This was my abstract (co-authored with David B. Weishampel) for the Walker Symposium at the SVP 2009 Meeting. It got a bit complicated as my Romer session was concurrent with this one...
Flight morphology and launch dynamics of basal birds, and the potential for competition with pterosaurs
Birds inherited a bipedal gait and feathered airfoils from their theropod ancestry. These features produce specific tradeoffs with regards to launch, maximum size, lift coefficient, and limb disparity. There are subtle effects related to the use of feathered wings, such as the ability to utilize separated wingtip slots and extensive span reduction, which have also influenced avian flight evolution. Combining information from structural mechanics, aerodynamics, and phylogeny, we conclude that the basal state for avian takeoff was a leaping launch, not a running launch. We find that several morphological features of early birds, inherited from theropod ancestry, predisposed them to radiation in inland habitats. We find that Archaeopteryx could sustain substantial loads on both its forelimbs and hindlimbs, but structural ratios between the forelimb and hindlimb of Archaeopteryx are indicative of limited volancy. Limb strength in Confuciusornis was modest, suggesting an emphasis on cruising flight and limited launch power. We find little evidence to support extensive competition between birds and pterosaurs in the Mesozoic. Prior literature has suggested that pterosaurs competed with early birds for resources and may have helped shape the early evolution of birds. There is some evidence of partitioning between pterosaurs and birds in ecological space. Evidence from the Jehol fauna suggests that pterosaurs dominated near coastlines during the Early Cretaceous, while birds were more diverse and important inland. However, flight is not a single, compact character. Flight mechanics vary considerably across volant animals. Some flyers experience only limited competition for resources with other flying species, and might compete most intensely with non-flying taxa. As a baseline for understanding the interactions between Cretaceous birds and pterosaurs, the flight dynamics of the two groups need to be compared in a quantifiable framework. Birds and pterosaurs inherited different morphologies, and this impacted their flight regimes. Comparing the two systems provides a basis for hypotheses related to competition in the Cretaceous, and the influences on early avian evolution.
Showing posts with label Launch. Show all posts
Showing posts with label Launch. Show all posts
Sunday, May 20, 2012
To Quad or Not to Quad
I received a really good question about the relative advantages of bipedal and quadrupedal launch at the Royal Tyrrell Museum. Essentially, they asked what the relative performance tradeoffs of each one might be, and why pterosaurs might have ended up locked into a quadrupedal launch style.
As it turns out, there are really two parts to the answer. In terms of which launch mode ends up as the dominant method of takeoff within a clade, it is likely that phylogenetic inertia plays an important role: birds inherited an obligate bipedal stance from their ancestors, and so every bird (so far as we know) has been a biped and launches bipedally, at least from the ground (more on this later, but some birds are quad launchers from the water, which is pretty neat stuff).
Bats seem to have inherited an obligate quadrupedal lifestyle. Pterosaur origins are more fuzzy, but they probably arose from one of a few different groups where bipedal to quadrupedal transitions were more common, and so their early evolution may have been more phylogenetically plastic with regards to stance. They eventually ended up as obligate quadrupeds, with most species placing most of their weight on the forelimbs (this is apparent because manus tracks from pterosaurs are typically deeper than the pes prints).
In terms of the relative performance advantages, it turns out that we can solve the question algebraically:
1) Both forms of launch will start as a leap (or a run ending in a leap). This means that the immediate post-launch cycle is ballistic. That's handy - ballistic math is easy.
2) Quad launch adds an upstroke immediately after push-off. Bipeds can (and do) raise the wings as they toe-off, so they can engage the first downstroke as soon as the wings have clearance.
3) Quad launch adds more power for initial push-off, so this goes into the ballistic equation. This will mean more height and speed, but at the cost of the added upstroke (the extra upstroke is accomplished with folded wings, so it's quite quick).
So, all we have to do is have an idea of max acceleration and unload time for takeoff, which gives launch speed, combined with the launch angle. We can vary these a bit to get a range of plausible values, and these give us the ballistic trajectory. So, for example, the maximum height gain can be calculated from the launch velocity squared x sin(launch angle) squared, divided by 2 x gravitation acceleration.
It turns out that for just about any flying animal, quadrupedal launch does better in nearly every way. They get a lot more power (because the flight muscles are so strong and can add to the launch in a quad takeoff, whereas in a biped takeoff they add very little). This means more clearance, ballistic time, and speed. It also allows for a greater range of starting wing attack angles, and is essentially "safer" because of the much greater clearance for the wings and body (larger margin of error, as it were). The extra time in the air from the greater push more than makes up for the extra upstroke time. For example, even in a giant like Quetzalcoatlus northropi, the initial upstroke would only take about a tenth of a second. It would have almost a third of second to reach the top of the ballistic leap, however, giving plenty of time to spare.
