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Re: Homing Pigeons? Try Homing Crocodiles...

John Scanlon writes:

Dann Pigdon wrote:

... signals sent by cryptochromes in the eyes of garden warblers are
processed by the optic lobe in the brain:

And we might thus imagine that birds do indeed 'see' the magnetic field,
perhaps as 'lines of force' projected onto the visual field. But earlier,
Dann also said this was

not something you'd expect in largely nocturnal crocs

and I beg to differ: if there's something like a virtual heads-up display of lines of force it would be much more accessible (contrasting with the visual background) at night, when it would also be more useful (migratory birds do a lot of night-flying, do they not?)

Good point. Although the German authors suggest that the orientation of magnetic fields may show up as alternating light or dark bands in a bird's field of vision. I wonder if the contrast would be all that useful at night (dark bands on a dark image). Then again, bright bands on a bright day-time image may have an equally poor contrast. This is of course assuming that magnetic fields aren't interpreted as a fifth visual band (after red, green, blue and UV), although the article suggests that cryptochromes stimulate existing photoreceptors in the eye (with no mention of a whole new type of photoreceptor).

Before we start making any sweeping statements about dinosaurs based on phylogenetic bracketing though, let's keep in mind that such homing abilities have only been demonstrated in estuarine (sea-going) crocs. It could be that fresh water species have lost these abilities (if indeed they *don't* have them), but it could be equally likely that sea-going crocs evolved the ability themselves independantly of birds (or a shared archosaur ancestor), and use very different physiological tricks to achieve similar goals. Until it can be demonstrated that all crocodilians either have advanced navigating abilities or have secondarily lost them (and the same is determined for birds), then using a phylogenetic bracket to give dinosaurs the same abilities might be a bit of a stretch.


Dann Pigdon
GIS / Archaeologist              geo cities.com/dannsdinosaurs
Melbourne, Australia             heretichides.soffiles.com