[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Great in the air, not so good underwater

----- Original Message ----- From: "Michael Habib" <mhabib5@jhmi.edu>
To: <dinosaur@usc.edu>
Sent: Friday, December 08, 2006 3:45 PM
Subject: Re: Great in the air, not so good underwater

I mostly agree with you, except for one small item:

So, for example, large seabirds are loading limited to the extent that they have to be able to hit steady state from a running start and they can only run so fast.

No, they don't have to be able to hit steady state directly from a running start. All they have to hit is an anaerobic burst window, and have the ability to accelerate to steady state before the burst power runs out.

And swans use a very different technique that involves (in visual extreme -- for purposes of qualitative illustration) a 'hovercrafting' technque combined with 'jet thrust augmentation' during a portion of the flapping cycle, where in the latter portion of the downstroke, they sweep the forearms forward with both elbows and wrists quite flexed, while cupping the outer wings more and more downward, aftward, and inward, creating a 'skirt' that traps air underneath them, bringing it in through a fairly large opening in front of the wings and exhausting it aftward through a much smaller opening with both openings contracting as the process continues. This is followed by a rapid lifting of the wrists and a rotation and extension of the outer wings that sets the procedure up for repetition and initiation of the next downstroke. Wing thrust is provided during the mid portion of the downstroke and leg thrust operates throughout the cycle . Since they do this slightly asymmetrically in order to sychronize with the leg strides across the water, the visual appearance is quite remarkable, especially when seen in slow motion from directly in front of the animal as it runs across the water. I have a film of this process that contains about 110 frames during a single wingbeat cycle.

Granted, that's not really a pure wing loading problem (they could sidestep by improved cursor ability), but it is an implicit factor and I suspect that the largest pseudodontorns were near the size limit for their particular morphotype.

I agree with both comments. However, my real point is that if flight is truly important to the animal, it or its decendants will partially sidestep the launch issue. A couple of ways to accomplish this are cliff launching and front limb assist during launching (I'm not sure how a bird would accomplish that latter, but bats and pterosaurs can). Given enough time for evolution to operate, I suspect there are hundreds of other ways as well.

Burst-takeoff specialists (like galliforms),......... My suspicion is that both authors were right to a point, but that the tradeoff systems (and thus, maximum sizes) change depending on launch specifics.

Yeah, me too.