Re: Birds ignore physics to fly in 3rd gear

[Michael Habib <mhabib5@jhmi.edu>]

On Monday, July 23, 2007, at 03:57  PM, Colin McHenry wrote:

> ... The Red Queen is running just to stay in place," Tobalske  
> says."Evolution does the same with birds: keeping them in constant  
> evolutionary motion but never allowing them to get any closer to  
> aerodynamic perfection"
> Well, I choked on my coffee at that one.  It's an example of  
> phylogenetic constraint, I say to myself.  For the Red Queen  
> hypothesis to be supported by large-scale, biomechanical analysis  
> would be news indeed.  So I had a look at the original paper on PLoS  
> http://biology.plosjournals.org/archive/1545-7885/5/8/pdf/ 
> 10.1371_journal.pbio.0050197-L.pdf
> and this chap Tobalske isn't on the author list.  Says an earlier part  
> of the the news article:
> "Often people think that animals are optimised," says US bird flight  
> researcher Bret Tobalske, a professor at the University of Portland  
> <http://www.up.edu/>. "[But] they evolved from ancestors that didn't  
> fly."

It is indeed an example of phylogenetic constraint, and my opinion is  
that Tobalske is taking it a bit out of proportion.  Modern birds are  
actually pretty close to optimization in a number of ways; you don't  
see birds with a given planform flying in a manner inefficient for that  
wing shape (example: we don't observe many, if any, low AR marine  
soaring birds).  If birds didn't at least approach some kind of  
optimization for their given flight dynamic, then wing shape and  
ecology would not correlate (which they do).  If Tobalske's comment  
were correct, then birds would never have achieved the level of flight  
optimization that they already have.  Of course, they're not _entirely_  
optimized for flight (plenty of other constraints, such as the need to  
molt and fold the wings), and some species are more efficient than  
others.

The problem here is that Tobalske misinterprets what phylogenetic  
signal means, I think (or, at least, he doesn't explain it well in that  
particular sound byte...which is admittedly not an easy thing to do).   
It is worth noting that the actual authors of the manuscript do not  
make the same mistake.  Statistically, the phylogenetic signal detected  
in the dataset just means that birds with a particular planform and/or  
flight dynamic tend to be phylogenetically related.  That doesn't  
actually mean that birds are especially inefficient for their given  
mass or planform, only that certain planforms and gaits tend to be  
unavailable to particular clades because the flight characters are only  
so plastic.  So, some clades produce lots of fast soaring seabirds,  
others produce highly maneuverable arboreal species, etc.  That's not  
quite the same as saying that species using a given dynamic are  
inefficient at it, however.

> An interesting statement in itself, but anyway, I'm hunting down  
> Carrolian heresies by now, so I let that one go and scan the PLoS  
> paper for mention of the Red Queen.  Guess what - not a dicky bird.  
> The paper seems (on first read) to be a competent comparative analysis  
> of wing-loading and flying speed across a range of different birds...

I also thought it was pretty competent, for what that's worth.  The  
authors also note that the departures from the expected scaling trends  
are not altogether surprising in a biological context: large species  
travel at slightly slower speeds than expected based on aerodynamic  
efficiency, and smaller species travel at slightly faster speeds than  
expected.  Various gait changes (bounding gaits, intermittent flight,  
etc) help to make these "off-speeds" more efficient, and the departures  
are not huge.  Basically, the optimal speeds for very large species are  
probably prohibitively fast, and those for small species are  
prohibitively slow.  This is interesting, but it is not quite what the  
popular article implies.

> But now I'm even more curious as to how the (Swedish based) authors of  
> this paper feel about their work being used by a US colleague to push  
> a particular line on macroevolutionary processes.  Are they  
> comfortable with their work being represented in the English language  
> science press in this way?

Good question.

Cheers,

--Mike H.



Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181
habib@jhmi.edu