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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 launching. --MH


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.

Yes. --MH

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 feet. --MH


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 un-assisted crossing.)

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.

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E). Which in many environments increases its rate of forward motion, in the non-theoretical sense.
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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
mechanics.


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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


Cheers,

--Mike H.