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.

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.

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

Bennettazhia humerus

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.

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.

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.

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