Wednesday, May 30, 2012

Off to the field

I'm headed to New Mexico for a bit of field work. Won't be posting again until June 5th as a result, but here's a post at H2VP about the field work (Justin is our expedition leader):

I also have a guest post at Archosaur Musings:

See everyone when I return!

Sunday, May 27, 2012

Beijing Pterosaur Meeting

This was one of four abstracts I was on for the Beijing Pterosaur Meeting a couple of years back.  The next pterosaur meeting is schedule for Rio - more on that in the not-to-distant future.

Soaring efficiency and long distance travel in giant pterosaurs
Authors: Michael Habib and Mark Witton

Azhdarchid pterosaurs include the largest known flying animals, with the largest species reaching a potential mass of over 250 kg.  Prior work suggests that several features of azhdarchid anatomy could be associated with a soaring-dominated lifestyle, including large size, burst-flapping adapted pectoral girdle and proximal forelimb, moderate to high wing aspect ratio, and exceptional pneumaticity.  However, long-range flight ability of azhdarchid pterosaurs has not been quantified in the literature. Furthermore, while the flight of giant pterosaurs has been modeled for a range of large species (Hankin and Watson 1914; Bramwell and Whitfield 1974; Brower 1983; Chatterjee and Templin 2004) and researchers have invariably concluded that they were capable of flight, some recent studies have called into the question the flight abilities of pterosaurs at large body masses (Chatterjee and Templin, 2004; Sato et al. 2009), especially the relatively ‘heavy’ masses in the recent literature (Paul 1991, 2002; Witton 2008). Here we present the results from a quantitative analysis of long-distance travel efficiency in azhdarchid pterosaurs, demonstrating that the largest pterosaurs should not only have been effective flyers, but had the potential to be the furthest-traveling animals known to science.

Power analysis indicates that the largest pterosaurs needed to reach external sources of lift, following launch, before they exhausted anaerobic muscle endurance.  Following climb out, even large azhdarchids should have been capable of staying aloft by using external sources of lift.  A quantitative framework already exists for estimating maximum migration range in soaring birds using thermal lift.  We have extended this framework to pterosaurs by altering existing models to accommodate the membrane wings of pterosaurs and uncertainty in potential muscle physiologies.  Maximum fuel capacity (stored as fat and additional muscle) was estimated by taking the difference between body masses scaled from skeletal strength (maximum) versus mass for maximum wing efficiency (maximizing lift coefficient according to reconstructed aspect ratio).  This new migration model indicates that the largest azhdarchid pterosaurs had the capacity for non-stop flights exceeding 10,000 miles.

The ability of large pterosaurs, especially azhdarchids, to effectively reach external sources of lift was great augmented by 1) adaptations for a powerful launch (Habib, 2008) that would allow them to exceed stall speed without utilizing excessive amounts of valuable anaerobic capacity, and 2) adaptations for rapid generation of full circulation on the wing, which would have substantially reduced the time and energy expenditure of climb out. Approximately 2.5 chord lengths are usually required before a wing develops full steady state circulation, known in the literature as the “Wagner Effect” (Wagner, 1925). Analysis of the tensile support in azdarchid wings suggests a potential for rapid translation and twisting of the outboard wing, which would be promoted by the T-shaped cross section of the wing phalanges.  Such rapid translation can develop full circulation up to five times faster than otherwise possible and greatly reduce the flapping cycles needed to reach maximum circulation during climb out, an observation previously made by at least one other pterosaur worker (Cunningham, pers comm.) but previously unmentioned in the formal pterosaur literature. These improvements to the efficiency of the initial climb out from launch would have extended the required proximity to external lift sources, and broadened the potential habitat range of giant pterosaurs. 

Literature Cited

Bramwell CD, Whitfield GR (1974). Biomechanics of Pteranodon. Philosophical Transactions of the Royal Society of London 267: 503-581.

Brower, JC (1983). The aerodynamics of Pteranodon and Nyctosaurus, two large Pterosaurs from the Upper Cretaceous of Kansas.  Journal of Vertebrate Paleontology. 3: 84-124

Chatterjee S. and Templin RJ (2004).  Posture, Locomotion and Palaeoecology of Pterosaurs. Geological Society of America Special Publication, 376, 1-64.

Habib, M.B. 2008. Comparative evidence for quadrupedal launch in pterosaurs. Zitteliana, B28, 161-168.

Hankin, EH and Watson DMS (1914). On the flight of pterodactyls. Aeronautical Journal, 18, 324-335.

