Re: pterosaurs, bats, flying theropods (fwd)

[Michael Bruce Habib <mbh3q@virginia.edu>]

> Except that, if 
> http://www.cmnh.org/dinoarch/1996Aug/msg00050.html is
> correct, bats were there first--and were happy in the diurnal niche until
> nasty raptors came along.

Those bats that forage diurnally probably are secondarily 
diurnal.  That is, nocturnality is probably plesiomorphic; 
some bats have derived diurnal habits, and this is then 
repressed if raptors show up (unless the bats are large).

There is no evidence I know of to show that bat species 
present before the divergence of raptors had a higher 
degree of diurnality than later species.

> True.  But to what does each owe this advantage--not to foraging ability,
> but to predation avoidance or lack of it.  This is what the evidence seems
> to suggest...
> Maybe speed is the thing--a raptor swooping on a flock (right word?) of
> bats...
> Well, a creature needs to move faster in the daylight when the predator can
> see it...

I see the difference in sensory equipment and flight styles 
as being the most important factors keeping most birds 
diurnal and most bats nocturnal. 

The average flight speeds 
of birds are quite a bit higher than those of bats.  

This has certain obvious advantages, and works fine IF an 
animal is visually-based.  Light-based vision has good 
range and excellent resolution. Of course, it doesn't work 
very well at night.

Bats have lower top speeds (in general), but often better 
manueverability.  This makes sense given that they use a 
navigation device that is relatively short ranged and 
coarser in resolution than vision.  Many bats forage in 
clutter, and these have impressive navigation skills (and 
larger brains to go with it, as it turns out).

Keep in mind that bats do not only have ecolocation, they 
have _fantastic_ hearing in general, ( at high frequencies, 
anyway) which is paramount for good night feeding.  

Therefore, birds have stronger navigational abilities in 
lit environments, but can't hold a candle to bats at night 
(for the most part).  Defense differences would be just one 
effect of this.

Also keep in mind that raptors do not need to eat bats into 
nothingness to keep them nocturnal; they simply have to be 
good enough at it to give nocturnal bats an advantage over 
diurnal bats.  It's really competition within bats that 
keeps them nocturnal, after all.

Mike Habib