Record-Breaker

Among flying birds, some of the strongest wings (structurally speaking) belong to peregrine falcons (that measurement comes from my own work).

Here's why: http://www.airspacemag.com/flight-today/falcon.html

Frightful, the world record holder, exceeds 242 mph in a stoop, and can pull out of such dives carrying a lure equal in mass to herself.  The best comparison I can think of is this: drop down the middle of a spiral stairwell, and catch yourself on the railings at the bottom with your arms.  With a compact car attached to your back.

Clap and Fling

One really cool mechanism used by some flying animals to quick-start lift on the wings is called a "clap and fling": the wings are clapped together above the animal on the upstroke, and then peeled apart.  This forces the vorticity to start on the wings almost immediately, and produces a counter vortex above the animal that results in a handy low-pressure zone above their body.  Insects used this mechanism the most, but some birds do, too.  The photo at left shows a pigeon using a clap and flight during launch.  This is why pigeon takeoff often produces a clapping sound. 

(In the photo, if you look closely, you'll see that the pigeon is just finishing toe-off.  As usual, the legs produce most of the launch power, then the wings will engage immediately - thanks to the clap and fling, the wings will hit max lift almost immediately, and that allows a very steep climb-out for the pigeon after it leaves the ground).

The photograph was taken by Joe Hancuff.  You can check out his work here and here.  He's also on twitter (@joehancuff).  He has a particularly extensive gallery of dancers.

Feathers vs Membranes

A recent discussion arose on the Dinosaur Mailing List that included some questions regarding the relative merits of membrane wings and feathered wings, mostly in the context of pterosaurs vs birds. In that spirit, I thought I'd give a little rundown of the relative advantages/costs of each type of vertebrate wing.

Avian Wings
Birds are the only flying vertebrates to use keratinized, dermal projections (i.e. feathers) to form their wings.  Feathers have the distinct advantage of being potentially separate vortex-generating surfaces, meaning that a bird can split its wing up into separate airfoils, thereby greatly changing its lift and drag profile as required (Videler, 2005).  Tip slots are the most obvious example of this mechanism, whereby the tip of the wing is split into several separate wingtips by spreading the primary feathers of the distal wing.  The alula, which lies along the leading edge of a bird’s wing, and is controlled by digit I, is another example of a semi-independent foil unit (Pennycuick, 1989; Videler, 2005).  The splayed primaries of a slotted avian wingtip passively twist nose-down at high angles of attack (and therefore at high lift coefficients), and this feather twist reduces the local angle of attack at the distal end of slotted avian wings, preventing them from stalling (Pennycuick, 2008).  Slotted avian wingtips may therefore be nearly "unstallable", though this does not prevent the overall wing from stalling (Pennycuick, pers comm.).  Feathered wings can also be reduced in span without an accompanying problem of slack and flutter – the feathers that form the contour of the wing simply slide over one another to accommodate the change in surface area.  Despite these advantages, feathers have some costs as wing components, as compared to membranous wings.  Feathered wings are relatively heavy (Prange et al., 1979) and cannot be tensed and stretched like a membrane wing (which has ramifications for cambering). Theoretically, avian wings should not be able to produce maximum lift coefficients as high as an optimized membrane wing (Cunningham, pers comm.), but experimental data to determine if transient, maximum lift coefficients actually differ significantly between bats and birds are not yet available (Hedenstrom et al., 2009).