Paul. G. S. 1991. The many myths, some old, some new, of dinosaurology. Modern Geology, 16, 69-99.

Paul GS (2002) Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. John Hopkins University Press, Baltimore. 472 p.

Wednesday, May 23, 2012

V is for vulture

On a bus headed towards Washington, DC and watching some turkey vultures soar in the distance. They are easy to pick out because the fly with their wings in a vertical "v" shape - this is called a dihedral. A dihedral tends to stabilize a flyer in roll (though this depends on the sweep of the wings). In strong gusts, vultures will roll back and forth passively, which we call canting. Just a fun aero fact for the day.

Monday, May 21, 2012


Another in the series of abstracts.  This was my abstract for the think-tank conference at the Konrad Lorenz Institute in Vienna, Austria in September 2010.  These are invite-only sessions on various hot topics related to evolutionary biology.  Ours was on the "Constraints and Evolution of Form" - basically an Evo Devo related gig.  I was the resident biomechanist for this one.

Emergence of convergent forms under fluid load in plants and animals
Very few biomechanists examine both plants and animals in parallel, apparently under a tacit assumption that the rules of shape determination must differ substantially between such distantly related groups.  However, convergent structures suggest that the rules of shape governing these groups are largely the same.  Such similarities suggest that environmental constraints are important in determining shape, and/or that genomes are more plastic and prone to morphological convergence than often accepted.  I suggest that reference to physical first principles should be made whenever shape is examined in multi-cellular organisms, regardless of their phylogenetic position.  As a case example, I report on the presence of highly convergent structures related to resistance and passive yield under aerodynamic fluid load in plants and animals.  I utilize examples from both living and fossil forms, including broad-leafed trees, neornithine birds, and azhdarchid pterosaurs.

Sunday, May 20, 2012

Walker Symposium

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.

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...

Frogmouth Pterosaurs

I have (finally) some new material to post.  In addition to the new stuff, I've decided to post some of my past abstracts that might not have been easily accessible to everyone.  Here is my abstract from the GSA Northeast Conference in 2011.  I gave a platform talk on the biomechanics of anurognathids (some of you will already know that Mark Witton and I have a manuscript nearing completion on the topic, as well).

Functional Morphology of Anurognathid Pterosaurs
Anurognathid fossils include several exceptionally well-preserved specimens, some of which include extensive soft tissue preservation.  This exceptional amount of morphological information makes anurognathids prime candidates for functional biomechanical analysis.  Furthermore, anurognathids displayed a suite of unusual characteristics that make them of particular interest for functional study.  These traits included extensive pycnofiber coverings, fringed wing margins, shortened distal wings, shortened faces, and enlarged orbits. Prior authors have suggested that anurognathids were adapted to catching small insects on the wing.  I present a quantitative analysis that supports this general behavioral inference, and provides details regarding probable anurognathid locomotion. Results indicate that anurognathids were exceptionally maneuverable animals.

Bone strength analysis in Anurognathus ammoni reveals that each proximal wing was capable of supporting nearly 22 body weights of force.  The wing spar of A. ammoni was substantially stronger in bending than that of an average bird of the same size (residual of 0.72).  The calculated relative bone strength overlaps significantly with that of living birds that capture prey on the wing (p>0.92) but differs significantly from all other avian morphogroups (p<0.04).  Overall humeral robustness is similar between A. ammoni and megadermatid bats.

Anurognathid launch appears to have been particularly rapid and steep. Once airborne, anurognathid pterosaurs could likely generate high lift coefficients.  Leading edge structure in Jeholopterus suggests that anurognathids were capable of generating a leading edge vortex (LEV) as observed in some living bats and swifts.  Analysis of flapping efficiency suggests that the expansion of the proximal wing, coupled with reduction of the distal wing elements, would have increased flapping power at the cost of increased drag.  The proportions of the wing and details of the shoulder may be indicative of the ability to hover for brief intervals; power analysis also supports this conclusion. These results are consistent with reconstructions of anurognathids as highly maneuverable flyers, preferentially foraging in cluttered habitats on small aerial prey.

Wednesday, May 16, 2012

Gettin' Gigantic

I wrote up a short post about the potential selective advantage of giant size in pterosaurs over at the blog.  You can check it out here.  I'm finishing the Chatham Maymester right now, so that's all for the moment.  Perhaps a bit more later today.

