[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Fw: Dinosaurs and birds
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 _
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
By contrast, I quite agree that generating lift forces in other
directions, or in short pulses, might have advantages for agility or
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.
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.
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.
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
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 selective process.
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.
7). Huh? If you are using a model, you have to take responsibility for
the underlying assumptions.
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.
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
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.
11). But probably the best living models we have, yes?
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.
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.
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.
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.
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.
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?
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?
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.
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.
--- Sigh. "Rarely"?
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.