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Re: Fw: Dinosaurs and birds
Note: I first began thinking about the evolution of flight about 1965, and my
ideas jelled about 1995, and have changed little since. I see 2 major problems
specific to ground-up flight scenarios: 1) the establishment of a phenotype
that allows synchronous flapping of the forelimbs while simultaneously using
the hind limbs in alternating fashion. 2) crossing the threshold between zero
and minimally beneficial forelimb aerodynamic locomotive assistance. The
comments below assume those magical thresholds have been previously breached.
Also I try not to use the term "wing-assisted" because I feel the need to
remember that the process discussed _culminates_ in a wing... so fore-limb
assisted is the term I use, and is synonymous here w/ "wing-assisted". Comments
etc., are numbered for clarity.
From: Michael Habib <email@example.com>
Sent: Thursday, April 5, 2007 11:22:11 PM
Subject: 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.
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. They are _not_ features of the model I use for assessing the
possibility of advantage gained by increase of maximum speed through use of the
forelimbs. The model I use is qualitative and involves multiple cases for
hindlimb traction, incline, and density of the locomotive environment (both
atmospheric and vegetative). In the case of vegetation, one must consider the
vertical gradient; it is very different to walk through waist-deep grass as
opposed to ankle-deep, for instance. Perhaps you meant to say something else.
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.
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.
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.
By contrast, I quite agree that generating lift forces in other
directions, or in short pulses, might have advantages for agility or
> 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.
3). So? We are discussing a selection-driven process that turns forelimbs into
forward flapping wings. An advantage is an advantage. Acceleration counts,
particularly in combination w/ advantageous change in vertical position.
If the animal wants to run faster,
it is more efficient to speed up the hind limbs than to add thrust.
4). That depends _entirely_ on the ambient environment, bodyplan and lifestyle
of the animal in question. 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. Animals
in the process of optimizing the hindlimbs for speed have 'chosen' an
evolutionary path that is unlikely to result in flapping flight. Animals that
need to approach a prey item from a slightly elevated position in
less-than-perfect traction (for example) have an entirely different take on
Thus, regardless of the gait, aerodynamic thrust is not helpful for
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.
It can be useful for increasing _acceleration_, and thus
getting to a given speed more rapidly.
6). Which, btw, is "a cursorial mechanism by which forward progress is directly
enhanced by wing oscillation", and significant from the standpoint of a
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.
7). Huh? If you are using a model, you have to take responsibility for the
Is there a reason that you
separate mathematical/mechanical analysis from evolutionary analysis?
I am generally used to melding the two together.
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.
_For instance, consider this quote: "If the animal wants to run faster, it is
more efficient to speed up the hind limbs than to add thrust." -- Mike H._
I agree that is a _generally_ true statement relative to the physics and theory
of locomotion, but it is not necessarily relevant to selective regimes on
specific bodyplans, or in specific environments.
> 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.
9). "Particularly helpful" implies that you are capable of quantifying
selective advantage, and determining a threshold of evolutionary significance
for same, and I don't think you or anyone else can currently do that. Advantage
is binary relative to construction of a specific evolutionary scenario, ie, it
is "helpful", or it is "not helpful". 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
the gravitational force, and 2) substrate conditions are optimal for hindlimb
And why would I leave out leaping? If it works, it works. Prey capture
scenarios come to mind, as do refugia.
> 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.
10). "Less (...) feasible" strikes me as being in the same category as
"particularly helpful". And again I ask, "What is the traction?"
> Analogies drawn from adult modern birds are not very useful in
> constructing quadruped-to-neornithine evolutionary scenarios, in my
> 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.
11). But probably the best living models we have, yes?
They have a very advanced upstroke system
(even compared to other living birds), and fairly hefty pectoralis
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.
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
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
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.
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.
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?
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?
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
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
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.
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
---- Yes, they are.
and that maximum
speed can rarely be enhanced by wings.
--- Sigh. "Rarely"?
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).