Had to work all night last night, so a quick post today. Figured I'd do a quick technical note.
One of the terms you'll see me use quite often here is 'viscosity'. The formal definition is that viscosity is the resistance of a fluid to changes in rate of shear. In colloquial terms, this means how thick and sticky the fluid feels (high viscosity means 'thicker' fluid). What you might not know is that the effects of viscosity aren't constant across body sizes: little things experience a stickier world. As it turns out, in biology, it is the relative viscosity that matters. For a tiny insect, air feels thick and heavy - the smallest flying insects basically paddle through the air, rather than fly in the normal sense.
I'll be talking more about this soon, but it's a great item to keep in mind as it explains a lot of biology with only a few simple rules.
Monday, April 30, 2012
Sunday, April 29, 2012
Microraptor: Odds and Ends
The top image on the left is from Hone et al. (2010) and shows the holotype of Microraptor gui under UV light. The image below, by Mick Ellison, shows a life restoration of Microraptor, and was taken from here (note: the hindlimbs could not actually get into the position shown in the image; that was done to show off all of the airfoils at once for comparative purposes). One of the key questions regarding flight in Microraptor is whether it evolved flight independently of avialans, or if it represents a morphology that was a more direct precursor to flight in birds proper.
One thing I noticed a few years back is that it seems that Microraptor had a different set of "solutions" to the problem of aero control, as compared to living birds. I have since put some math to it, and the calculus bears out the intuition. Myself, Justin Hall, David Hone, and Luis Chiappe are writing this up now (see earlier cryptic blog post), but Justin has given a couple of talks on the hindwing use recently and some of you out there know that that I have been murmuring about the tail being used in aero control. All will be revealed in the full manuscript (WFTP moment) but I do think it is quite interesting that the aero control surfaces in Microraptor took advantage of pre-existing maniraptoran anatomy. In other words, you don't have to do much to your average dromaeosaurid to get it into the air.
This is a potentially critical observation. For one, it suggests that the origin of flight in dinosaurs may have been more simple than previously supposed. It also suggests that flight control may have had more to do with the gains and losses of aerodynamically active morphology we see near the origin of birds than simple weight support. I am sad to say that most paleontologists don't seem to have a particularly good grip on what lift actually is, how it is used, and how it is generated. Many of my colleagues also seem to struggle with how drag fits into the whole scheme. Of course, I have lots of gaps in my knowledge, too, so I can't go pointing fingers. Nonetheless, I suspect that we are going to see a major overhaul of the models for dinosaur flight evolution in the year or two.
The Ellison image is associated with a recent paper by Li et al. (2012) in Science. The authors favor display characteristics for some of the feathered morphology, particularly the tail fan. I don't discount this function at all, but it should be noted that it doesn't take much to provide a decent stabilizer or control surface for a mid-sized flying animal, and display surfaces don't have to be aerodynamically useless or costly. (Just to shore a common myth, that is not the same as saying that tail fans, crests, flaps, etc would act as rudders on flying animals. As a general rule, rudder use does not work well for a non-fixed wing flyer. Even fixed-wing aircraft do not initiate turns by using rudders; the rudder systems are for stabilization).
References
Hone DWE, Tischlinger H, Xu X, Zhang F (2010) The Extent of the Preserved Feathers on the Four-Winged Dinosaur Microraptor gui under Ultraviolet Light. PLoS ONE 5(2): e9223. doi:10.1371/journal.pone.0009223
Li Q, Gao KQ, Meng Q, Clarke JA, Shawkey MD, D'Alba L, Pei R, Ellison M, Norell MA, Vinther J. 2012. Reconstruction of Microraptor and the Evolution of Iridescent Plumage. Science 335 (6073): 1215-1219
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.
Back in Action
After over a week away (EB2012 and writing a grant proposal for the Office of Naval Research) Aero Evo is back in action. Multiple posts planned for later today - stay tuned.