So, on the whole, quad launch is just "better" - with one exception. A bipedal launcher with short wings and a very short flapping time can switch from ballistic phase to flapping phase a bit earlier. This is not as efficient as the quad option, but it can mean a steeper and more immediate climb-out. This is only useful for a burst-launching specialist at moderate or small sizes (at giant sizes quad launch is king), but it is perhaps noticeable that this exact set of morphological features and takeoff strategy is extremely common among living birds - it is highly typical of galliform birds (pheasants, grouse, etc), pigeons and doves, and many of the passerines. It is also perhaps telling that there are no particularly short-winged pterosaurs. For a quadrupedal launching animal, very short wings don't do nearly as much good. The closest example might be anurognathids, and as noted in my GSA abstract, they are quite unique among pterosaurs. More on that later...
As it turns out, there are really two parts to the answer. In terms of which launch mode ends up as the dominant method of takeoff within a clade, it is likely that phylogenetic inertia plays an important role: birds inherited an obligate bipedal stance from their ancestors, and so every bird (so far as we know) has been a biped and launches bipedally, at least from the ground (more on this later, but some birds are quad launchers from the water, which is pretty neat stuff).
Bats seem to have inherited an obligate quadrupedal lifestyle. Pterosaur origins are more fuzzy, but they probably arose from one of a few different groups where bipedal to quadrupedal transitions were more common, and so their early evolution may have been more phylogenetically plastic with regards to stance. They eventually ended up as obligate quadrupeds, with most species placing most of their weight on the forelimbs (this is apparent because manus tracks from pterosaurs are typically deeper than the pes prints).
In terms of the relative performance advantages, it turns out that we can solve the question algebraically:
1) Both forms of launch will start as a leap (or a run ending in a leap). This means that the immediate post-launch cycle is ballistic. That's handy - ballistic math is easy.
2) Quad launch adds an upstroke immediately after push-off. Bipeds can (and do) raise the wings as they toe-off, so they can engage the first downstroke as soon as the wings have clearance.
3) Quad launch adds more power for initial push-off, so this goes into the ballistic equation. This will mean more height and speed, but at the cost of the added upstroke (the extra upstroke is accomplished with folded wings, so it's quite quick).
So, all we have to do is have an idea of max acceleration and unload time for takeoff, which gives launch speed, combined with the launch angle. We can vary these a bit to get a range of plausible values, and these give us the ballistic trajectory. So, for example, the maximum height gain can be calculated from the launch velocity squared x sin(launch angle) squared, divided by 2 x gravitation acceleration.
It turns out that for just about any flying animal, quadrupedal launch does better in nearly every way. They get a lot more power (because the flight muscles are so strong and can add to the launch in a quad takeoff, whereas in a biped takeoff they add very little). This means more clearance, ballistic time, and speed. It also allows for a greater range of starting wing attack angles, and is essentially "safer" because of the much greater clearance for the wings and body (larger margin of error, as it were). The extra time in the air from the greater push more than makes up for the extra upstroke time. For example, even in a giant like Quetzalcoatlus northropi, the initial upstroke would only take about a tenth of a second. It would have almost a third of second to reach the top of the ballistic leap, however, giving plenty of time to spare.
So, on the whole, quad launch is just "better" - with one exception. A bipedal launcher with short wings and a very short flapping time can switch from ballistic phase to flapping phase a bit earlier. This is not as efficient as the quad option, but it can mean a steeper and more immediate climb-out. This is only useful for a burst-launching specialist at moderate or small sizes (at giant sizes quad launch is king), but it is perhaps noticeable that this exact set of morphological features and takeoff strategy is extremely common among living birds - it is highly typical of galliform birds (pheasants, grouse, etc), pigeons and doves, and many of the passerines. It is also perhaps telling that there are no particularly short-winged pterosaurs. For a quadrupedal launching animal, very short wings don't do nearly as much good. The closest example might be anurognathids, and as noted in my GSA abstract, they are quite unique among pterosaurs. More on that later...
