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bird heads

There seems to be some confusion about my comments on bird head
movements...  When birds "bob" their heads while walking, the
important part of the motion (in the hypothesis I described last week)
is not the sudden forward jerk -- it's the relatively long periods of
time during which the head is stationary.  Betty appears to think
she's speaking against the hypothesis when she writes:

> If bobbing the head was integral to seeing during flight, these
> would not happen.

when in fact everything she wrote supports the hypothesis that the
promotion of retinal image stabilization is an important aspect in the
control of bird head movements.  I wrote last week that I knew of no
firm data collected to test the hypothesis I outlined.  I still don't,
but I have discovered that Richard Dawkins believes it to be accurate;
he wrote about it in _Unweaving the Rainbow_.  I haven't yet written
to him to ask if he knows of any relevant data...

As for the suggestion that the scenario be rigidified via
quantification of eye vs. skull size and degree of bobbing, I don't
think such an approach would be fruitful.  Yes, there is variation
among birds of eye size vs. skull size, and more importantly there is
variation among birds of eye size vs. orbit size (for some reason,
cormorants, hornbills and penguins have eyes that for birds are
relatively small in their sockets).  However, to bob or not to bob is
not likely to be a quantitative trait.  Note that not all birds jerk
their heads while walking.  For instance, many hop, which is
conceptually like bobbing their whole bodies.

Anyhoo, getting back to extinct archosaurs, you'd have the problem of
determining eye size vs. orbit size.  You could get a reasonable idea
of the diameter of the iris from specimens with scleral ossicles.
However, many birds have tube-shaped eyes, so the total eye volume
isn't specifiable from one linear measurement.  Next (although I could
be wrong and welcome corrections) it's my impression that the orbits
of dinosaur skulls are not as well-defined as you might want for such
a purpose.  That is, it looks to me like there is a lot of space which
in principle could house an eye in, for example, an Allosaurus skull,
but how can you tell how much of that space was occupied by eyes as
opposed to extra-ocular muscles (or something unrelated to vision)?  I
suspect it would be more fruitful to ask why bird eyes are so big.  As
I just mentioned to somebody on the list, the largest eyes known from
a terrestrial vertebrate can be found in the skull of an ostrich.  I
found that kind of surprising given that ostrich heads seem so puny
relative to their bodies.  But there you go.  My feeling is that if
you want to firm up the contention that other theropods did not bob
their heads you'd want to firm up a) the hypothesis that head-bobbing
arose because it promotes retinal image stabilization and b) when in
the ancestry of birds did the eyes begin to fill up their sockets
(hence making it more difficult for birds to move their eyes within
them).  Was it before or after they split with other theropods?  My
guess would be after, but at the moment I can't say with any real
conviction, and off the top of my head I'm not even sure how you could
determine this.  Perhaps with a combination of approaches from
molecular, evolutionary and developmental biology...

Mickey Rowe     (rowe@psych.ucsb.edu)