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Re: COMPETING BATS, BIRDS & PTEROSAURS



Hi!

Some comments on my suggestion as to why birds outcompete bats when there is
daylight have been sent on the list. I'll discuss them all in the same
message.

I said:

"I believe birds do better than bats in daylight because birds rely on
vision whereas bats rely on echolocation. Echolocation is neither as acute
nor as effective as seeing, if one has good eyes."

Adam Britton commented:

"Taking the latter statement, bird visual
acuity is good, but echolocation (in bats and dolphins) is at least its
equal in many respects (ie. in terms of information the bat receives about
its prey, to enable it to capture that prey). The organs involved are also
adaptable to other tasks: pressure for more sensitive ears, which can also
be used to listen for prey movements; adaptability of echolocation signals
for different environments (open spaces, cluttered forest, etc); the ability
of pressure waves to "detect" things that vision cannot (eg. dolphins
detecting food under sand) amplification of acoustic signals to stun prey
(in dolphins)."

I don't quite agree with you. For one thing, bat echolocation doesn't work
over distances as long as bird vision. This is because the intensity of the
returning signal is inveresely proportional to the fourth power of the
distance. Bats can make louder sounds, of course, but they have their
limits.

How many frames bats can sense per second depends on the frequency they send
their clicks. When they fly and haven't yet sensed pray, they make their
clicks at somewhat low rate. At this time, they probably "see" their world
like viewed through a stroboscope. When prey comes in "sight", the frequency
increases. It is not known why the top frequency isn't sustained all the
time; it may be so because making clicks at a great rate consumes energy.
Another problem birds don't need to worry about.

How detailed an image one can see depends on the wavelength of thewaves used
to scan the environment. Wavelengths of visible light, which is oscillation
of an electromagnetic field, are far shorter than those of sound waves,
which are oscillations of matter.

I know bats can peerform some quite impressive tricks and distinguish some
quite small details, but I'd still say their wecholocating equipment falls
short of avian eyes - if there is plenty of sunlight, and no mist.

Jarno Peshcier wrote:

"Maybe in reality that's the other way around? Birds already existed (let's
assume that bats evolved later than birds even thought that's probably far
from 100% certain) and already had good eyesight so they primarily dominated
the day time. Bats had to try and do their stuff at night to avoid direct
competition with the ever present bird during the day. This in turn was a
strong selective pressure that caused the bats to quickly "develop"
echolocation.

Chicken and egg situation? Personally, the "forced in the dark ->
echolocation" seems a bit more probably to me than "echolocation -> better
suited for the dark". But of course that's no more than a hunch."

To me, the exact opposite seems more probable. The very first mammals were
small, nocturnal things with, most likely, a degenerated eyesight. They had
adapted to use their sense of smell and hearing instead. They also had
whiskers. It is possible that they had some kind of rudimentary echolocation
too.

Now, it seems to me that if these were going to evolve into flying forms,
they couldn't have competeted with birds making their living in daylight;
for birds had retained their good eyes and even developed them further. The
mammals, in turn, had senses best suited for nocturnal life. So,
echoilocating, nocturnal bats evolved.

Best wishes,
Henri Rönkkö