Sunday, April 8, 2012

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.


  1. For years I have considered the posterior aerodynamic appendages of the critters in question to be more likely for control than for lift. Especially given that the aerodynamic feature disappears phylogenetically when the posterior undergoes its own aerodynamic transformation.

  2. Yes indeed. In fact, I believe you and I had a chat about this on a plane to SVP a few years back. You'll be happy to know that we (=myself, Justin Hall, David Hone, and Luis Chiappe) have a paper going to PNAS very soon (i.e. possibly next week) that actually puts numbers to the control hypothesis and makes a pretty solid case against the lift-for-support hypothesis.

  3. Interesting. I wonder if this could also apply to pterosaurs. Basal pterosaurs with long tails tend to have extensive hindlimb patagia supported by an elongated 5th toe. This seems to be very reduced in short-tailed pterodactyloids.

    1. Yes, it could also apply to pterosaurs, though in that case, as you note, there is a transition in the morphology. The broader uropatagium in the basal forms would be better positioned and shaped to do what the uropatagium does in launching bats (see recent post on the PLoS ONE bat tail paper) - a broad uropatagium can help support the posterior portion of the animal in flight and produce lift upon takeoff.

      The uropatagium in short-tailed pterodactyloids was a bit reduced, but more importantly, it was split - this means that sprawling the hindlimbs would produce a "four wing" scenario more akin to what we see in flying fish. That sort of morphology provides more options for control, particularly authority in roll and yaw.