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Re: COMPETING BATS, BIRDS & PTEROSAURS
I hope this isn't getting dangerously off-topic for the dinosaur list, but
bear with us.
From: "Henri Rönkkö" <email@example.com>
Sent: Sunday, March 04, 2001 3:37 AM
> Thanks for the information. Do you mean that in order to produce double
> pulses in a given time, a bat needs to breath twice as fast?
No. Given the linkage between wingbeat and respiration, it would be
impractical to vary respiratory rate - wingbeat does vary a little, but this
has more to do with flight dynamics than anything else. To increase pulse
emission rate, bats squeeze more signals into each expiratory phase of
respiration. This does start to cost more energy, so it's only something
they do when necessary (eg. checking out a potential prey item; getting
dangerously close to an obstacle).
> I remember reading the bats had some kind of a chirp radar, which means
> producing pulses at a greater frequency wouldn't be of harm; however, I'm
> not sure, as I don't remember very well what was stated in the text.
Bats employ a "gated time window" - a time during which a bat "expects" an
echo to return. This greatly minimises interference from other bats and
prevents jamming. Attentuation and directionality of ultrasound also help to
ensure interference is rarely a problem, although a consistently high pulse
repetition rate together with a high density of bats (eg. emergence from a
roost) would increase the chances of confusion. During such times, however,
bats rely more on their spatial memory of familiar locations.
Vision has one apparent advantage over echolocation: it is free. The linkage
of pulse emission with wingbeat makes bat echolocation "free" also, but it
does appear to constrain the ability of the bat to update information
frequently. In reality, this constraint is not apparent (high rates of prey
capture, no crashed bats), and the linkage between pulse emission and
wingbeat can be broken when necessary. Some bats (eg. hipposiderids) employ
a strategy of hanging on a branch and throwing the occasional pulse into the
surroundings. If a doppler-shifted echo is returned, it means an insect is
close, and the bat then takes off. Even here, where echolocation is not paid
for by flight muscles, net energy gain from high prey capture rates offsets
the apparent cost of these echolocation signals.
> I must say this is a very nice achievement for a bat. However, I still
> vision is, in overall, a better form of perception when there is no
> of light.
Don't get me wrong - I think vision is great (I use it all the time).
However, your statement illustrates exactly where it is limited. Look at it
this way: bats can hunt with equal ability regardless of ambient light
conditions, whereas birds are constrained severely by light conditions. Even
nocturnal birds cannot perform as well during daylight hours because their
eyes are tuned for sensitivity in low-light conditions. The only
environmental condition which would cause a serious problem for echolocation
would be heavy rainfall, but then birds don't fly in such conditions anyway.
I think we've got an apple and oranges scenario here. Each has its merits,
though I'll certainly go with you in agreeing that vision is better overall
for diurnal, aerial animals. It has other advantages as well, such as
receiving discrete visual signals.
> But anyway, the historical constraint of having become a
> nocturnal, color blind animal with dull vision results in an inability to
> evolve into a diurnal flier, when there are birds around.
Haven't we come full circle here? You originally argued that vision was
superior to echolocation and hence bats were unable to exploit the daylit
skies, whereas I responded that echolocation is just as effective as vision
in its ability to provide the bat with the information it needs to get
around. I'm not sure whether I succeeded in my argument, although I enjoyed
trying, but it brings me back to the idea that other factors were
responsible for bats not exploiting the daylit skies. For example, flying at
night avoids diurnal predators, and it also neatly gets around
thermoregulatory problems during the heat of the day. Megabats, which use
vision and not echolocation, also have similar thermoregulatory problems
forcing them to roost during the heat of the day.