Monday, April 30, 2012

Viscosity

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.

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.


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.

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.

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.

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.

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.


Monday, April 16, 2012

Wait, penguins can fly!?

In short, no, penguins cannot fly in the traditional sense.  But they can aquafly, which is today's topic.

There are relatively small numbers of flying animals that can also swim with the wings - most of these are birds, though some insects can perform this trick, as well.  When the wings are used for lift-based motion through the water this is termed "aquaflight".  Most aquaflying birds do also fly in the aerial sense, but penguins have abandoned aerial locomotion and use the wings exclusively for aquatic locomotion.  This evolutionary transition is among the most interesting functional morphology problems in ornithology.  Understanding how penguin morphology has changed over time in response to their aquaflying habits not only sheds light on aquatic locomotion, but it also gives us information regarding the constraints of aerial locomotion (which is primarily why I figured it was worth a discussion on Aero Evo).

The image at left is a reconstruction (by Chris Gaskin) of Kairuku, as described by Daniel Ksepka and colleagues.  The full paper is available here for free.  Kairuku hails from the Oligocene.  Penguins have quite a good fossil record, and we know from the record that early penguins were often quite large (Kairuku stood nearly five feet tall).  All of the known fossil penguins are also from clearly flightless animals - even early penguins like  Kairuku, or the even earlier Waimanu, possessed wings clearly dedicated to swimming at the expense of aerial locomotion.  However, we can deduce from phylogeny that penguin ancestors were flying animals, probably  amphibious flyers (those animals that use the wings to both fly and swim).  As mentioned above, there are some modern examples of amphibious flyers, and we can use them in functional comparisons with penguins (both living and fossil) to gain a better understanding of what mechanical changes have occurred in penguins.

It might be expected that using the wings underwater would dominate the physical forces acting on the wings of amphibious flyers.  If this were true, then the wing geometry and bending resistance of the wing bones in amphibious flyers (like puffins) should be quite similar to that of penguins.  However, this turns out not to be the case.  In 2010 I published a paper showing that the wings of penguins are tremendously "overbuilt", with regards to their bending strength, when compared to other birds - including other living birds that swim with their wings.  One of the figures from that paper is shown at left.  You will note that the residuals in humeral strength are much greater for penguins (Sphenisciformes) than for any other birds in the sample.  Puffins and auks, which all aquafly as well, are in the Alcidae.  The Procellariiformes include a sample of aquaflyers, as well.  As such, penguins are unique, even among aquaflyers.

In fact, this trend is not even limited to comparisons with aquaflying birds that still engage in aerial locomotion (i.e. those that "fly" proper).  Last November I published a paper with Gareth Dyke and Xia Wang (available from PLoS ONE here) that described new material of plotopterids and compared the functional characteristics of plotopterid structure to penguin morphology.  Plotopterids were flightless, aquaflying birds of the middle Cenozoic that were similar to penguins in many anatomical respects and which also reached large sizes (particularly in the Miocene).  Some of the material we described in that manuscript is pictured at left.

As it turns out, the wings of plotopterids appear to be more similar (in terms of geometric variables known to correlate with mechanical properties) to living auks than to living penguins.  This implies that the swimming stroke of plotopterids was more like that of auks than penguins, even though plotopterids were flightless.

There are more groups that need comparing in this way to generate a firm conclusion (Mancalline auks and Great auks, for example), but the emerging pattern is that the mechanics and evolutionary history of penguins is quite unique.  Their morphology, it seems, is not just about having lost flight - it is presumably the specific trajectory by which they left the aerial realm, as well their specific  phylogenetic history, that has placed penguins in a unique morphological space.  In the 2010 Biological Journal paper I suggested that at least one major factor of importance may be the mirrored aquaflying stroke of penguins - unlike other aquaflying birds, living penguins produce nearly the same magnitude of lift with the wings on the upstroke and downstroke (albeit with opposite signs, of course).  This mode of swimming greatly increases efficiency by reducing surge accelerations.  Where this feature entered penguin evolutionary history has not yet been examined, but may be a fruitful topic of future research.