Monday, May 14, 2012

Thrust, lift, drag

I have had a smattering of questions lately about the roles of lift and drag in animal flight. An extensive review would require a book, but here are some basics:

1) Lift is the component of fluid force that is directed perpendicular to flow. This need not always mean that lift is directed upwards. For example, thrust in animal flyers is actually a component of the generated lift. By angling the flight stroke such that the power stroke sweeps down and forward, flying animals point some of their lift forward as thrust. The distal part of the wing produces more thrust and proportionately less weight support. As you move further inboard (proximal) on the wing, weight support becomes more important and thrust contribution diminishes.

2) Drag is the fluid force component parallel to the fluid flow. Most flying animals fly at a lift:drag ratio above one. This will typically mean that they use lift as the primary source of weight support and propulsion, and that minimizing drag improves propulsive efficiency. However, the situation can be complicated. Drag can contribute to weight support, and very tiny insects fly at L:D ratios less than one - as such, they paddle through the air rather than use true, lift-based flight.

In the aquatic realm, both drag-based and lift-based propulsion is common. The former is particularly used in fast starts. For flyers, the same principle applies during their equivalent of a "fast start": namely, takeoff. I'll be writing about that basic derivation later.

Friday, May 11, 2012


A colleague of mine asked the other day what the upturned wing tips on many modern commercial aircraft are doing, and if they have any similarity to the split wingtips used by many birds.  A full explanation would take pages, but here's a quick and dirty one:

There is a price on producing lift called 'induced drag'.  Induced drag can be modeled a few different ways, but basically it involves some efficiency-reducing downwash at the wingtips involved in the formation of the wingtip vortices.  A wing without a tip does not suffer from induced drag, as a result, but in most real situations wings obviously must have tips (in a wind tunnel you can build a wing that goes from wall to wall to eliminate induced drag effects).

An upturn of the wingtip can, in certain speed regimes, reduce induced drag.  Splitting the wingtip into smaller, high aspect ratio wings can also reduce induced drag.  Fixed wing aircraft can be designed with the first of these two tricks.  Many birds use both: they spread the primary feathers in both the transverse and vertical planes - this means they get the multiple-tip bonus and the vertical displacement effect.

Induced drag is particularly problematic at low speeds, so these tricks are best for slow soaring, landing, and takeoff.  Of course, in a fixed-wing aircraft, the winglets are set in place and cannot be differentially deployed.  In animals, however, slotted wingtips are only deployed when it is most useful - that is, at low speeds.

Wednesday, May 9, 2012

The Bumblebee Myth

I had an interesting quick discussion on Twitter earlier today where the issue of the old "bumblebees can't fly myth" came up.  This is an old story that has turned into something of an urban legend.  There are all sorts of versions running around the internet (brief overview here), but they typically suggest that an aerodynamicist of some note showed that it was mathematically impossible for a bumblebee to fly.  Obviously, bumblebees do fly, and the story is often used in a derogatory sense to put down scientists and engineers or being intelligent yet (in the views of the story-tellers) unable to grasp something obvious to everyone else.  It is, as it were, another version of the Straw Vulcan argument to deride logical approaches to problems.

I could go on for a while about how ridiculous this sort of thing is, but I imagine it is self-evident to anyone that reads this blog, so I'll hold my typing on that front.  Needless to say, the story is based upon a flawed understanding of the situation.

What was shown at some point (though the details are foggy when this was done the first time) is that a standard steady-state, oscillating foil model using a rigid wing does not accurately describe bumblebee flight.  If bees had to fly like airplanes, they would indeed by grounded.  But, of course, bees don't fly like airplanes.  For one thing, insects have flexible wings.  The shape changes in the wings are critically to their lift production.  Second, insect wings carry a significant virtual mass of air on them as they move, owing to the "stickiness" of the air at that size scale.  This means the effective profile of the wing is not the same as the anatomical profile.  Thirdly, bees and many other insects with high wing beat frequencies seem to rely heavily on unsteady effects.  This means that the air never reaches equilibrium on their wings.  The flow is kept in a constant state of imbalance; this works best at very high wing beat frequencies and small size scales and can produce very high lift coefficients (well above steady state maxima).  Small birds and bats can use some of these tricks, too, it would seem, because the measured maximum lift coefficients are often quite large in small animal flyers (about 5 for flycatchers, for example, compared to the 1.6 theoretical maximum for a thick feathered wing with slight camber).

So, in short: yes, bumblebees can fly.  We understand how they fly and have for a long time, though there is plenty left to learn about insect flight overall. The story that someone proved bumblebees can't fly on paper is a myth (or, at best, a major misinterpretation).  Finally, suggesting that logic is a bad way of solving problems is silly.

