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ALULAS, BATS, PTEROSAURS



Attempt 1 to catch up on a backlog. So much cool stuff happening.

John Bois wrote:

> On Thu, 1 Aug 1996, Nick Longrich wrote:
> 
> > I can't think of any reason to call bats clumsy fliers, and the fact
> > that there are about 1000 species of them or something ridiculous
> > like that proves that they're good enough at it to suit Mother
> > Nature or natural selection.
 
> But they are almost completely outcompeted in the day time by 
> alula-bearing birds. 

'Almost completely outcompeted' is a bit of an oxymoron. Bats are probably not
nocturnal as a result of competition with birds - there is niche partitioning
even when insectivorous bats are day fliers (and they are on some Mediterranean
islands) - but because they are so vulnerable to predation. Where raptors are
absent, bats that are normally nocturnal become diurnal. Speakman et al. have
argued that the rise of predatory birds in the Eocene (at that time, early
raptors and owls together with predatory rollers) forced a return to nocturnal
habits in microbats (exaptation surely involved). Megabats are a different
story, and are diverse and very abundant frugivores. Though they are mostly
crepuscular or nocturnal, they exploit a food source not used on a similar scale
by birds. They have not therefore been 'outcompeted in the day time by alula-
bearing birds'. That they are not fully diurnal may relate to thermoregulation,
see below.

The fact remains that, while bats are the most species-rich group of mammals,
they *are* hopelessly outnumbered by volant birds.

> Any enhancement of aerial agility in a flying animal must have a
> telling selective edge.  Although this is self-evident, I also
> wonder whether other things such as increased heaviness due to
> having to carry offspring within, or fragility of bat's wings might
> also affect their day/night distribution?
 
Bats are furthermore restricted by their thermoregulatory abilities due to their
highly vascularised patagia. It's been suggested that, for all we know, the same
may have been true of pterosaurs. But their wing membranes appear to have been
comparatively robust structures (unlike single skin layer of bats) - of course
you can still argue about their thermoregulatory role. David Peters has
suggested that early pterosaurs used their wing membranes to thermoregulate
while basking on tree trunks, and has compared this with the evolution of wings
(as solar panels) in insects. Work on Palaeozoic stoneflies, however, has
resulted in the recent(?) suggestion that insect wings evolved as sails for
mobility on the surface film of fresh water.

The fragility of bat wings is almost certainly not a determining factor in bat
ecotype or diversity, for two reasons. (1) is that bats are able to avoid
injury, even in crowded shrubbery that we would consider 'dangerous' in view of
their thin wings, by virtue of their sonar. (2) Except in extreme circumstances
(severe damage), bats can function with damaged wing membranes, and any damage
does heal quickly. I wish someone would tell this to the animal hospitals that
have euthanised bats bought in with damaged wing membranes. Furthermore, bats
do well in habitats that we would predict as 'dangerous' in view of their thin
wings - many navigate around dense foliage in woodland, urban and bankside
habitats.

> Pterosaurs were very diverse, and they didn't have an alula.

Regardless of which wing configuration you believe to be correct (there are at
least three), pterosaurs have a mobile propatagium controlled by their pteroid
bone. This is analagous to the alula in that it controls stalling speed. Has no
leading edge slot though..

"Well maybe he doesn't answer to Chuck, try Charles"

DARREN NAISH