Re: Pterosaur wing membranes

[Michael Habib <mhabib5@jhmi.edu>]

> I remember seeing film of fuit bats soaring and I've  just bumped into 
> a ref
> about soaring flight in a "Flying Fox", a fruit bat. The  site says, 
> "It flies
> with a slow wing-beat, frequently glides and can also soar  on  
> thermals."
>
> http://www.animalinfo.org/species/bat/pterlivi.htm
>
> Here's a photo:  http://ibis.nott.ac.uk/Action-Comores/batflight2.jpg


I suspect you're referring to the Thomson (2002) paper on soaring in 
flying foxes, which I also find interesting.  However, the situation is 
very different from in pterosaurs.  Flying foxes, when they soar (and 
it's not that widespread, it would seem) are convective soarers (as the 
site says, referencing thermal utilization).  Low wing loadings are 
quite nice for convective soaring, which is why vultures have low wing 
loadings.  However, low wing loadings are terrible for dynamic soaring 
(which is why albatrosses have high wing loadings).  Pterosaurs were 
pretty obviously built for dynamic soaring (with incredibly high aspect 
ratios, for starters).

Bats are, in fact, not very good at gliding, all told.  Most cannot 
glide much at all, and the flying foxes glide very slowly.  At higher 
wing loadings, an animal glides faster (getting from point A to point B 
more quickly), without having to use more energy.  The amount of 
vertical travel remains unchanged, they just get to where they're going 
faster.  That doesn't mean gliding is no good to large bats; they 
obviously use it when appropriate and it saves energy.  Vultures also 
glide pretty frequently.  But, all told, they're not good at covering 
distance or riding shears.  Rather, such low-loading animals are good 
at loitering.

Pterosaurs, by contrast, were great at gliding and dynamic soaring, but 
probably pretty poor at loitering (compared to a vulture, that is, 
which is a very high bar).

Cheers,

--Mike Habib