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Re: COMPETING BATS, BIRDS & PTEROSAURS



Henri said:

"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."

I wasn't being particularly clear. What I meant was, echolocation is equally
effective as a tool for locating and catching prey. There are pros and cons
to each method, but they're best suited to the animals using them. However,
don't underestimate echolocation - it is capable of more than a visual
system in some respects, even though a visual system is a better overall
solution for a diurnal animal. Sound is limited by attenuation, yes, but
that provides essential filtering to prevent confusing echoes from reaching
the bat, and it's offset by the very high sensitivity of echolocation
compared with vision (a bat can detect a human hair at a distance of three
metres, for example). This compensates for the need to "see" long distances.
Also, birds are much faster than bats, so they can exploit the opportunity
of seeing prey at much greater distances.


"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."

Actually we do know. Bats produce one pulse per wingbeat, which means that
echolocation is "free" because it exploits the same muscles during flight
that power respiration. You don't necessarily need a constant stream of
information if you're searching for prey - vision would be "over-designed"
for the way bats hunt. There are several reasons why bats don't produce a
constant stream of clicks all the time: for a start it's unnecessary
(wasteful of energy with little useful information gain); it means bats
wouldn't be able to change pulse design to suit different discrimination
tasks (as they do); it would result in overlap of emitted pulse and
returning echo (bats "deafen" themselves as they produce each pulse, to
avoid damage to the auditory nerves) and hence loss of information; it would
overlap with pulses of other bats (jamming them), and it would likely alert
every prey item in the vicinity!


"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."

That's true, but it doesn't mean visual animals can actually use that
resolution functionally. I'd argue that bats are better at maximising the
functional potential of sound. High bandwidth pulses of sound carry large
amounts of spectral information which bats can utilise. A bat can
differentiate echo delays of around 0.5 microseconds, which means they can
successfully tell the difference between two bumps on a surface which differ
in height by 0.1mm (at a distance of 1 to 2 metres) and this has been tested
and confirmed. This helps to explain why bats can successfully differentiate
insect species (no matter what their orientation relative to the bat) during
flight from spectral information encoded in the echoes. Sound might not seem
an effective system to us, as we are primarily visual animals, but it is
equally effective at identifying and catching prey for bats. Most
insectivorous bats can catch around 1,000 to 1,500 insects per hour in the
peak following sunset, which is an impressive feat by any standards.

That's why I argued that echolocation and vision are more evenly matched
than you might think, and why there were probably other competitive reasons
why bats were forced into a nocturnal niche - echolocation came afterwards
(ie. in situations where a visual system is not the best option, or where
competition with a visually-orientated animal is too great, echolocation has
evolved as a highly effective solution).


"To me, the exact opposite seems more probable. The very first mammals were
small, nocturnal things with, most likely, a degenerated eyesight."

Why would that be the case? Most nocturnal animals today have excellent
eyesight - surely it's easier to enhance the acuity of an existing system
(eyesight) than throw it away and evolve a new one, unless there was some
really powerful selective force. Echolocation is not the only solution to
getting around in the dark, although it may be the best solution when there
is not enough light to navigate, and certainly a better solution when
hunting highly mobile, volant prey like insects.

Best wishes,

Adam