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Re: Fw: Dinosaurs and birds



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1). Huh? 'Cant-go-any-faster-than-the-hind-legs-can-run" is the model/analogy _you_ use to "refute" the idea that forelimb assistance can convey advantage by increasing maximum speed. The assumptions I listed are inherent to _your_ argument that fore-limb assistance can't increase maximum speed in the absence of incline.

Constant thrust force and parallel thrust force are not assumptions of the hind limb limited running mechanic. They are not assumptions inherent to my argument, and I thus I never meant to imply them. The animal can point the thrust in any direction it wants, and apply it in any series of pulses or continuous push that it wishes. If the hind limbs are the limiting factor in velocity, then aerodynamic thrust still does not speed them up. This is not only supported by the physics of the situation; we can gather data from a wide range of birds, and the vast majority do not use the wings at all while running. Those species that utilize the wings during running do so briefly, during acceleration or turning. This indicates an advantage in balance or turning radius. It also indicates a lack of utility in wing assistance to raise maximum speed (even though living birds can point aerodynamic force vectors in a number of directions).


In any case: (another) major flaw in the model that is used to 'prove' that inclines are required for fore-limb assist to be useful in forward locomotion is yet another underlying assumption; that of hard smooth ground. An animal that can create fore-limb thrust is _highly _ unlikely to
restrict itself to a theoretical parallel-to-earth-surface direction
for that thrust, and directing thrust slightly upward can increase
stride length and height w/out necessarily decreasing stride frequency.

I only assumed that the ground was firm enough that the feet do not slip. If they slip, then producing lift towards the substrate can be effective for some animals; this would then be effectively WAIR all over again. Producing both lift and thrust such that the animal increases height on each stride does not increase forward velocity during running; it will actually slow the animal. It will clear higher obstacles, however. Thus, I did not really assume a parallel-to-earth vector; I simply investigated the problem using that vector because it would be the most advantageous from a speed gain scenario, if such maximum velocity increases were possible. I simultaneously considered other vectors; but they slow the animal down intrinsically. The exception being a downward vector to increase friction coefficient, which is usually an incline situation and is the WAIR dynamic.


Conditions that reduce hindlimb traction even slightly such as muddy ground, shallow water, and certain types of vegetative cover therefore alter the benefit profile of fore-limb thrust considerably. I call this the 'tread-lightly factor'... and it is easy to construct scenarios where _maximum forward speed_ is increased through _fore-limb assistance_. "Sinking in is not a nimble thing to do" -- a song someone should have written, but didn't.

That's a reasonable point; though the vector would be generally be pointed down into the substrate to increase the frictional force on the feet (ie. WAIR dynamic). Only if the animal is sinking appreciably, or wading rather deeply, would producing a positive lift force on each stride be helpful. Deep wading will interfere with the forelimbs unless it is taxon with long legs, which then reduces the need for help from the forelimbs. Thus, there is a window of possible help there, but it is limited. Modern wading birds, even those that run on muddy ground, are rarely seen running while using the wings, unless they are launching. This indicates that the window of advantage is small among modern birds in those environments. However, they might not be good models for basal forms.


A model wherein the simplifying assumptions are hard, smooth, flat ground and a constant thrust vector at precise right angle to the gravitational vector is interesting as a first step, and indeed inevitably results in a faceful of dirt or reduction of velocity for the poor creature required to operate under those conditions. However, when the results are applied to the real world where other conditions exist, it is a clear-cut case of garbage in, garbage out.

I really think you are under the impression that the physical analysis I discussed was a much more limited than it is. The only thing I assumed was that the animal isn't slipping heavily. That is hardly "garbage in"; it is pretty realistic since most running animals don't slip constantly. It was not a narrow computer simulation, it was just mechanics.


By contrast, I quite agree that generating lift forces in other
directions, or in short pulses, might have advantages for agility or
maneuverability.

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2. "Might"?
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Yes, might. If there was a consistent advantage, then we would see more wing use by running birds. In addition, we need more information on the mechanics of the basal forms we're discussing to know if they could produce the advantages suggested above.


If the animal wants to run faster,
it is more efficient to speed up the hind limbs than to add thrust.

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4). That depends _entirely_ on the ambient environment, bodyplan and lifestyle of the animal in question.

