<< The basics of bat sonar are these:
"Bats that echolocate are of the suborder Microchiroptera within the mammalian order Chiroptera.>>
Mostly Microchiroptera, yes. There's one exception that I know of (the rousette fruitbat), although its not actually very *good* at it...
<< In layman's term that means that certainly not all bats use echolocation.
Indeed; about 170 or so species do without, mainly because their eyesight is perfectly good and they have the decency to sleep through the night and only fly about when its light enough to see what they're doing :-)
<<Those that do use many different forms of echolocation. >>
Yes; there's a fair degree of variation, which its reasonable to assume we haven't figured out in its entirety yet.
<< Some of you have pointed out that bats can fly in dense formations apparently without colliding. If this could be achieved only by sonar, it would be strong evidence that bats can use their sonars effectively without being disturbed by all the other bats' sonars. >>
Analysis of their sonar pings indicates they do all use different frequencies when flying in a group, so its difficult to see how they could be disturbed (indeed, why would they bother using their sonar at times like this, if it didn't work?). This has been demonstrated, for example, with the Noctule bat.
<<the oft-repeated statement that bats are blind is untrue, megachiroptera have good eyesight,>>
But don't generally use sonar :-)
<< microchiroptera have unimpressive sight but are not blind. >>
In a pitch black cave, eyesight of any quality isn't going to help...
<<However, if a sonar-using species of bat prefers to hunt in large flocks, that would be strong evidence that bats are not significantly disturbed by each others sonars. >>
The Greater Spear-Nose bat sometimes hunt in large flocks, for one. I'm sure there are others.
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