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Re: Fw: Dinosaurs and birds

1). Huh? Can you clarify? Because the statement above sounds like good old-fashioned double-talk, in the context of my "good argument".

I thought you made the argument well, and the result you described seems intuitive. However, like many issues in locomotion (or mechanics in general), the limits in this case are not particularly intuitive. --MH

Mike claims this is physically impossible, even for humans equipped with high-tech rocket packs, due to something called the "limiting hindlimb mechanic". To quote; "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...." --MH.

"If", indeed. Surely you did not mean to posit your conclusion as a pre-condition to your argument.

Of course not, and I don't think I did so. My conclusion is that the animal does not gain maximum running speed by flapping the forelimbs. The pre-condition is that the hind limbs are already at maximum capacity. If the hind limbs are not actually limiting, then perhaps the animal can run faster. However, I find it hard to see how this would be the case. It seems unlikely based on four factors:

1) The mechanics, when calculated, suggest a hind limb limitation. Animals cannot exceed physical laws, even with the wide variety of possible body plans and lifestyles, so the mechanics suggest pretty firm limits.

2) Running birds do not flap continuously while cruising at high speed. I know you consider adult birds to be a poor model, but many of them are cursorial, and they should be using the wings to increase sustained running speed if that were possible. Roadrunners, secretary birds, and seriemas should all flap continuously while chasing prey if that dynamic adds top speed. All three of those species flap while running in some cases, but it's usually during fast starts turns, and climbs. I have yet to see any running bird sustain a sprint and flap continuously while doing so.

3) To the best of my knowledge, neither neoflightless cursorial birds nor precocial offspring of volant species flap continuously while sprinting. Personal observations, along with literature images and videos, suggest to me that they do flap in a hard pulse while fast starting or maneuvering tightly. If forelimbs increased maximum speed, we would expect the forelimbs of neoflightless birds to be nearly as strong as those of volant cursors (since they would be producing large amounts of aerodynamic power with the forelimb system).

4) Personal experimentation: tying yourself to a car or running downhill does not add top speed, it just creates unbalance.


In any case, the hind limbs are NOT the limiting factor, at the rate of forward motion that can be achieved by running; THRUST is the limiting factor.
This conclusion requires only 2 assumptions; 1) the stresses on the leg are less at the front of the stride cycle than at back, and 2) it is possible for the hind limbs to increase cycle frequency when not generating the primary thrust necessary to maintain forward motion. Both assumptions appear to me to be manifestly true.

Can you demonstrate that the hind limbs are not limiting? If total thrust alone is the limiting factor, that means that hind limbs are not limiting in their oscillation rate or length, which seems unlikely. Incidentally, I would imagine that the stresses on the leg are not lower on the front of the stride cycle than the back except on the first stride. --MH

The maximum speed a biped can generate through unassisted use of the hind limbs does NOT constitute some magical theoretical barrier that cannot be exceeded.

It's not magical, it's just a mechanical barrier. As David M. said: "I'd bet money it's very close to that barrier, which is not magic at all but depends on length and mass of the hindlimbs in total and the arrangement and size of the muscles that move them."

We also need to keep the footfall time in mind: if our near-bird increases its travel speed during aerial phase with the forelimbs, then it will be going beyond the velocity sustainable by the hind limbs when it makes footfall at the end of that aerial phase. The critter is going to fall over at that point unless it can speed its footfall rate to keep up (essentially skipping), but that requires lowering the duty factor. If the hind limb power is constant, but the duty factor decreases, then the total force imparted on the substrate will decrease. --MH

2). Total power relative to forward motion is not coming from the hind limbs in this situation, yet evidently there is a term in your equation that "...requires higher hind limb power (that we've already limited)". [???] Why is that? In fact, relative to the theoretical situation as posited, NO forward thrust is required from the hind limbs... nor is any aerodynamic lift required, as the legs serve to counteract gravity. Note, that can be done mechanically, w/out thrust generation other than that required to pull a given leg forward into the front of the stride cycle...

The term requires higher hind limb power because force imparted and limb oscillation speed both depend on total power. If power is limiting, then both of those variables have an upper limit irrespective of assistance from forelimbs. --MH

Not quite what I meant.  If the animal is supporting its full weight
with lift, and doing so by overcoming drag with thrust (from the
wings), then it is flying.

3). Flight requires a flight-control phenotype. Simple thresholds of lift and thrust production do NOT mean FLIGHT-CAPABILITY in any relevant ecological or evolutionary sense. And lift is not relevant to my argument.

Thresholds of lift and thrust are paramount to flight capability: they define the ability to fly. As such, thresholds of lift and thrust are of great importance in an ecological or evolutionary sense. Of course, a number of adaptations are required to meet the prerequisite lift and thrust needs. I'm not sure what you mean by a "flight-control phenotype". Do you simply mean that flying animals must be able to control their flight path? --MH

Positive lift is counter-productive for running speed.

3). Thrust. NOT lift.  Not necessarily counter-productive.

Well, the thrust problem is the same problem as before: exceeding hind limb capacity and crashing. However, I am also curious as to what dynamic you are proposing to produce only thrust from the forelimb. Part of the resultant force will be perpendicular to the flow, and result in lift. It could be minimized by pushing largely with the outboard wing, but avian wings generally produce both thrust and lift (which are really two vectors of the same fluid force in flapping flight). --MH

4). _If_ my understanding of the hypothesis is correct, WAIR, when advanced as a _primary_ flight evolution scenario, overemphasizes foot traction by oversimplifying lifestyle, substrate issues, and traction enhancing devices, and in so doing discounts the numerous advantages attainable in a vast number of situations in which those simplifying assumptions do not hold, yet forelimb assistance can be postulated. This may be a result of WAIR's use of extant animals w/ a specific life style as models, or natural academic competitiveness.

WAIR has some problems, but oversimplification of lifestyle, substrate, and traction are not really major faltering points. The WAIR mechanic was studied with live birds on natural substrates (ie. branches), so those factors are actually all included. WAIR in and of itself also does not exclude or reject other uses of the forelimbs, but, like you said, the authors of the WAIR description emphasize its importance over other dynamics (and probably take that emphasis a bit far). In any case, WAIR as a primary flight evolution scenario misses many other important dynamics, just as you pointed out, and also suffers from certain mechanical shortcomings (ie. the mechanics of WAIR might not have been available to basal birds). --MH

Sure, I can expand.  Sorry for shifting the subject a bit last time and
causing confusion.  If the normal force on the foot is reduced by
producing positive lift, then the foot will both 1) experience lowered
friction, and 2) impart reduced force against the ground.  Not a
particularly useful tool for running rapidly.

5). That depends on environment, lifestyle and bodyplan. Not a universal law. Reduced foot force against the ground can highly advantageous to many exploits, _especially when it is discretionary_.

It's not a law, but it is a result of laws (physical ones), and thus is going to be a pretty firm rule most of the time. Reduced foot force against the ground can be advantageous, but not for running speed. --MH


--Mike H.