P.S. If you love penguins then you need to check out the definitive fossil penguin blog by Daniel Ksepka: http://fossilpenguins.wordpress.com/


References
Ksepka, DT, Fordyce RE, Ando T, and Jones CM. 2012. New fossil penguins (Aves, Sphenisciformes) from the Oligocene of New Zealand reveal the skeletal plan of stem penguins. Journal of Vertebrate Paleontology 32: 235-254

Habib M. 2010. The structural mechanics and evolution of aquaflying birds. Biological Journal of the Linnean Society. 99(4): 687-698

Dyke GJ, Wang X, Habib M. 2011. Fossil plotopterid seabirds from the Eo-Oligocene of the Olympic Peninsula (Washington State: USA): descriptions and functional morphology. PLoS ONE 6(10): e25672. doi:10.1371/journal.pone.0025672

Friday, April 13, 2012

Bennettazhia humerus

video
Quick post this evening because I have been out at the museum and a monthly social gathering most of the day/evening.  This a quick video showing a CT scan series through the proximal half of the humerus of the mid-sized pterosaur Bennettazhia oregonensis.  This animal was one of the critical specimens in the original quadrupedal launch work, and is also interesting in that it was found in Oregon (pterosaurs from the Northwest U.S. are quite rare).  If you look closely, you can make out some of the internal bracing.

Thursday, April 12, 2012

Munich Archaeopteryx

Perhaps the most iconic fossil of a (potentially) flying animal is Archaeopteryx.  Debates on the flight ability of Archaeopteryx abound in the literature.  One of the more recent investigations of this issue considered feather strength, which is an interesting approach, but may suffer from significant error is feather measurements or body mass estimates are imprecise.  I take my own approach to the problem, which is to look at the bending strength of the bones, rather than the feathers.  I've done this for a wide range of living birds, but only a couple of specimens of Archaeopteryx, which is why I have yet to formally publish the results (though I have given a conference presentation on them at SVP).  One of the specimens I have data for is the Munich Archaeopteryx, shown in the photo at left.  It's not the best photo, but I managed to grab it quickly while measuring the specimen at the BSPG in Munich, Germany.  Regardless of the ultimate flight status of Archaeopteryx, working with a specimen of this historical importance was a real treat.  My special thanks goes out to David Hone, who organized the meeting where I examined the specimen.

I will be writing more on Archaeopteryx and other species relevant to the origin of birds in the coming months.  I have quite a few posts lined up on the topic; some more quantitative than others.  For now, however, it is back to my teaching duties, so today's installment is necessarily brief.


Wednesday, April 11, 2012

Insect Takeoff

At left, a small parasitic wasp (Brachyserphus sp.) launches herself into the air. This species attacks nitidulid beetles, and the photograph is again a wonderful shot by entomologist and photographer extraordinaire, Alexander Wild (as before, go to http://www.alexanderwild.com for more awesome shots).

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

Insect Flight and Human Health

Many of the practical applications of understanding animal flight relate to areas like robotics and aeronautics.  However, there are implications for human health, as well.  Since flying animals experience intense skeletal loads and have rather strict muscle output requirements, many of them (particularly vertebrate flyers) make good models for understanding the limits of biological tissues, which in turn can play a role in future breakthroughs for biomaterials, tissue engineering, and the like.

However, there is a more direct link with human health, as well: many of the world's most serious diseases are carried by flying insects. Parasite life cycles can be quite complex, and many of them include more than one host.  It is common for one of these hosts to be a smaller, more mobile organism, and that is often an insect.  The more mobile host is typically called the vector, though in truth the definition of a disease vector is more specific than that.

