Here There be Dragons

As my previous teaser post suggested, I am going to spend some time on dragons over the next week.  This is mostly because dragons are fun, but also because the shapes of things humans have imagined to fly highlight some myths about animal motion and anatomy. To start off with, though, I would like to look at some real dragons.

Draco volans: Flying Dragon
There are actually 20-30 species in the genus Draco (depending on your preferred taxonomy).  Most members of the group have elongate ribs that they can extend, forming airfoils that allow gliding to varying degrees.  The most famous of the group is Draco volans.  This small lizard can extend gliding surfaces on both the trunk and the neck, and while the aspect ratio of the wings is pretty low (so the glide angle isn't great), these animals can manage glides of tens of meters or more.

The wings, when extended, have an elliptical shape, and while this is not the only way to get an elliptical lift distribution (which is typically desirable) it is one way this can happen.  The relatively short span also keeps the inertia low during turns; anecdotal accounts indicate that Draco are actually quite maneuverable. 

The photograph at left  (taken from here) gives an idea of the scale of the lizards and the structure of the wing. Note that the ribs seem to be relatively compliant; part of each spar is actually cartilage. We might suppose the the planform in Draco is just the result of constraint - in other words, we might suspect that there just aren't many shapes that are viable for a lizard (or other lepidosaur) using ribs to make wings. 

However, the fossil record shows that other wing shapes are viable.  Icarosaurus is pictured at left (image by Julius Csotonyi; note the obvious copyright notice and watermark - respect the copyright, please.  You can find it on his website here).  Icarosaurus hails from the Triassic.  Note that the wings have a high aspect ratio shape and that the overall span is relatively much greater than in Draco.  Interestingly, Icarosaurus was also a substantially larger animal than the largest Draco.  As a result, even with a greater relative span, Icarosaurus sported a higher wing loading.  With the greater AR, it would have had a smaller glide angle (gone further for a given amount of lost height), and with the greater wing loading, Icarosaurus would have glided faster than Draco.  Both of these likely came at the expense of lower maneuverability.  The shape of the spars (i.e. the ribs) supporting the wings in Icarosaurus curved posteriorly near the tips, particularly the ribs the near the mid-section of the wing.  This gave a broad, backswept tip shape to the wing of Icarosaurus.  To the best of my knowledge, the specific aerodynamics of this tip shape have not be investigated in the literature for Icarosaurus.

If you are interested in reading more about Draco gliding, and the evolution of gliding in other, similar, fossil forms, then I recommend reading this paper by McGuire and Dudley.  Sadly, a subscription is required.


Chrysopelea: Gliding Tree Snakes
Yup, that's right, gliding snakes.  They may not have the dragon namesake, but many of the historical reconstructions of dragons show flying, serpentine animals (see text from my most recent blog post before this one).  The closest thing to such an animal among real species are the snakes of the genus Chrysopelea.  Jake Socha and his group have done most of the leg work on understanding gliding in these animals.  You can check out his lab page here.  I have already blogged about these critters here, so I won't go into it at length, but there image below (by Tim Laman, from here - again, respect the copyright please) gives a great shot of how these snakes flatten their bodies during gliding. 

 Vortices are spun off either side of the body in these snakes during glides, and this produces a decent lift profile that allows glides averaging 10 meters of horizontal distance (Socha et al., 2005: available here).  This is particularly impressive because the snakes must use a rather unusual launch method (hanging and dropping from the tail) to take off; all other gliders are able to leap to begin flight, and that helps a great deal.


Next time: a look at some pterosaurs, then we begin building a fantasy dragon and consider the limits of size in vertebrate flyers.

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

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.

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.

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.