One cool note of relevance to animal flight from EB2012: apparently the arborescent ganglia of honeybees become considerably more robust once they begin flying regularly. The onset of flight is associated with a metabolic boost, and that might be related to the expansion of the neural architecture. However, it also possible that the visual stimulus is important (flying animals need to keep track of rapidly changing horizon lines, etc). More on this in a bit.
One cool note of relevance to animal flight from EB2012: apparently the arborescent ganglia of honeybees become considerably more robust once they begin flying regularly. The onset of flight is associated with a metabolic boost, and that might be related to the expansion of the neural architecture. However, it also possible that the visual stimulus is important (flying animals need to keep track of rapidly changing horizon lines, etc). More on this in a bit.
Saturday, April 21, 2012
EB2012
Greetings all! I am en route to Experimental Biology 2012 in San Diego (actually started today but I had to teach yesterday). I will not be able to post much substantial until I get back.
I would like to take this moment to suggest that my fellow paleontologists also make a point of attending conferences with a neontological theme. I know that a handful of other paleontologists will be there at EB2012, but it would be great to see more of you at such gatherings. I also suggest that those who spend a significant amount of time working as theoreticians (myself included) take opportunities to exchange notes with experimentalists as much as possible.
Sure, some of us do a bit of experimental work, but I'm talking about the real down-and-dirty experimentalists. The ones that know their model organisms better than their own house. That's where most of our raw data ultimately arise (the other chunk comes from hardcore field biologists, who deserve similar respect).
Cross those discipline boundaries folks! See some of you in San Diego.
I would like to take this moment to suggest that my fellow paleontologists also make a point of attending conferences with a neontological theme. I know that a handful of other paleontologists will be there at EB2012, but it would be great to see more of you at such gatherings. I also suggest that those who spend a significant amount of time working as theoreticians (myself included) take opportunities to exchange notes with experimentalists as much as possible.
Sure, some of us do a bit of experimental work, but I'm talking about the real down-and-dirty experimentalists. The ones that know their model organisms better than their own house. That's where most of our raw data ultimately arise (the other chunk comes from hardcore field biologists, who deserve similar respect).
Cross those discipline boundaries folks! See some of you in San Diego.
Giant Flyers: to the limit (or not)
I am preparing for my departure to the Experimental Biology Conference in San Diego (starts today), so another brief post today.
One of my pet projects over the last couple of years has been to make a go at estimating the maximum mechanically allowed size for each of the groups of powered vertebrate flyers: birds, bats, and pterosaurs. This is a rather difficult task for many reasons, not the least being that working out the "weak link" in any given scaling problem is time consuming. Pterosaurs provide a special challenge since they provide no living representatives to work on.
I have a preliminary model now, however, and I am rather excited about it. Because it's still rough, I am not going to get into too much detail, but here are some of the punchlines if I turn out to be right with my approach (emphasis on "if"):
- None of the flying clades ever produced an animal at the mechanical flight limit for the group, which means that ecological limits or other constraints have historically set the maximum size.
- Of the three clades, birds came the closest to reaching their mechanical limit (Argentavis)
- Giant pterosaurs, even though they were the largest flying animals, were further from their mechanical limit than the largest flying birds.
- Bats have the lowest absolute limit, but they also have the greatest gap between observed max size and potential max size. As a result, ecological constraints on size might be particularly strong for bats. A 3 meter wingspan bat does not seem impossible with the numbers I have right now. That would more than double the maximum span of the largest bat on record.
All of these limits are estimated using the most giant-friendly morphology in each clade: teratorns for birds, azhdarchids for pterosaurs, and megachiropterans for bats. There could be altogether novel morphologies out there for each group that would push the limit higher, though for various reasons I can get into later, I think the limits I am calculating are somewhat generalizable.
One of my pet projects over the last couple of years has been to make a go at estimating the maximum mechanically allowed size for each of the groups of powered vertebrate flyers: birds, bats, and pterosaurs. This is a rather difficult task for many reasons, not the least being that working out the "weak link" in any given scaling problem is time consuming. Pterosaurs provide a special challenge since they provide no living representatives to work on.
