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 Leaping. Show all posts
Showing posts with label Leaping. 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...
Sunday, April 29, 2012
Spiders Incoming
Vadas Gintautas and I have done some more data collection on the spider aero-control work at this point, and the results are proving to be pretty darn exciting.
To bring everyone up to speed, here's the gist. We are working on the issue of aerial righting in spiders. The way in which spiders right themselves after a fall from a ledge, branch, etc. has never been reported in the literature. It is an interesting problem, because:
1) Spiders can't twist around in the air like cats and other mammals can.
2) Spiders that pursue prey over uneven terrain, rather than using burrows or webs for capturing food, are bound to fall sometimes.
3) Most spiders are small enough that they are going to fall in an intermediate Reynolds number regime - i.e. the air will be pretty sticky for them, but they won't just be "sinking" like a tiny gnat would.
4) Spider body plans are already of interest in robotics designs for movement over land. Now imagine that the spider-bot can turn itself in the air using wake capture so that it always lands on its feet. Yup, it's awesome.
What's particularly interesting here, though, is that we are not using the spiders we would expect to be particularly good at aero control. We are using Black Hole Spiders, Kukulcania hibernalis, which hunt and move mostly over the ground. They can climb, though, so we figure they might still have some kind of aerial righting response. If they do, then it would suggest that aero control appears in even those species with limited arboreal habits. A photo of K. hibernalis is shown at left. It comes from the site of Ken the Bug Guy here, which is where we purchased our spiders.
What's cool is that we have found that our little spider stuntmen can, indeed, right themselves in the air, and they do it using a nice, simple trick: when dropped, the spiders immediately assume a leg position that makes them passively unstable in roll while inverted, but stable in roll when upright - this means that the spiders automatically flip right-side-up if they happened to be falling upside down, and then they stay that way for the rest of the trip. All the spiders have to do is hit the critical position and hold it. We are working on the algebraic solutions that demonstrate why the righting position works, but our simulation study already predicts their motion quite well (i.e. it matches our imaging studies of the real spiders).
The aerial righting response we have captured is somewhat similar to that seen in other arthropods, such as ants and stick insects. Since we have found it in a mostly terrestrial animal, it is looking ever more likely that this is a method of aerial righting that is extremely widespread in terrestrial arthropods, and that's very neat stuff for understanding arthropod evolution (which is handy because arthropods represent most of the animal species on Earth).
We are not the only group working on falling spiders. As I mentioned recently in an earlier post, Robert Dudley of Berkeley has a student working on arboreal spiders from the neotropics that, not surprisingly, are even better at aero control (I won't get into details because they have a paper pending, but it's awesome).
Vadas and I are setting up to work on some additional species of spiders, in order to take a more comparative approach. We will include some arboreal spiders that we expect will be able to actually glide. Not kidding; gliding spiders. There are all sorts of other aerial behaviors we can find in spiders, as well, that take into account the use of silk for producing drag lines and parachuting. There is an entire realm of aerial acrobatics in spiders that has been rather understudied (ballooning being the only one of the bunch to get much play in the literature). So stay tuned for spinning, dropping, gliding, righting, leaping, parachuting spiders.
To bring everyone up to speed, here's the gist. We are working on the issue of aerial righting in spiders. The way in which spiders right themselves after a fall from a ledge, branch, etc. has never been reported in the literature. It is an interesting problem, because:
1) Spiders can't twist around in the air like cats and other mammals can.
2) Spiders that pursue prey over uneven terrain, rather than using burrows or webs for capturing food, are bound to fall sometimes.
3) Most spiders are small enough that they are going to fall in an intermediate Reynolds number regime - i.e. the air will be pretty sticky for them, but they won't just be "sinking" like a tiny gnat would.
4) Spider body plans are already of interest in robotics designs for movement over land. Now imagine that the spider-bot can turn itself in the air using wake capture so that it always lands on its feet. Yup, it's awesome.
What's cool is that we have found that our little spider stuntmen can, indeed, right themselves in the air, and they do it using a nice, simple trick: when dropped, the spiders immediately assume a leg position that makes them passively unstable in roll while inverted, but stable in roll when upright - this means that the spiders automatically flip right-side-up if they happened to be falling upside down, and then they stay that way for the rest of the trip. All the spiders have to do is hit the critical position and hold it. We are working on the algebraic solutions that demonstrate why the righting position works, but our simulation study already predicts their motion quite well (i.e. it matches our imaging studies of the real spiders).
The aerial righting response we have captured is somewhat similar to that seen in other arthropods, such as ants and stick insects. Since we have found it in a mostly terrestrial animal, it is looking ever more likely that this is a method of aerial righting that is extremely widespread in terrestrial arthropods, and that's very neat stuff for understanding arthropod evolution (which is handy because arthropods represent most of the animal species on Earth).
We are not the only group working on falling spiders. As I mentioned recently in an earlier post, Robert Dudley of Berkeley has a student working on arboreal spiders from the neotropics that, not surprisingly, are even better at aero control (I won't get into details because they have a paper pending, but it's awesome).
Vadas and I are setting up to work on some additional species of spiders, in order to take a more comparative approach. We will include some arboreal spiders that we expect will be able to actually glide. Not kidding; gliding spiders. There are all sorts of other aerial behaviors we can find in spiders, as well, that take into account the use of silk for producing drag lines and parachuting. There is an entire realm of aerial acrobatics in spiders that has been rather understudied (ballooning being the only one of the bunch to get much play in the literature). So stay tuned for spinning, dropping, gliding, righting, leaping, parachuting spiders.
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
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