Friday, May 4, 2012

The Strength of Color

Had a great extended dinner meeting with a friend and collaborator this evening to get a project rolling on the micromechanics of feathers.  One of the key features will be sorting out how pigments and structural colors affect the mechanical properties of feathers.  It's well documented that some pigments (melanins, particularly) strengthen feathers - but we don't know yet how much, by exactly what mechanism, and the relative effects on performance.  Other pigments probably also have an impact, but that's even more mysterious. 

Why do we care how feathers work?  Well, as a biologist with a strong interest in the evolution of flight in birds, I obviously have a personal stake in knowing more about feather mechanics.  But here are some traits of feathers that might make them interesting models for those with a more applied interest:

- Feathers have a high strength to mass ratio (particularly with regards to bending)
- Feathers are abrasion resistant
- Feathers are good thermal insulators
- Feathers are fast to replace - they are manufactured quickly with precision
- Feathers absorb impacts well
- Feathers are water resistant
- Feathers have notable aerodynamic properties (duh)

That's a rather solid set of attributes for a single biological structure.  With growing interest in biomaterials, we expect that feathers might hold some very intriguing clues about efficient material use and pigment effects.  Here's hoping!

Wednesday, May 2, 2012

Soaring is Good

Qualitatively, soaring flight is typically associated with large size in living flyers.  Only relatively large bats have been recorded soaring often, and soaring flight over long distances is well documented for many large birds (vultures, gannets, albatrosses, eagles, etc).  Soaring flight is less well documented for small birds, and it has typically been presumed that this is because gliding and soaring is less energetically useful for small birds than big ones.  But, as it turns out, migrating with long gliding phases is an efficient way to go even for relatively small birds - or at least, for one species of small bird.

Sapir et al., in a neat paper in PLoS ONE showed that bee-eaters run much lower heart rates when gliding and utilizing soaring flight than in continuous flapping flight (see figure from their paper at left).  If heart rate measures metabolic expenditure they way they suggest, then this means bee-eaters still get quite a good deal using unpowered flight mechanisms over long trips.

Now, this does not mean that soaring flight is not still more important to the biology of large flyers.  One reason that soaring might still be more critical to the evolution and ecology of giant flyers compared to average-sized ones is that long bouts of continuous flapping flight simply aren't available to large flying animals.  As size increases, flying animals start to face problems with mass-specific power scaling.  This is solved by laying down large fractions of anaerobic (i.e. "fast twitch") muscle.  Those high-powered muscle fibers have low endurance, however, so large flyers necessarily can only flap for short bursts - then they have to switch to unpowered phases of flight, and therefore use external sources of lift.  It is not surprising then, that large flyers also, more often than not, have more adaptations related to soaring than small sized flyers, and this means that large flying animals are probably better at soaring that small ones most of the time. 

Still, papers like Sapir et al. (2010) are important in dispelling our myths about the effects of size in animal flyers.  It is a greatly misunderstood area of biology, and one fraught with centuries of engrained concepts based on human intuition instead of careful measurement and analysis.  It's good that modern researchers are taking a second look at the biology of size in animal flyers.

Sapir N, Wikelski M, McCue MD, Pinshow B, Nathan R (2010) Flight Modes in Migrating European Bee-Eaters: Heart Rate May Indicate Low Metabolic Rate during Soaring and Gliding. PLoS ONE 5(11): e13956. doi:10.1371/journal.pone.0013956

Tuesday, May 1, 2012

Soap Box Moment

One of the quirks of being an animal flight specialist is that there are not many of us.  If you consider those that focus on fossil taxa, there are even fewer.  If I focused my publications to communicate only to other members of my field, I'd be talking to about 12 people, tops.  I might as well just send postcards.

But, as it turns out, there are thousands (if not millions) of people out there that find information on animal flight fascinating or even practical (see: robotics and aeronautics engineers).  So, I have a large, but dispersed and eclectic audience out there to reach.  How on Earth can I get to them all?

Easy: Open Access Publishing. 

Many of you already know that I'm rather firmly in the OA camp.  I admit that I have not done my part to promote OA in the same way as individuals like Andy Farke, Matt Wedel, and Mike Taylor (hats off to you guys!) but I at least favor OA journals like PLoS ONE for my publishing needs and give a nudge here or there when I can.

On that note, there is a great article out by Michael Eisen here that gets right to the core of the wimpish way that universities have dealt with a growing problem.  I would point out, as well, that while the academic institutions are the largest offenders in this case, other businesses have also fueled the fire (see: biomed engineering companies, etc).

Read and enjoy.