile galliforms are not particularly good models. They have a very advanced upstroke system (even compared to other living birds), and fairly hefty pectoralis mass. But yes, they do use the wings briefly for some maneuvers on the ground. These should be kept in mind during evolutionary analysis, but we must also be aware of what mechanical limitations exist for basal birds and near-avians. Juvenile galliforms have many derived characters not seen in basal forms.


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.

Of course; I agree with all of the above. The question then becomes which sorts of assistance are mechanically feasible and which are not. Increasing maximum running speed happens to be a low feasibility dynamic. By contrast, changing acceleration, improving turning radius, or performing incline runs are all mechanically feasible ways of using incipient wings, though how helpful they are depends largely on the flight apparatus in place. With an understanding of the flight apparatus and structural strength of basal birds, we can narrow the range of possibilities to further evolutionary analysis.


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...

I wouldn't be surprised; they're probably using their wings to get to top running speed more quickly, or aiding balance (or both). I have observed a rather wide range of birds, juvenile and adult, in both the wild and captive situations. I have seen precocial chicks use wings for acceleration when startled from standing still. I have not seen any evidence that they reach higher speed. I'm willing to bet that maximum speed is not increased in the quail you observed, either. Nonetheless, better acceleration is helpful (which is probably what they are gaining), and another reasonable trait for selection of flapping. Again, I only was referring to the hypothesis of increased maximum speed previously, not any other advantages of wings on the ground, of which there are a few.


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.

I'm not sure if they are more convincing or not. I am actually somewhat critical of the WAIR hypothesis, as I mentioned in my original post. Inclines are required for the specific WAIR dynamic, but not for other advantages of incipient wings.


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".

What are you suggesting they are "not necessary" for? If you mean maneuvers, clearing obstacles, burst accelerations, or balance, then I agree. But those do not usually _directly_ enhance forward progress, with the exception of burst acceleration forward. I should have included that exception in my original statement. That said, my point was that inclines are important to WAIR dynamics, and that maximum speed can rarely be enhanced by wings. Those are the two kinematics I had in mind with regards to forward progress being directly enhanced, which is why I said inclines were important. I did not mean to imply that inclines are required for the evolution of flight from a cursorial ancestor at any general level.


(If, and only if, "wing oscillation" is what we called 'flapping' down on the farm. If not, what the heck does it mean?)

Yes, it means flapping, essentially. I said wing oscillation to be a bit more broad, since flapping usually means a true flight stroke (though it doesn't actually have to, it does have that connotation).


Cheers,

--Mike H.