The conditions under which vectors are the most mobile will therefore often be the conditions under which disease spreads most quickly. As it turns out, insect flight is rather sensitive to atmospheric conditions, on account of insect muscle and respiratory physiology, as well as their typically size range.  Many insects, such as mosquitos, sit in a size range where changes in atmospheric density or temperature can change flight performance quite abruptly.  At left is a wonderful photograph of an Aedes triseriatus eastern treehole mosquito taking a blood meal, by Alexander Wild. To see more of Alex's spectacular photography (and trust me, he has stuff you've never seen before, guaranteed) go to http://www.alexanderwild.com/

The limits on vector performance may getting more relevant in some locations. For example, in Hawaii, avian malaria has decimated many of the endemic bird populations.  Many (if not most) of the lowland species present before European habitation of the islands are now extinct, or nearly so. However, the highlands have been protected from this effect in the past because they were cool enough to keep the malarial loads inside the mosquitoes low, and also because the cooler air and lower oxygen density inhibited the malaria-carrying mosquitos (different mosquito species take over at high altitudes).  Unfortunately, Freed et al. discovered a about six years back that more mild high altitude temperatures were allowing malaria to spread up the mountains.  They attribute this mostly to malarial loads within mosquitos, but it is likely that flight performance of the mosquitoes themselves is also improving at higher altitudes.  The Freed et al. study can be found here.

Avian malaria has been used as a model for understanding human malaria for decades (in fact, two of the Nobel Prizes for malaria research were given for discoveries in avian malaria).  It is reasonable to suspect that human health will also be influenced by the impacts of climatic changes on vector mobility.  In the coming weeks, I will post more examples of how insect flight performance is critical to human health and economies - both for better and for worse.

References

Freed LA, Cann RL, Goff ML, Kuntz WA, Bodner GR. 2005. Increase in avian malaria at upper evelvation in Hawai'i.  The Condor 107: 753–764


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.

Sunday, April 8, 2012

Extra Wings?

I was looking over the 1994 review article by John Davenport (in Reviews in Fish Biology and Fisheries) on flying fish aerodynamics, and he makes a very interesting and astute observation:

"The expanded, flat pelvic fins of four-wingers have evolved, not to increase wing area, but to function as tailplanes or stabilizers well behind the centre of gravity, with an area some 20–35% of the total lateral fin area, and an angle of incidence less than that of the cambered pectoral fins."

In other words, the big forward pectoral fins, which are the main wings, are cambered and produce the vast majority of the weight support during gliding.  The "hindwings", i.e. the pelvic fins, are control-related.  Readers should note that the pelvic fins in flying fish are also much shorter and broader in overall shape (i.e. low aspect ratio) - this makes sense for airfoils used to control but not as primary gliding support surfaces.

Those that work with me regularly know where I am going with this, as there is another group of critters with "hindwings".  More on that some other time.

Saturday, April 7, 2012

Royal Tyrrell Lecture

My talk at the Royal Tyrrell Museum on pterosaur mechanics has been posted online here.


Look Ma, No Limbs!

While I personally spend more time working on powered flight than dedicated gliding, I do have to say that some of my favorite animals are the gliding snakes in the genus Chrysopelea.  Yup, that's right - gliding tens (or even hundreds) of meters without any limbs at all.

The world expert on gliding in snakes is Jake Socha; he worked under Mike LaBarbera at University Chicago years ago, and now has his own biomechanics group at Virginia Tech.  

Jake's snake work was featured on National Geographic, and the program is actually available in full version on YouTube now, albeit with some ad breaks: Snakes That Fly

As a quick synopsis, essentially what happens is that Chrysopelea individuals can glide by flattening themselves (spreading their ribs) and moving in a repeated S-curve that spins counter-vortices over both sides of their bodies, creating a low pressure zone over their backs that allows the snakes to glide.  My understanding is that the snakes essentially drop to launch, building speed and then pulling out in a large J-hook, though they may add some spring to the initial launch by unloading from a coiled position.  It should be noted that this is the exception that makes the rule: every other flyer, both gliders and powered flyers, that has been examined in the lab uses some kind of leap or run to initiate flight.  Snakes, of course, are a weird case because they lack limbs with which to do these things.