But it is true for the vast majority of known body plans, lifestyles, and ambient environments in which avian locomotion has been studied, because rather unusual circumstances have to be in place for it not to be true. I am obviously not certain of the precise body plans or lifestyles of basal birds, but what we know of their structure so far suggests to me that my statement would hold for them as well. Until evidence shows otherwise, it seems the more strongly supported hypothesis. Again, we can observe quite easily that volant running birds do not flap while running except during fast starts and launches.


Again, there is the implied assumption is that the process of evolving flapping forward flight through forelimb-assistance begins w/ some sort of bipedal cheetah analog running on hard smooth ground

That assumption is not made, actually. The results are the same with a modest runner on any reasonably level surface (ie. not a tree trunk) where the animal doesn't slip a great deal.


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Thus, regardless of the gait, aerodynamic thrust is not helpful for
faster speed.

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5). Again, that depends on the capabilities of the hind limbs, the direction of thrust, and the physical qualities of the substrate. If by 'chance,' some benefit occurs, evolution toward flight continues.
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My statement does not depend on the capabilities of the hind limbs, or the direction of thrust. It only depends on substrate to the degree that they animal is not slipping heavily. I suppose flight evolution could have been closely tied to locomotion on slick mud surfaces, but I doubt it, based on the range of environments in which near-avians and basal-birds are preserved. In addition, a "slick-running" model has the same problems as the WAIR model with regards to flight apparatus prerequisites.


It can be useful for increasing _acceleration_, and thus
getting to a given speed more rapidly.

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6). Which, btw, is "a cursorial mechanism by which forward progress is directly enhanced by wing oscillation", and significant from the standpoint of a selective process.
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Yes; I noted this exception to my original statement in my last post.

They are not required for mathematical analysis, actually, and I did
not mean to imply such assumptions.

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7). Huh? If you are using a model, you have to take responsibility for the underlying assumptions.
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True, I am. But I did not make a number of underlying assumptions that you original thought I might have made.


Is there a reason that you
separate mathematical/mechanical analysis from evolutionary analysis?
I am generally used to melding the two together.

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8). You just have to pay attention to which form of analysis is subjugated to which. For instance, I think you sometimes forget that the relative efficiency between competing iterations of a given system at a given time is what is relevant in the competitive context, as opposed to the theoretical efficiency of the total system relative to the ambient environment.

I keep both in mind; I'm trained as an evolutionary biologist, first and a biomechanist, second. Obviously, what is important is whether the animal has improved relative efficiency compared to its competitors. However, this can be evaluated by asking whether or not there is an overall increase in efficiency from a given behavior, or if a given hypothesized dynamic is mechanically feasible. If a dynamic is not feasible, or can never be efficient compared to ancestral state, then it will not have an advantage as a competitive variant over a given iteration, regardless of the playing field.


Advantage is binary relative to construction of a specific evolutionary scenario, ie, it is "helpful", or it is "not helpful".

I'm not sure this is actually true, but it's a reasonable simplification for the case at hand. Really, advantage is probalistic. If the probability of advantage for each individual is high, then the probability of overall expansion of the trait in question is high. If the probability of advantage is low per individual, then the trait might still expand (bet-hedging traits, frequency dependence, etc. might help it along) or it might disappear. This is the case even if the trait is sometimes "helpful". However, a binary state is probably a good approximation here because we're discussing limits.


If you find that it is helpful, it must be included in consideration of the effects of selection on heritable morphological variance. If you say "not helpful to increase stride length", then it seems to me inescapable that you are using
the unrealistically limiting assumptions: 1) thrust is always at exactly right angle to
the gravitational force, and 2) substrate conditions are optimal for hindlimb traction.

I am saying that it will not be helpful to increase stride length (at least in a manner that increases forward speed) in most realistic cases. I am not assuming anything about thrust direction with that statement (although thrust is actually defined as horizontal; the term we should be using there is the 'resultant force'), nor am I assuming that substrate conditions are optimal. I merely assumed that they were not extremely sub-optimal.


And why would I leave out leaping? If it works, it works. Prey capture scenarios come to mind, as do refugia.

