NARRATOR:Listen to part of a lecture in a biology class.
FEMALE PROFESSOR:So, that is how elephants use infrasound.Now, let's talk about the other end of the acoustical spectrum—sound that is too high for humans to hear--Ultrasound
Ultrasound is used by many animals that detect—and, some of them, send out—very high-frequency sounds.So, what's a good example? Yes, Carol.
FEMALE STUDENT:[matter-of-factly, stating the obvious] Well, bats—since they are all blind, bats have to use sound for, you know, to keep from flying into things.
FEMALE PROFESSOR:That's echolocation.Echolocation is pretty self-explanatory: Using echoes—reflected sound waves—to locate things…As Carol said, bats use it for navigation and orientation… and what else? Mike?
MALE STUDENT:Well, finding food is always important—and I guess not becoming food for other animals.
FEMALE PROFESSOR: [light laugh]Right, on both counts.Avoiding other predators—and locating prey—uh, typically insects that fly around at night.Now, before I go on, let me just respond to something Carol was saying—this idea that bats are blind…Actually, there are some species of bats, the ones that don't use echolocation that do rely on their vision for navigation, but it is true that for many bats,their vision is too weak to count on.
OK, so quick summary of how echolocation works.The bat emits these ultrasonic pulses—very high-pitched sound waves that we can’t hear—and then: they analyze the echoes—how the waves bounce back.Here, let me finish this diagram I started before the class.So the bat sends out these pulses, very focused bursts of sound, and echoes bounce back.You know, I don't think I need to draw in the echoes, your reading assignment for the next class—it has diagram shows this very clearly—so anyway as I was saying…
By analyzing these echoes, the bat can determine, say, if there is wall in a cave that needs to avoid, and how far away it is.Another thing it uses the ultrasound to detect, is the size and shape of objects.For example, one echo they quickly identified is one they associate with moth, which is common prey for a bat, particularly a moth beating its wings.However, moth happened to have major advantage over most other insects: they can detect ultrasound; this means that when the bat approaches, the moth can detect the bat's presence.So, it has time to escape to safety, or else they can just remain motionless.Since, when they stop beating their wings, they will be much harder for the bat to distinguish from, oh a leaf or some other object.
Now, we have tended to underestimate just how sophisticated the abilities of animals that use ultrasound are.In fact, we kind of assume that they were filtering a lot out—uh, the way a sophisticated radar system can ignore echoes from stationary objects on the ground.Radar does this to remove “ground clutter”—information about, um, hills or buildings that it doesn’t need…But bats, we thought they were filtering out this kind of information, because they simply couldn't analyze it.But, it looks as if we were wrong.Recently, there was this experiment with trees and a specific species of bats—a bat called the lesser spear-nosed bat.
Now, a tree should be a huge and acoustical challenge for bat, right? I mean it got all kinds of surfaces with different shapes and angles.So, well, the echoes from a tree are going to be a mass of chaotic acoustic reflections, right? Not like the echo from a moth.So, we thought, for a long time, that bats stopped their evaluation at simply “that’s a tree.”Yet, it turns out that-that bats, or at least this particular species, can not only tell that it’s a tree, but can also distinguish between, say, a pine tree and a deciduous tree—like, a maple, or an oak tree: just by their leaves—an-and when I say “leaves,” I mean pine needles, too.Any ideas on how it would know that?
MALE STUDENT:Well, like with the moth, could it be their shape?
FEMALE PROFESSOR:You are on the right track.It’s actually the echo off all the leaves—as a whole—that matters.Now, think: A pine tree—with all those little, densely packed needles… those produce a large number of faint reflections in wh-what’s called a-a “smooth” echo—the waveform is very even …But an oak which has fewer but bigger leaves with stronger reflections, produces jagged wave form, or what we called a rough echo.And these bats can distinguish between the two, and not just with trees, but with any echo that comes in a smooth or rough shape.
Ultrasound is used by many animals that detect—and, some of them, send out—very high-frequency sounds. So, what’s a good example? Yes, Carol?
文章中教授说“Ultrasound is used by many animals that detect”，然后让学生举例，学生说了bats,整篇主要内容是在讲蝙蝠如何利用超声波进行定位等一系列行为。因此选B。