Wednesday, April 11, 2012
Insect Takeoff
One thing that you may note, if you are feeling observant, is that the wings are just coming down as the walking limbs are completing their push off of the substrate. This seems to be a rather general trend - the legs are used to initiate launch and the wings engage relatively late in the launch cycle (Nachtigall and Wilson, 1967; Nachtigall, 1968; 1978; Schouest et al., 1986; Trimarchi and Schneiderman, 1993; 1995). A "leap first, flap second" modality is also seen in vertebrates, and the generality of the trend in flying animals probably derives from the fact that pushing off the substrate provides much greater efficiency in achieving high accelerations from rest than would be accomplished by first engaging the wings. It is also worth noting that slow flight, which would include the first moments after takeoff, typically requires greater wingstroke amplitudes than fast flight - so clearance for the wings can be a problem for a launching animal. The easiest solution is simply to jump first, fly second.
References
Nachtigall and D.M. Wilson. 1967. Neuro-muscular control of dipteran flight. J. exp. Biol. 47: 77–97
Nachtigall. 1968. Elektrophysiologische und kinematische Untersuchungen über Start und Stop des Flugmotors von Fliegen. Z. vergl. Physiol. 61: 1-20
Nachtigall. 1978. Der Startsprung der Stubenfliege Musca domestica. Ent. Germ. 4: 368-373
Schouest LP, Anderson M, Miller TA. 1986. The ultrastructure and physiology of the tergotrochanteral depressor muscle of the housefly, Musca domestic. J. Exp Zool. 239: 147-158
Trimarchi JR and Schneiderman AM. 1993. Giant fiber activation of an intrinsic muscle in the mesothoracic leg of Drosophila melanogaster. J. Exp. Biol. 177: 149-167
Trimarchi JR and Schneiderman AM. 1995. Initiation of flight in the unrestrained fly, Drosophila melanogaster. J. Zool. Lond. 235: 211-222
Head Over Heels
I know that it is now technically Wednesday, but this was supposed to be the Tuesday post - got a little held up with work at the Carnegie Museum.
In any case, given that I posted a bit about how bats launch, it seems only fair to also point out that great work has been done on how they land, as well. Dan Riskin and colleagues have a great paper in the Journal of Experimental Biology (freely available here) where they examined the tricky business of landing on ceilings. As it turns out, there are a few ways to do it, and one of them basically involves a cartwheel in the air to bring the feet, and then the hands, in contact with the inverted substrate.
This, in turn, brings me to a point that I have been making at conferences lately: landing and launching from ceilings is tricky business. The exact kinematics of ceiling-launches in bats have not been elucidated in great detail, but it is known that they are quite acrobatic. This is important, because one of the bits of rebuttal I hear rather often about large fossil flyers (mostly pterosaurs) is: "why couldn't they just drop off a cliff for speed?"
The answer is that this turns your average giant pterosaur in a rather elaborate lawn dart. Dropping head-downward to takeoff is actually a pretty inefficient way to go unless one happens to live on ceilings (like bats). Consider this: to successfully launch, one of the giant flyers (such as a large pterosaur or teratorn bird) would probably need around two g's of acceleration upwards (closer to three would be ideal). If they start by falling, then they are accelerating in the wrong direction at a g, not to mention that the poor critter is oriented in a very compromising way.
Launching like a falling stone is tough going - bats do it, but it's a very specialized trick. So, please make myself and other flight folks stop wincing: don't drop your pterosaurs.
Monday, April 9, 2012
Bounding Bats
There is a wicked paper out in PLoS ONE on how bats can actually use their uropatagia and tails to get a little extra lift during slow flight and launch. The paper is freely available (like all PLoS papers) here.
Adams et al. (2012) show some really neat dynamics for the uropatagium and tail in bats. They also just get some great shots of bat launch in fringed myotis. What's particularly interesting here is that previous work on ground launch in bats has focused on the species with the most powerful takeoffs: vampire bats and New Zealand short tailed bats. Those models have been very informative, and I have studied the experimental data on vampire bats (Desmodus) extensively in my reconstructions of pterosaur launch. However, most bats are not built like vampires. The genus Myotis is a large group of rather "typical" bats: while the genus is hardly uniform, it is essentially comprised of small, insectivorous species that rarely come to the ground.
On the whole, the launch in Myotis works about the same as in Desmodus, but I do note one really neat difference: if you take a look at the figure I've pasted here from Adams et al. (2012), you'll note that in the first panel (bottom) the bat is pushing off at the wrist followed by the wing fingers. It's actually unfurling the wing part of the way early on (instead of late, as in Desmodus) and letting the highly compliant fingers in the wing bend to produce a pushing surface. That's not just bending at a joint, mind you, that's the actual bone that's flexing. Spectacular stuff.
