Saturday, April 21, 2012

Giant Flyers: to the limit (or not)

I am preparing for my departure to the Experimental Biology Conference in San Diego (starts today), so another brief post today.

One of my pet projects over the last couple of years has been to make a go at estimating the maximum mechanically allowed size for each of the groups of  powered vertebrate flyers: birds, bats, and pterosaurs.  This is a rather difficult task for many reasons, not the least being that working out the "weak link" in any given scaling problem is time consuming.  Pterosaurs provide a special challenge since they provide no living representatives to work on.

I have a preliminary model now, however, and I am rather excited about it.  Because it's still rough, I am not going to get into too much detail, but here are some of the punchlines if I turn out to be right with my approach (emphasis on "if"):

- None of the flying clades ever produced an animal at the mechanical flight limit for the group, which means that ecological limits or other constraints have historically set the maximum size.

- Of the three clades, birds came the closest to reaching their mechanical limit (Argentavis)

- Giant pterosaurs, even though they were the largest flying animals, were further from their mechanical limit than the largest flying birds.

- Bats have the lowest absolute limit, but they also have the greatest gap between observed max size and potential max size.  As a result, ecological constraints on size might be particularly strong for bats.  A 3 meter wingspan bat does not seem impossible with the numbers I have right now.  That would more than double the maximum span of the largest bat on record.

All of these limits are estimated using the most giant-friendly morphology in each clade: teratorns for birds, azhdarchids for pterosaurs, and megachiropterans for bats.  There could be altogether novel morphologies out there for each group that would push the limit higher, though for various reasons I can get into later, I think the limits I am calculating are somewhat generalizable.


  1. I find this information very interesting and wait eagerly a more detailed account. On a side note I want to thank you for your work.
    I’ve a question/interrogation. It seems to me strange that all the flyer animals (and the gliders) use a membrane or a membrane like structure for the wings (eg uniform structure) except the bird which use assemblage of feathers. The convergent evolutions of all the others species make me think that’s because feathers were use for something else at first (insulation or display) before been selected for fly. I’ll be happy to have your opinion on this subject.

    1. Thank you for the kind words and support. I hope that a more detailed account can be presented before the end of the year. The avian wing is indeed unique in that it uses complex outgrowths of the integument for the flight surface. Membranes are probably more common because the development of a membrane requires relatively few developmental changes - membrane production, is, at it were, very plastic in macroevolutionary time. I agree that the exaption of feathers from prior functions may partially explain their uniqueness, as well. On the other hand, some flight membranes may have had alternative functions prior to flight, as well. Overall, I think the take-home message is that membrane production is more developmentally repeatable between lineages, and therefore more likely to show up as a convergent trait over time in numerous clades. Great thoughts all around; thanks for reading!

  2. Thanks for your quick answer and comments . I might also add another thoughts : these difference could be linked to another flight origin for feather wing versus membrane wing (tree vs ground origin for example).