Chiropteran Wings
Bats have a wing surface formed primarily by a membrane stretched across the hand, antebrachium, brachium, and body down to the ankle.  Unlike birds, which have a limited number of muscles that produce the flapping stroke (two, primarily: m. pectoralis minor and m. pectoralis major), bats have as many as 17 muscles involved in the flight stroke (Hermanson and Altenbach, 1983; Neuweiler, 2000; Hedenstrom et al., 2009).  The membranous wings of bats are expected to have a steeper lift slope than the stiffer, less compliant wings of birds (Song et al., 2008).  This results from the passive cambering under aerodynamic load that occurs in a compliant wing: as lift force increases, the wing passively stretches and bows upwards, producing more camber, and thereby further increasing the lift coefficient and total lift.  While there are some advantages for a flying animal in having such a passive system, bats presumably must mediate this effect with the many small muscles (and fingers) in their wings – tensing the wings actively while under fluid load will mediate the amount of camber that develops.  This would be important to mediate drag and stall, though no empirical data currently exist to indicate exactly how bats respond to passive cambering.  The work by Song et al. (2008) also indicates that compliant, membrane wings achieve greater maximum lift coefficients than rigid wings, but data have yet to be collected demonstrating that this holds in vivo for bats and birds.  Compared to birds, the distal wing spar in bats is quite compliant (Swartz and Middleton, 2008).

Pterosaur Wings
The structure and efficiency of pterosaur wings is obviously not known in as much detail as those of birds or bats, for the simple reason that no living representatives of pterosaurs are available for study.  However, soft tissue preservation in pterosaurs does give some critical information about their wing morphology, and the overall shape and structure of the wing can be used (along with first principles from aerodynamics) to estimate efficiency and performance.

It is known from specimens preserving soft tissue impressions that pterosaur wings were soft tissue structures, apparently composed of skin, muscle, and stiffening fibers called actinofibrils, though the exact nature and structure of actinofibrils has been the topic of much debate (Wellnhofer 1987; Pennycuick 1988; Padian and Rayner 1993; Bennett 2000; Peters 2002; Tischlinger and Frey 2002).  Associated vasculature is also visible in some specimens, especially with UV illumination (Tischlinger and Frey, 2002).  Recent work on the holotype of Jeholopterus ningchengensis (IVPPV12705) seems to confirm that the actinofibrils were stiffening fibers, imbedded within the wing, with multiple layers (Kellner et al., 2009).  The actinofibrils were longer and more organized in the distal part of pterosaur wings than in the proximal portion of the wing, which may have implications for the compliance of the wing going from distal to more proximal sections.  The inboard portion of the wing (proximal to the elbow) is called the mesopatatgium, and was typified by a small number of actinofibrils with lower organization, which would have made this part of the wing more compliant than the outboard wing.

The outer portion of the wing, which was likely less compliant the mesopatagium, is termed the actinopatagium (Kellner et al., 2009).   Because pterosaurs had membrane wings, they could presumably generate high lift coefficients, but exactly how high depends on certain assumptions regarding their material properties and morphology (pteroid mobility and membrane shape being two of these factors).

Now, for some punchlines...
Based on the structural information above, we might expect the following regarding pterosaurs and birds:

- Pterosaurs would have a base advantage in terms of maneuverability and slow flight competency.

- Pterosaurs would also have had an advantage in terms of soaring capability and efficiency

- Pterosaurs would have been better suited to the evolution of large sizes (though this was affected more by differences in takeoff - see earlier posts about pterosaur launch).

- Birds will perform a bit better as mid-sized, broad-winged morphs (because they can use slotted wing tips and span reduction).

- Birds would have an advantage in steep climb-out after takeoff at small body sizes (because they can work with shorter wings and engage them earlier).  This might pre-dispose them to burst launch morphologies/ecologies.

Interestingly enough, the fossil record as we currently know it seems to back up all of these expectations.  For example, the only vertebrates that seem to have been adapted to dedicated sustained aerial hawking in the Mesozoic were the anurognathid pterosaurs.  Large soaring morphs in the Mesozoic were dominated by pterosaurs, also.  On the other hand, mid-sized arboreal forms in the Cretaceous were largely avian.

Full references for all of the above literature is available upon request.  I'll post the full refs here as soon as I have a chance, but just email me in the meantime if need be (currently traveling in Boston).

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.


References

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

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

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

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.