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Re: COMPETING BATS, BIRDS & PTEROSAURS



Hi!

I'll continue discussing the avian and bat ways of perception.

Adam said:

> 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).

Well, I believe birds are capable of seeing even farther, as you say here:

> Also, birds are much faster than bats, so they can exploit the opportunity
> of seeing prey at much greater distances.

Adam told why bats produce pulses at variable rates:

> 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);

Thanks for the information. Do you mean that in order to produce double the
pulses in a given time, a bat needs to breath twice as fast?

> 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!

I remember reading the bats had some kind of a chirp radar, which means that
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.

I said:

> "How detailed an image one can see depends on the wavelength of the waves
> 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."

Adam said:

> That's true, but it doesn't mean visual animals can actually use that
> resolution functionally.

Well, that's possible. I don't know.

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.

I must say this is a very nice achievement for a bat. However, I still think
vision is, in overall, a better form of perception when there is no shortage
of light.

I said:

> The very first mammals were
> small, nocturnal things with, most likely, a degenerated eyesight."

Adam commented:

> 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.

You're right. They can see in very dark places, and so on. However, their
eyes aren't as good for looking around in daytime as are the eyes of birds.
This is due to the loss of color vision and sharpness of the eyes - these
things, in turn, result from the loss of cone cells, while rod cells have
increased in number. On the other had, bird eyes aren't too good for seeing
in darkness. 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.

Best wishes,
Henri Rönkkö