I don't leave them out, either. They just weren't relevant to my initial comments. I'm not arguing against a "ground-up" scenario on the whole, just a maximum speed increase advantage.


We must be careful, however, because even juvenile galliforms are not
particularly good models.

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11). But probably the best living models we have, yes?
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Probably not, actually. The juveniles of other groups might be better. In particular, the juveniles of species that are not burst takeoff specialists would be more informative, for a number of reasons.


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12). The wing kinematics changes have been studied through the entire maturation process? Or are you extrapolating from adult birds? This comment (unlike other sections of this post) is not in any way adversarial... I am genuinely curious, and your expertise is impressive, to say the least.
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The flight-related anatomy has been studied throughout the maturation process for galliforms, rails, geese, and a few other groups. Wing kinematics have been studied in juveniles of galliforms to some degree, at various ages, though a reappraisal might be helpful. In any case, I was using data from actual juveniles, albeit mostly qualitative data with regards to the kinematics. The structural data is more amenable to quantitative analysis, and happens to give the same answer in this case. I'll dig up some references for you. Thanks for the compliment, btw, that was very kind of you.


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13). In my opinion, that ("feasibilty" of increasing max run speed) cannot be determined by theoretical analysis; once the possible/impossible threshold is breached, subject to reasonable assumptions, it is time to take data.
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The analysis for modern avian running dynamics isn't quite as theoretical as you may have guessed. There it is more known mechanics and computation involved in my original appraisal than theoretical biology. In any case, getting data is always a good step, and one reason that I am confident of my conclusions is that the behavioral data and structural data from modern birds supports those conclusions. However, which observations and calculations are relevant to basal birds is a tricky business, and then we are in theoretical analysis.


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14). So your opinion is that within a cohort of same-age quail, those whose feathers are continually pruned will continue to arrive at point B at the same time as those who have begun to receive thrust assistance from decreasing 'limb-load'? Obviously, divergence will occur. But you feel it will not be measurable before the un-pruned birds attain flight?
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No, I think those with feathers intact will tend to get to point B faster, but only because of increased fast start acceleration. Thus, the further the animals run, the less relative time will be spent flapping and the smaller the difference in time to get to the finish. This should be measurable, though the differences might be small and large samples therefore might be required. I would also recommend using a continuous vortex gait flyer, if possible. Juveniles from precocial ducks might be a better model than young galliforms. Incidentally, decreasing limb load would be disadvantageous to running speed, because it would mean the hind limbs were producing less force in their interaction with the ground. Did you mean to say "increasing", or were you referring to some other loading?


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15). I have not "suggested" anything. I have clearly and unequivocally stated, from the get-go, inclines are not necessary for valid fore-limb assisted (= "wing-assisted"), ground-up evolutionary scenarios. I paste in the original statement and your original response for those who might be confused about what started this:


I wrote in reply to one of Scott's posts: "Not sure I understand, from the perspective of a 'ground-up' selective
process that can transform a terrestrial mud-lover into a barn swallow,
where the line between volancy and various forms of wing-assisted
running is (inclines are NOT necessary, in my opinion)."


Mike H. replied-- "The incline is necessary, because without it there is no requirement to
produce a lift force towards the substrate, which is the critical
aspect of wing assisted running."


My position-- Inclines are not necessary for constructing valid ground-up scenarios wherein forward flapping flight evolves; that includes, but is not limited to, scenarios involving selective advantage conveyed by forelimb generated thrust that increases the maximum speed of the animal.
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Good call going back to the beginning (think we've lost people yet?) I misinterpreted your original comment about inclines not be necessary as saying that the animal would attain better forward speeds even if there were not an incline. I did not mean that inclines are required for the evolution of flying forms from cursorial ancestors, though I see exactly why you thought that's what I meant. Sorry for the confusion.


and that maximum
speed can rarely be enhanced by wings.

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--- Sigh. "Rarely"?
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Well, yes, because the animal would need to be running under rather unusual conditions to create a situation wherein the forelimbs can enhance maximum running speed. Under most conditions, running speed is not enhanced. And maximum running speed is never truly enhanced, since gaining traction on mud or a tree trunk only allows the animal to reach a speed closer to its normal maximum, and does not allow the animal to exceed its normal maximum speed.



Cheers,

--MH