托福official69听力lecture3 the fiddler crab's visual field原文解析+翻译音频

[00:00.00]Professor: Okay so we've seen some examples of animals whose vision is very different from our own. [00:11.45]Now I’d like to play around with this idea and animal’s visual perception, the way it sees the world has a profound effect on the way it behaves. [00:22.70]And when we try to understand the behavior of an animal whose perception is different from ours, we're interpreting their behavior from our perspective. [00:32.32]And for that reason, some of our current thinking about animal behavior, theories which have not taken perceptual differences into account, may be biased. [00:42.92]So, before we get started, who can remind us of some of these differences, yes jack.
[00:49.84]Student: Well, last week we talked about insects with compound eyes and well, I remember this picture in the book that show you how each eye was actually seeing hundreds of images of an object when we see only one.
[01:04.88]Professor: I'm glad you brought up that example, because the animal I want to discuss today also has compound eyes and that’s the fiddler crab. [01:13.76]The fiddler crab has been studied quite a bit by researchers interested in this connection between perception and behavior. [01:25.05]One reason is that it's quite easy to observe, it lives out in the open on mud flats and has a very limited territory. [01:33.94]In fact rarely leaves this mud environment. [01:37.21]Now, the fiddler crab has an unusual way of seeing, as you notice from this picture, its eyes located on stalks that point straight up, this gives the crab a panoramic visual field.
[01:50.21]Here let me show you what I mean, so the fiddler crab can see three hundred sixty degrees all around in every direction. [01:59.44]And the end of each stalk is divided into about ten thousand little eyes. [02:04.97]So, it has compound vision similar to what some insects have. [02:09.59]Another unusual feature is that this crab’s vision is sharpest on the periphery on the outer edges of its visual field. [02:18.73]Actually, this makes a lot more sense when you realize that is on the edge of its visual field on the ground, where the other crabs are likely to be, that they have to make the finest, distinctions, like between rival and mate. [02:33.89]But even the sharpest part of the crab’s visual field isn’t all that great, it'd be like oh, imagine looking at a newspaper, and say you saw nothing but a blurry white object, maybe with a few black spots on it, ok.
[02:49.52]Now another important thing, animals have to be able to see is predators. [02:55.34]For fiddler crabs, the most important predators are typically birds and the very center of the fiddler crab’s field vision is, surprise, the sky. [03:05.56]And here's the thing, the crab’s visual field is even blurrier in the center upon the sky than it is on the edges.
[03:13.82]Student: But that doesn't make sense, if the center of the crab’s visual field is so blurry, then how can they ever defend themselves, I mean, how do you know they would, mistake, I don't know, something, anything, in the air for bird.
[03:28.49]Professor: Well, this goes back to what I said earlier, we need to stop thinking from our own visual perspective and think about it from the animal’s point of view. [03:37.11]Because everything it sees is blurry, the fiddler crab’s world view is very simple one, everything in the sky is assumed to be a predator and everything on the ground is assumed to be a crab. [03:50.47]This is the most basic distinction that needs to make for its survival. [03:54.41]So, what for us would be a disadvantage, blurry vision, actually turns out for the crab to be advantage to its survival. [04:03.86]This way of seeing simplifies the way the crab needs to respond to whatever it sees.
[04:09.58]Student: But how can we know that for sure.
[04:12.31]Professor: Actually, researchers have proven it. Here’s what they did. [04:16.09]They placed a cylinder on a mudflat inhabited by fiddler crabs, the crabs treated the object like another crab, fighting it, ignoring it or even sizing it up as a potential mate. [04:28.77]But, when the same cylinder was tossed up in the air over the mudflats, all the crabs ran for the safety of their burrows. [04:37.59]So you can see how the crab's behavior is affected by its vision and object on the ground, any object as long as it's on the periphery of its visual field is perceived as another crab. [04:50.00]And that same object when seeing in the air, at the center of its visual field, is perceived as not a crab and therefore a potential predator.