Re: FUCHSIA and the Ostrom Symposium Volume (long...)

[Graydon <graydon@dsl.ca>]

On Sat, Aug 10, 2002 at 08:11:18PM +0200, David Marjanovic scripsit:
> ----- Original Message -----
> From: "Graydon" <graydon@dsl.ca>
> > Forward stroke to downward stroke is an angle change; there's an obvious
> > advantage to a cursorial predator in being able to apply the predatory
> > stroke over a wider range of angles.  (Since you can adjust for dinner's
> > desperate deeking better that way.)
> 
> Okay, but will the prey ever be directly under the predator, and if so,
> won't the latter rather use its feet?

Probably not, but you don't need to go from <straight in front> to <down
like a wing stroke> in one pass; all you need is an escalator, so that
<down enough to be noticeably better at grabbing the little hopping
furball for lunch> shades into <down enough to be better for making
those hard right turns to avoid breaking a foot in the furball hole>.
So long as those overlap, natural selection can produce a wide range of
forearm aerodynamic motions, some of which can eventually become the
flight stroke.

> > If you're looking at _why_ a running animal would flap, well, presumably
> > a cursorial predator needs to apply the predatory stroke _while it's
> > running_.
> 
> I should have got that idea myself :-] -- but see below...
> 
> > That being the case, it really isn't a stretch to have the range of
> > flapping behaviours expand; all it takes is some advantage -- in
> > turning, or in traction, or in clearing an obstacle -- accrue to
> > 'premature' use of the predatory stroke while outside of striking range
> > in pursuit.
> 
> But all cursorial birds tend to fly as rarely as possible, or not at all,
> arguing against the idea that flight is an advantage for a cursorial animal.
> (This is Ebel's main argument against cursorial models.)

Ebel should chase chickens.

Leaving aside that no modern cursorial bird is a predator with a grasping
manus, you've got the wrong end of things, there; they _can_ fly, or are
descended of ancestors who could fly; you're looking at highly derived
behaviour as an explanation of the basal behaviour, and that won't work.

Note that chickens and other semi-flightless domestic birds will use
their wings to steer and to generate lift to help them over obstacles;
if you're a small opportunistic-omnivore theropod being chased by a
slightly larger opportunistic-omnivore theropod, being able to jump six
feet of brush in a frenzy of flapping, compared to not quite being able
to do so, is doubtless a cause of differential breeding success.  It
_starts_ with getting lunch more reliably or hopping gopher holes
better, but eventually the escalator will get you to a pure flight
locomotion regieme.  Modern cursorial birds have abandoned that regieme
_to volant competitors_; that's not a situation that would apply with
early birds, since there is little reason to expect that they'd be in
direct competition with pterosaurs.

> > Early use in turn can be explained by normal behaviour variation; some
> > predators are more optimistic than other predators, and figure it's
> > worth a try even when the odds are bad, and sometimes it helps balance
> > to stick out a limb;
> 
> Agreed.
> 
> > either or both of those suffice to get onto that
> > particular gradual ramp of moving toward the flight stroke from the
> > predatory stroke.
> 
> But then... will they use the flight stroke?

Sure.  Becuase the flight stroke is just 'turn real sharply' or 'don't
fall off this rock face'; there's a smooth escalator, observed in modern
birds, between 'waves arms, very showy' (ostriches) and the arctic tern,
migrating from pole to pole on long tapered wings.

Also, don't forget that flapping has _defensive_ value; as anyone who
has seen a chicken run, or a grouse take off (or do a threat display)
knows, it can take a minute to figure out what, in all that motion,
might be lunch, and how to grab it without getting clouted.  Predators
are _cautious_.

-- 
graydon@dsl.ca   |  Hige sceal þe heardra, heorte þe cenre,
                 |  mod sceal þe mare þe ure maegen lytlað.
                 |   -- Beorhtwold, "The Battle of Maldon"