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.

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