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Re: pterosaurs, bats, flying theropods (fwd)



> Date: Fri, 27 Dec 2002 20:30:22 -0800 (PST)
> From: Jaime A. Headden <qilongia@yahoo.com>
> To: dinosaur@usc.edu
> Cc: jbois@umd5.umd.edu
> Subject: Re: pterosaurs, bats, flying theropods
>
> John Bois (jbois@umd5.umd.edu) wrote:
> <Niches are available to all body plans>
>
>   But identical niches may not be available to different bodyplans. One of
> the things I am arguing about, as was Jim Cunningham, is that the anatomy
> of bats, birds, and pterosaurs are quite distinct from one another and
> actually provide constraints. Animals that specialize to the nighttime or
> even during crepuscular hours, have specializations to these in their
> sensory anatomy.

I agree...with the proviso that niches are infinitely complex and that I was
using the term in a very broad sense.  In other words, the daytime niche is
"available to all body plans" means that there is no niche barrier
preventing bats from being diurnal.  Yes, they have sensory adaptations for
the night...but this doesn't prevent them becoming secondarily diurnal--just
as wings in birds don't prevent the evolution into the flightless niche when
it is available!

>...the effective significance of the relative
> separation of these birds from these bats cannot in any way be established
> as a performance superiority.

If it were established we wouldn't be discussing it.  I'm arguing for higher
predatory escape performance in birds relative to bats.  There _is_ evidence
for this--in microbats, at least: where raptors exist, diurnal microbats do
not.

> How can not bats have outperformed the
> birds? Does being diurnal mean something better than being nocturnal? How?

Absolutely not!  Bats are good at some things, birds are good at other
things.  However, inasmuch as predation avoidance is an omnipresent and
tyrannical arbiter, birds may have proven their superiority in one aspect:
they can fly faster.  But then bats are absolutely superior at exploiting
the nocturnal niche--at least they have a body plan which enables them to do
so in much greater numbers and diversity than any other winged vertebrate.

> I do not think anyone has ever been able to come up with a
> suitable or even agreeable ecological mechanic in which birds and bats
> arrived at their present distributions and ecologies as a result of
> interacting throughout the Eocene--Recent timeline. If anyone can suggest
> how any mechanic would have happened that agrees with the fossil record
> and can thus be observed in anyway, I would love to hear it.

Another question might be: _if_ bats are kept out of the diurnal niche by
predation today, is there anything in the fossil record that suggests
anything about this mechanic in the past (either pro or con).  If not, the
present cannot arbitrate the past (I agree), but it does provide a pretty
good first order hypothesis--not only for birds and bats, but for birds and
pterosaurs as well.  Failing that, what are the other feasible hypotheses
for small pterosaur extinction?  We have anything _over_ some arbitrary mass
going extinct at the K/T.  Are we suggesting everything under some mass that
snags the small pterosaurs and dinosaurs during the Cretaceous?

> >Then we look at reasons why birds have it over bats. As you mention, they
> >both share a wide range of food resources; and it's hard to imagine that
> >one species could simply _out-eat_ a competitor.>

> Why not? Compared to the always-on-the-wing lifestyle birds have versus
> bats, which fly for only a few hours of their life, or the more efficient
> chiropteran digestive system, compared to the "slip right through me"
> avian one, birds need to eat a lot more than bats do, have extensive
> migratory, breeding, and competitive behavior, whereas nearly all bats are
> closely associated, communal animals that share body heat, sometimes food,
> etc.. We're also dealing with completely different animals, whose
> comparative biology is different from their comparative ecology and
> sociology. And that most bats tend to fall along similar lines, whereas
> birds are extraordinarily far more diverse. You don't see many birds
> exploiting some bat ecologies, either, such as chief nocturnal insect
> eaters, or the "vampire" ecology.

Aren't these mostly arguments why bats are better competitors at night.  The
question remains: why not bats in the daytime?  Do birds out-eat bats for
the daytime niche?  And don't you think that if some bolide from hell
extinguished all bird life, that bats would become soaring bats, emu bats,
etc., i.e., their morphological diversity is currently held in check by the
limitations of their niche.