This is not the first time that bat tails have been implicated in flight control. Another paper, also in PLoS ONE predicted the role of the tail in flight control previously (Gardiner et al., 2011). It's a nice little theoretical paper and it is neat that a theory-based work and an experimental one on the same bit of morphology hit in back-to-back years.
If you want to check out what the vampire version of bat launch, you can turn your cursors here for the manuscript in the Journal of Experimental Biology (Schutt et al., 1997). You can also check out a video of a vampire bat running here.
References
Adams RA , Snode ER , Shaw JB (2012) Flapping Tail Membrane in Bats Produces Potentially Important Thrust during Horizontal Takeoffs and Very Slow Flight. PLoS ONE 7(2): e32074. doi:10.1371/journal.pone.0032074
Gardiner JD, Dimitriadis G, Codd JR, Nudds RL (2011) A Potential Role for Bat Tail Membranes in Flight Control. PLoS ONE 6(3): e18214. doi:10.1371/journal.pone.0018214
Schutt, W. A. Jr., Altenbach, J. S., Young, H. C., Cullinane, D. M., Hermanson, J. W., Muradli, F., and Bertram, J. E. A. 1997. The dynamics of flight-initiating jumps in the common vampire bat Desmodus rotundus. The Journal of Experimental Biology, 200, 3003-3012.
On the whole, the launch in Myotis works about the same as in Desmodus, but I do note one really neat difference: if you take a look at the figure I've pasted here from Adams et al. (2012), you'll note that in the first panel (bottom) the bat is pushing off at the wrist followed by the wing fingers. It's actually unfurling the wing part of the way early on (instead of late, as in Desmodus) and letting the highly compliant fingers in the wing bend to produce a pushing surface. That's not just bending at a joint, mind you, that's the actual bone that's flexing. Spectacular stuff.
This is not the first time that bat tails have been implicated in flight control. Another paper, also in PLoS ONE predicted the role of the tail in flight control previously (Gardiner et al., 2011). It's a nice little theoretical paper and it is neat that a theory-based work and an experimental one on the same bit of morphology hit in back-to-back years.
If you want to check out what the vampire version of bat launch, you can turn your cursors here for the manuscript in the Journal of Experimental Biology (Schutt et al., 1997). You can also check out a video of a vampire bat running here.
References
Adams RA , Snode ER , Shaw JB (2012) Flapping Tail Membrane in Bats Produces Potentially Important Thrust during Horizontal Takeoffs and Very Slow Flight. PLoS ONE 7(2): e32074. doi:10.1371/journal.pone.0032074
Gardiner JD, Dimitriadis G, Codd JR, Nudds RL (2011) A Potential Role for Bat Tail Membranes in Flight Control. PLoS ONE 6(3): e18214. doi:10.1371/journal.pone.0018214
Schutt, W. A. Jr., Altenbach, J. S., Young, H. C., Cullinane, D. M., Hermanson, J. W., Muradli, F., and Bertram, J. E. A. 1997. The dynamics of flight-initiating jumps in the common vampire bat Desmodus rotundus. The Journal of Experimental Biology, 200, 3003-3012.
Thursday, April 5, 2012
Swimming Eagle
This video has been making the rounds, so many of you have probably seen it: Swimming Eagle of Baton Rouge.
What I find particularly rewarding about this little clip is that the quantitative model I built over the last year to estimate water launch in pterosaurs also predicts that eagles (and some other birds) should be able to do this, as unusual it is. Always validating to see expectations met.
I will post more about water launch in pterosaurs later, but the basic gist is this: the folded wing pivot of a bird (wrist) or pterosaur (base of fourth finger) can produce quite a bit of flat plate drag in the water, if the wing is still mostly folded. Combined with the powerful flight muscles, this provides a mechanism for generating substantial forces in the water without compromising flight anatomy.
What I find particularly rewarding about this little clip is that the quantitative model I built over the last year to estimate water launch in pterosaurs also predicts that eagles (and some other birds) should be able to do this, as unusual it is. Always validating to see expectations met.
I will post more about water launch in pterosaurs later, but the basic gist is this: the folded wing pivot of a bird (wrist) or pterosaur (base of fourth finger) can produce quite a bit of flat plate drag in the water, if the wing is still mostly folded. Combined with the powerful flight muscles, this provides a mechanism for generating substantial forces in the water without compromising flight anatomy.
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