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Re: Fw: Dinosaurs and birds
On Tuesday, April 10, 2007, at 03:32 PM, don ohmes wrote:
Quick re-cap of the debate (started w/ a resolved miscommunication) as
I understand it (corrections welcome); I say ground-up flight
evolution scenarios that pre-suppose flat terrain and advantage
conveyed by increases in maximum speed generated by "wing-assisted"
(=forelimb assisted) aerodynamic thrust are feasible, and even likely,
due to the presence of other potential advantages such as acceleration
and maneuverability, all on a variety of substrate scenarios. Mike H.
says increasing maximum speed w/ aerodynamic thrust on flat ground is
not possible because either the hind limbs will fail before the
minimum threshold of advantage is reached, or use of the 'wings' while
running will result in a reduction of overall speed. He is skeptical
of such advantage factors as acceleration and maneuverability on
probabilistic/relative efficiency grounds. I think this is a correct
summation of positions...
The only correction is that I am not particularly skeptical of
acceleration and maneuverability as an advantage important to the
evolution of avian flight. I am merely skeptical that basal birds were
able to generate the same degree of advantage in those realms as the
juvenile galliform birds that are becoming popular as models. I also
echo Jim Cunningham's comments about the importance of utilizing energy
from gusts. That's a separate conversation though, really. --MH
A). A running bipedal animal can use an incremental increase in
forward thrust to increase stride length, w/out decreasing stride
frequency, therefore increasing forward velocity.
This seems reasonable, but it doesn't work out, even if the thrust is
increased incrementally. If the hind limbs are running at their limit
for a given gait, then adding stride length will by definition reduce
the stride frequency or the force imparted per stride. In either case,
the animal cannot exceed its previous velocity, and will have to push
against drag on top of it. I can post the computations, if you would
like. However, it is actually reasonably intuitive if you look at the
constraints. In order to increase stride length without losing
frequency or imparted force per stride the animal must move faster
through the air, which means either pushing harder with the feet
(already disallowed) or reaching a speed during the airborne phase via
forelimbs that is above the animal's normal running speed, in which
case it falls over when it comes back down. There is no solution
possible where stride length increases without reducing stride
frequency or imparted force, if you don't let the hind limbs work any
harder and the animal cannot actually fly. This solution only becomes
possible at the point where the animal is actually holding itself up
with the forelimbs, and helping to push with the hind limbs, which is
I take the gist of your comments on "friction coefficient" to indicate
you calculate that an upward thrust vector (or any reduction of the
gravitational vector) reduces the ability of the feet to impart thrust
to the substrate, and therefore reduces speed.
1). Consider a running bipedal animal w/ clawed hind feet on hard flat
ground. At some point, imparting more thrust w/ the feet becomes
counter-productive w/ respect to speed because traction is broken, and
energy is wasted. If forward thrust from the forelimbs is applied at
this point, the foot thrust required to maintain speed is reduced,
reducing slippage and reducing energy waste. In other words, the hind
leg is returned to it's optimal state relative to thrust vs slippage,
which obviously varies by substrate.
In a sense, but if the animal pushes itself forward with the forelimbs
while the hind limbs are slipping, it will probably fall over. A more
efficient solution, most of the time, will be the WAIR type solution:
impart momentum to the air such that your resultant force vector is
mostly down (ie. negative lift force) and thus increase friction on the
In fact, reduction of the gravity vector on the hind foot can
_increase_ the thrust potential of same, rather than reducing it.
(Anyone who doubts this should run a zip-line (or 'zip-rail", to coin
a term) across a plowed field such that some body weight can be
supported by arms, and compare the time of crossing the field to an
The reason that the zip line passenger moves faster is that they trade
in gravitational potential more effectively and/or reduce loss of
momentum at the end of strides. Even if the zip rail is horizontal,
the individual is gaining speed by pushing and gliding: it's a lot like
inline skating at that point (just on the hands instead). If the zip
rail is positioned such that the person on it is actually suspended
partially, and thus cannot put their full weight against the ground,
then they will actually be slowed down (cannot push as hard). They
might be able to get up to a higher speed if they have enough distance,
but pushing little by little and gliding to build up momentum. This is
not analogous to the situation at hand, however. (It is analogous to
an animal that is fulling gliding and pushing off the substrate at
intervals, such as a flying fish). --MH
C). We agreed that adult living birds are not good models. They can
fly, and the behavioral phenotype reflects that. They don't _need_
wing-assisted running, why would they seek to use it? When they need
more speed, they launch.
Some volant birds hunt on the ground, and would presumably utilize the
wings to increase running speed after prey if they could. I can think
of several other cases where adult birds would use wing assisted
running to reach higher velocities if it worked. Besides, we can show
quantitatively that the birds cannot increase their top running speed
with the wings. --MH
BTW-- the tails are important to the traction/sub-optimal substrate
debate (see comment 1), and also (it seems to me) allow (when used in
conjuncture w/ the fore-limb "wings') the hind limbs to be more
free/effective/important in (prey) capture/fighting...
Tails can be very important in some situations, and very unimportant in
others. I think you're correct though; tail evolution was probably
important in early avian flight evolution, even though some of the
earliest birds heavily reduced the aerodynamic importance of the tail
(ie. Confuciusornis). --MH
It will clear higher obstacles, however.
E). Which in many environments increases its rate of forward motion,
in the non-theoretical sense.
Absolutely. I think that would be a great use of incipient wings.
Especially for a dromeosaurid. Think of all of the advantages to
hunting in tall undergrowth for a good leaper. Modern analogs are
numerous and include both mammals (foxes, felids) and birds (some
raptors, gruiforms, and others). --MH
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
F). Sure they do. Every step they take they slip, their entire lives.
Not enough to make the incipient wings worth it, most of the time. If
the animal is using the forelimb forces to increase traction, then it
is pushing against drag to do so. The increase in traction has to be
worth the drag costs. Thus, the slipping has to be heavy for it to be
helpful. This might be the case during fast starts and unusual
substrates. I think the fast starts are probably the more important of
the two, statistically.
In addition, we are assuming that the upstroke is powerful for the
traction increase model; that's probably not a good assumption for
basal birds. --MH