Re: Doin the Theropod Nod

[Mickey Rowe <rowe@psych.ucsb.edu>]

"James R. Cunningham" <jrccea@bellsouth.net> writes of the head
motions of many birds:

> I've wondered if they do it to help enable a time-delay form of
> stereoscopic vision.  They also do it when they're just standing
> around.  Total speculation, though.  I'm clueless about the real
> reason.

First off, I think Ed's confusion (evidenced in another message) would
have gone away if he had kids young enough to appreciate "Sesame
Street" -- Bert (of "Ernie &" fame) calls this dance "Doin' the
coo-coo pigeon".  Anyhoo, to the best of my knowledge nobody has
really tried to test any hypotheses as to why many birds bob their
heads as they walk, but I think there is a more likely hypothesis than
the one suggested by Jim above.  Relative to most other animals, birds
have huge eyes.  That is, their eyes take up a larger percentage of
their skull volume than do the eyes of other animals.  One of the
things that birds have largely reduced in order to make space for
their eyes are the extra-ocular muscles, the muscles that move the
eyes within their orbits.

When you walk (assuming you're not a bird) your head wiggles up and
down.  Reflexes linking the semi-circular canals in your ears to the
muscles around your eyes cause your eyes to move down and up to
compensate.  It's relatively easy for you to look at a particular
point far in front of you and keep that point focused stably on your
retina as a consequence of these (and other) reflexes.  Lacking the
requisite muscles, birds cannot do this.  Thus, the only way for a
bird to keep a stable image on its retina is to keep its head in a
stable position.  By keeping its head in one place as it moves its
body forward relatively slowly, and then jerking its head forward to
another temporarily fixed position the bird thus does the best it can
to keep its retinal image stable while still allowing the animal to
move.  In effect, you could say that the bird's neck muscles are doing
the job its eye muscles used to do (though that would only be partly
accurate since the neck muscles of animals with better developed
extra-ocular muscles also have compensatory neck-muscle reflexes which
appear to function mainly as an aid to retinal image stabilization).

To bring this back to the animals most of us are here to talk about, I
would suggest that most dinosaurs -- even most theropods -- did *not*
"do the pigeon".  More probably they (like us) retained the
extra-ocular muscles that evolved before the evolution of calcified
skeletons, thus making these head-bobbing movements unnecessary.

-- 
Mickey Rowe     (rowe@psych.ucsb.edu)