Re: Homing Pigeons? Try Homing Crocodiles...

[Dann Pigdon <dannj@alphalink.com.au>]

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:
>  
>  http://www.abc.net.au/news/stories/2007/09/27/2044505.htm
> ------------------------------------------------------------- 
>
> 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
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