Friday, April 6, 2012

Pterosaur Water Launch: Preliminary Results

Back again for more water launching goodness.  These results were presented at SVP 2011; with luck they will be finalized and appear in a formal journal (PLoS ONE) this summer.


Here's what I get for Anhanguera, using the technique from yesterday's post: 

The initial escape phase could be accomplished with a net remaining acceleration of 17.6 m/s2, and an acceleration of up to 39.2 m/s2 on the subsequent, unhindered propulsive bound. Sufficient contact area to provide a useable propulsion phase would require that the wing finger be opened 15-25 degrees.  This contact area was greatly augmented by the exceptionally broad MCIV-PHIV joint.  The escape phase would require exceptional shoulder adduction musculature, and I note that Anhanguera appears to have had such expanded musculature: the orientation and enlargement of m. subscapularis appears to be of particular importance, along with the reinforcement of the joint between the scapula and notarium (Bennett, 2009). My model predicts that Anhanguera would have used a series of repeated propulsions when launching from water (unlike terrestrial launch), which would have occurred as a series of “hops” across the water surface.   

This repeated propulsion is required because the animal loses energy to the initial escape from surface tension, and because no lock and release is available on the wing finger joint during water launch, which somewhat reduces power output.  The quadrupedal water launch still greatly outperforms any kind of bipedal launch, however.  In fact, bipedal launch from the water was almost certainly impossible for pterosaurs of practically any size (bipedal terrestrial launch was likely impossible, as well for most species - not to mention inferior in performance in just about every way).  This is particularly true given the recent work on floating position done by Dave Hone and Don Henderson.




One important note is that this water launch model makes predictions about morphological features one should expect to find in pterosaurs adapted to water launching.  In this way, it makes testable predictions from the theoretical model.  This is important, as we will presumably never get water launch trackways.  

Based on this water launch model, we can expect that the following features should be better represented in marine taxa than terrestrial taxa:

expanded scapula 
reinforced scapular-notarial joint
expanded deltopectoral crest 
Warped deltopectoral crest or expanded tip of dp crest
extra-broad MCIV-PHIV wing finger joint
limb length disparity 
expanded posterior brachial musculature

So far, this pattern appears to hold.  While azhdarchids do have expanded dp crests, and somewhat expanded triceps they lack most of the other features in the list (at least comparatively speaking).  So far, only marine pterosaurs exemplify all of the above simultaneously.


Thursday, April 5, 2012

Water Launch: Some Nuts and Bolts

I've been asked about the nuts and bolts of my pterosaur launch models at quite a few meetings.  I have a few papers that should be out later this year which include more of the computational details than prior publications (which focused on the comparative differences in maximum load potential, rather than specific performance).  In the meantime, though, here are a few of the bit and pieces that go into the water launch model (similar for the terrestrial launch model):

Flapping frequency
Based on the expectations from Pennycuick (2008) flapping frequency varies roughly as body mass to the 3/8 power, gravitational acceleration to the ½ power, span to the -23/24 power, wing area to the -1/3 power, and fluid density to the -3/8 power:

f = m3/8g1/2b-23/24S-1/3p -3/8

The result is the expected flapping frequency in hertz; taking the inverse gives the expected flapping time in seconds.  This provides a framework for estimating wing motion speed.  The wings would have begun launch folded, in the water.  As a result, the model begins using the density of saltwater and the folded span and area.  I then transition the model through a time-step series in which most of the animal is raised above the water (air drag), but the wing finger pivot and feet still experience subaqueous drag. 