I have a preliminary model now, however, and I am rather excited about it. Because it's still rough, I am not going to get into too much detail, but here are some of the punchlines if I turn out to be right with my approach (emphasis on "if"):
- None of the flying clades ever produced an animal at the mechanical flight limit for the group, which means that ecological limits or other constraints have historically set the maximum size.
- Of the three clades, birds came the closest to reaching their mechanical limit (Argentavis)
- Giant pterosaurs, even though they were the largest flying animals, were further from their mechanical limit than the largest flying birds.
- Bats have the lowest absolute limit, but they also have the greatest gap between observed max size and potential max size. As a result, ecological constraints on size might be particularly strong for bats. A 3 meter wingspan bat does not seem impossible with the numbers I have right now. That would more than double the maximum span of the largest bat on record.
All of these limits are estimated using the most giant-friendly morphology in each clade: teratorns for birds, azhdarchids for pterosaurs, and megachiropterans for bats. There could be altogether novel morphologies out there for each group that would push the limit higher, though for various reasons I can get into later, I think the limits I am calculating are somewhat generalizable.
Thursday, April 19, 2012
Microraptor - Brief Note
I am putting together a paper for PNAS with three coauthors on flight dynamics in Microraptor gui. One thing that is becoming increasingly apparent as I combine my various calculations is that Microraptor was probably pretty darn good in the air, overall. It is not known if it had powered flight ability, but even if Microraptor was an unpowered gliding animal, it has an awful sophisticated set of control surfaces (including some that you don't see in modern birds). This was a highly maneuverable little beast.
I will be posting more about Microraptor and other paravians once our paper is out (assuming that all goes well in review). I may also be giving a talk on the subject at SVP, and Justin Hall (who is also on the paper in question) has recently given some talks on the subject at conferences on the West Coast.
I know that this blog is quite new, and readership is obviously still limited (though growing!), but I would like hear what your most pressing questions are regarding early avian (and near-avian) flight. I am hoping to do a relatively sophisticated series on the topic here once the semester wraps up and my teaching obligations are complete for the summer.
I will be posting more about Microraptor and other paravians once our paper is out (assuming that all goes well in review). I may also be giving a talk on the subject at SVP, and Justin Hall (who is also on the paper in question) has recently given some talks on the subject at conferences on the West Coast.
I know that this blog is quite new, and readership is obviously still limited (though growing!), but I would like hear what your most pressing questions are regarding early avian (and near-avian) flight. I am hoping to do a relatively sophisticated series on the topic here once the semester wraps up and my teaching obligations are complete for the summer.
Tuesday, April 17, 2012
Gliding Spiders
Quick update this evening. Vadas Gintautas and I did our first trial runs today in our new project on spider aero-control. The goal is to determine how spiders stabilize and control their descents following a fall. Work by Robert Dudley, especially, has shown that many animals without wings (especially arthropods) can control their falls (and even glide) using their bodies, heads, and limbs as basic control surfaces. This seems to be particularly well developed in arboreal (tree-living) animals, as would be expected.
Rob has primarily worked out the dynamics of wingless gliding in tropical ants. Vadas and I expect that climbing spiders can use similar dynamics, but this has yet to be shown to in the literature - so we're dropping spiders for science! Our preliminary results suggest that spiders take on stereotyped aerial positions that confer aerial stability. We have done runs with a range of sizes/ages, but so far have only tested one species. More to come on this area of work.
Rob has primarily worked out the dynamics of wingless gliding in tropical ants. Vadas and I expect that climbing spiders can use similar dynamics, but this has yet to be shown to in the literature - so we're dropping spiders for science! Our preliminary results suggest that spiders take on stereotyped aerial positions that confer aerial stability. We have done runs with a range of sizes/ages, but so far have only tested one species. More to come on this area of work.
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