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Re: Homing Pigeons? Try Homing Crocodiles...

Dann Pigdon <dannj@alphalink.com.au> wrote:

> There's no reason to suspect that the homing abilities of birds and crocs 
> are due to the same physiological adaptations - especially since it's now 
> been shown that birds can see magnetic fields via adaptations in the eye 
> itself (not something you'd expect in largely nocturnal crocs).

Two things of import... first a quasi-philosophical issue... it is
probably not correct to say that birds "see" magnetic fields.  The
fact that light receptors in their eyes appear to be involved in
magnetic field detection doesn't make that detection anything like
vision and hence seeing.  We have photoreceptors in our eyes that help
to regulate daily fluctuations in hormone levels... receptors that
re-set biological clocks as it were.  Since these receptors do not
appear to play any role in our conscious visual perceptions, it
wouldn't be correct to say that we see the time of day.  Similarly, if
birds have any introspective capabilities, it's not likely that
magnetic field detection is like sight even if the transduction
process (converting magnetic field strength/orientation into a neural
signal) takes place in the eye.

Second, at least some of the relevant magnetosensory apparatus that
appears to work in birds also works in salamanders.  The basic
transduction mechanism may thus span a much larger phylogenetic
bracket than Archosauria.

> Marine turtles also have phenominal homing abilities, yet I wouldn't
> suggest that they got them from the common ancestor of turtles and
> archosaurs.

I haven't read the latest literature, but based on what I know from a
few years ago I'd be surprised to find that even within birds there
was only one magnetic field transduction mechanism.  Marine creatures
are probably more likely to use a magnetite based mechanism, but even
that could be conserved between birds and turtles.  I would *not* be
so quick to discount homologous relationships among various components
to the homing systems of birds and turtles.

Mickey P. Rowe     (mrowe@lifesci.ucsb.edu)

P.S. Thanks to all for not pointing out my previous misspelling of the
word "yolk".  I'm sure you all cringed at it just like I did.