Minimum Launch Speed
There are a couple of possibilities for the speed at the end of the launch cycle.  The first model forces the launch to provide horizontal speed equivalent to the stall speed (Vmin=2*WL*(1/(1.23*CLMax))0.5) thereby propelling the modeled pterosaur to steady state from the launch alone.  The second approach allowed a flapping burst once airborne, and allows the pterosaur in question to accelerate to steady state within its anaerobic window. These estimates can be checked against expectations of Strouhal Number limitations (see Reconstructing the Past post from yesterday).  Burst flapping after launch should produce a relatively high Strouhal number (for the size regime of the animal in question), but is still constrained.  As a rough rule of thumb, a Str up to 200% of the optimal cruising value is pretty realistic (higher freq, higher amplitude, lower speed). Launch acceleration is calculated as the simple average over the course of the launch cycle.  The launch time can then be varied from the starting values to obtain a range of possible launch accelerations, and therefore a range of potential power requirements. 

Power estimates are used with ballistic motion model to determine the initial acceleration, velocity, and height during launch.  My model species for water launch has been Anhanguera santanae. The contact areas were estimated by mapping muscles onto a laser surface scan of AMNH 22555, which is a particularly nice uncrushed specimen.

Anhanguera input parameters
Flight muscle fraction: 20-26%
Hindlimb muscle fraction: 10-12%
Wingspan: 4.01m
Anaerobic Power: 300-400 W/kg

Taking the above input parameters, taking a conservative estimate of muscle power from living archosaurs, using the motion speed from part 1, and then adding in the reconstructed contact value and an estimate of flat plate drag coefficient for propulsive force efficiency (not actually that difficult as flat plate coefficients are measured for a wide variety of shapes) yields an estimate of potential acceleration.  

In my next post, I will briefly discuss what sort of results this yields for Anhanguera.


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.

Wednesday, April 4, 2012

Giant, Feathered Tyrannosaur

Alright, it doesn't fly, but this is relevant to the evolution of feathers and, more importantly, is sheer awesome in fossil form.  The embargo on this literally ended 14 minutes ago.  It is hot off the press.

A giant, feather dinosaur from the Lower Cretaceous of China

Also a blog description over at Archosaur Musings here.


Reconstructing the past

One of the things I hope to do with Aero Evo is to give some insights into how biomechanists like myself manage to make estimates of performance for fossil animals.  In particular, of course, I will focus on how we know things about extinct flying animals.

Three of the parameters of typical interest are the speed at which a flying animal flew, the rate at which it flapped its wings, and how much room it needed to flap them: velocity, flapping frequency, and flapping amplitude, respectively.

These three factors are all correlated, and can be summarized by the Strouhal Number.  The Strouhal Number is equal to fA/U, where f is frequency, A is amplitude, and U is velocity.  As it turns out, the range of strouhal numbers at which a flapping wing or undulating fin can operate efficiently is pretty darn narrow.  For creatures as seemingly different as dolphins, birds, fish, and dragonflies, Str in cruising locomotion only varies from about 0.2-0.4.  This pattern occurs because of the narrow range of motion in which vortex shedding is efficient.

The classic paper on the topic is by Taylor et al., and can be found here.  That link will take you to the abstract; those on campuses should be able to get the full paper, though it is behind a paywall.  Fortunately, Nudds, Taylor, and Thomas wrote a followup paper with much of the same information (specifically on birds), and it is open access here.

Now, it should be noted that the limits on Str mentioned above only apply to cruising - that is, the animal is moving steadily at its efficient, long-distance gait.  Burst performance is different, and on average, the Str will be higher during things like rapid climbs (say, just after takeoff) or bursts after prey.  Nonetheless, we can get a good idea of how extinct animals worked by using Str.  I'll post more on how to use Str, and how to combine it with other equations to solve for multiple variables, soon.


PhyloPic

You don't know about PhyloPic?  Shame on you.  PhyloPic is an open database of life form silhouettes, and includes plenty of flying creatures, both living and fossil.  The fantastic silhouette of Archaeopteryx that current graces the header of the Aero Evo page comes from PhyloPic, and was rendered by the site's creator, T. Michael Keesey.  The equally impressive Tupandactylus imperator is by Evan Boucher, and the fantastic little Drosophila (obviously not to scale) is by Ramiro Morales-Hojas.  Check out their work on PhyloPic. All three of those vector images are freely available.  There are hundreds more on the site.

Go check it out using the link above or clicking here.


Distance Champs

For those that haven't seen it, there is a press release for the description of Guidraco here.  This is yet another example of a large pterosaur whose closest known relative was half a world away (other examples include the anhanguerids/ornithocheirids of Brazil and Europe, giant azhdarchids of Europe and North America, etc).  This is rather exciting for me, personally, as I have been suggesting for the last couple of years that large pterosaurs might have had cross-continental travel abilities.  I even got to babble about it on NPR: you can check it out here.


Do the Twist

Arguably the most adept fliers in the world are those animals who carry flight as their namesake: the dipteran insects, i.e. the flies.

Alexander Wild (whose photography you should check out if you are not already familiar) recently posted some new shots of fruit flies approach a fungal feeding patch.  I am particularly fond of this shot.

Note how the plane of the wings are almost perpendicular to the direction of travel at the moment the shot was taken.  The degree of wing rotation used by insects, particularly during landing and takeoff, can be quite extraordinary.  Sadly, the precise effects and roles of wing rotation in animal flight are poorly understood.  Some good work has been done with bees and flies, but even there we are still quite naive.  According to Sharon Swartz of Brown University, next to nothing is known about the role of wing twisting in bats.  The knowledge base situation is only marginally better in birds.

Those looking for a great experimental project on animal flight: think about working on wing twist.  Theoreticians [which I suppose includes myself, though my work is about a 50/50 split] have some good ideas of what should happen, but we need experimentalists to play it out and get the real nuts and bolts.


Pterosaur Acrobatics

I received a fun message from David Hone of Archosaur Musings fame yesterday.  Here's the relevant bit:

"A biology teacher in Ireland tweeted me to ask if I thought a pterosaur could pull off a loop-the-loop or victory roll or similar. My guess was 'probably' especially something like and ornithocherid, but I thought you'd love it as a thought problem..."  -- David Hone

It's a fun idea to ponder.  The precise maneuvers available to fossil animals are can be difficult to work out with much confidence, but there are a few things that can be said with confidence:

- The most maneuverable pterosaurs were probably anurognathids, and they could certainly pull off a loop-the-loop or just about anything else you could want from a flying animal.  In fact, anurognathids were probably among the most agile flying animals of all time, right up there with living vesper bats, swifts, and the like.  I talked a bit about their abilities at Pterosaur.net here.  Mark Witton dealt with them more while debunking the vampire anurognathid concept here.

- For larger pterosaurs, things get a bit trickier, but the inertial problems would still be pretty minor.  The real question becomes whether the animals could handle multiple extra body weights of force on the wings.  Based on work I've done on pterosaur wing strength, as well as work by Colin Palmer, it seems that at least pterosaurs as large as Anhanguera (4-5 meter wingspan) could have handled major rolls or loops.  I have yet to check the numbers for something bigger, like Quetzalcoatlus northropi, but I might have to give it a shot.  I may post a more extensive conversation on it the idea at H2VP soon.


Welcome to Aero Evo

This is a new blogging endeavor of mine, and my first solo run at it.  Those that have worked with me in the past know that I sporadically post to H2VP and the Pterosaur.net Blog.

This blog will differ substantially from what I have done previously.  First and foremost, I will be specifically discussing animal flight - particularly the evolution of flight.  The format is also going to be different from what I have done in the past. I have designed this blog to be a rapid-fire, regularly updated feed.  I expect to post something almost every day (holidays and such excepted, of course). Posts will typically be short - when I have something more lengthy to say, I will link over to H2VP or Pterosaur.net.    This can therefore be seen as a form of micro-blog.

On to the wings!