托福official48听力lecture3 Mutualism原文解析+翻译音频

[00:00.00]NARRATOR: Listen to part of a lecture in a biology class.
[00:03.71]FEMALE PROFESSOR: OK. Today I'd like to spend some time going into more detail about symbiosis. [00:09.09]Symbiosis. What is it? Anyone?
[00:13.00]MALE STUDENT: I thought it's when two organisms are in a relationship that they both benefit from, [00:18.18]well, at least that’s what I thought it was until I did the reading last night. [00:22.26]Now I'm kinda confused about it because the book used that definition to describe mutualism.[00:28.00]Could you explain the difference?
[00:30.00]FEMALE PROFESSOR: Good. I was hoping that someone would bring that up. [00:32.73]Sometimes scientists working in different fields use the term “symbiosis” to mean slightly different things, [00:38.90]and it can get confusing, [00:40.88]for example, when “symbiosis” is used as a synonym for “mutualism.” [00:45.20]But there are quite a few of us out there who think there should be a clearer distinction made between the two.
[00:51.40]Ok, where to begin... Um, the original definition of symbiosis is pretty simple. [00:58.40]It simply means living together. [01:00.90]So, any close relationship between two organisms of different species would be considered a symbiotic relationship, including positive and negative relationships.
[01:11.42]Mutualism then is a kind of symbiosis, a specific type of symbiotic relationship where both organisms benefit somehow. [01:22.08]So, your book is correct.
[01:24.66]Now, I want to make it clear that, um, the positive result from being in a mutualistic relationship doesn't have to be equal for both organisms. [01:33.28]It's not a one-to-one ratio here.
[01:36.00]Is everyone with me so far? [01:37.98]Symbiosis—general term, mutualism—a narrower, more specific kind of symbiosis.
[01:47.39]Now let's take a closer look at mutualistic relationships. [01:51.54]I'll start off by describing a case of mutualism that involves a certain butterfly species found in South Africa and Australia. [01:59.35]It’s a good example of how dependence on a mutualistic relationship can vary.
[02:04.32]OK, there's this butterfly family, and I'll spare you the fancy Latin name because it's not important for our purposes here. [02:12.21]I'll call them coppers and blues, well…because most members of this family have blue or copper-colored wings. [02:19.83]I think this is one of the most interesting cases of mutualism: These butterflies require the presence of ants to complete their life cycle—[02:29.22]their interaction with ants is obligatory.
[02:32.10]So this is what happens. [02:34.57]A female butterfly of these coppers and blues will lay eggs only on vegetation where there’re ants of a particular species. [02:42.95]The butterflies can smell…well, ants leave behind pheromones—a special chemical signal. [02:49.40]The butterfly recognizes the ants'pheromones on the plant…and then the newly hatched butterflies, the caterpillars, will feed on this plant after they hatch from the eggs. [03:01.34]As the caterpillar gets a little older and finds shelter under nearby rocks or stones to protect itself from predators, it’s always attended or escorted by ants. [03:09.54]And it always makes its way back to the host plant to feed, guided by the ants—the ant escort service—so to speak.
[03:22.00]Now why would the ants go through all this trouble? [03:25.32]What's their benefit? Mary?
[03:28.12]FEMALE STUDENT: It's probably related to food …
[03:30.63]FEMALE PROFESSOR: Uh huh, you’re onto something …
[03:32.40]FEMALE STUDENT: OK. Ants feed on sweet stuff, right? [03:35.93]So the caterpillar must have some kind of special access to honey, or sugars, or something like that. [03:42.93]Maybe caterpillars produce honey somehow. [03:45.80]On second thought…I'm probably way off.
[03:48.96]FEMALE PROFESSOR: You're pretty close, actually. [03:50.82]The caterpillars have a “honey gland”—an organ that secretes an amino acid and carbohydrate liquid. [03:56.93]The caterpillar secretes the liquid from the honey gland—rather large quantities—enough to feed several ants. [04:04.77]But what makes this relationship obligatory for the caterpillar? [04:09.16]Well, if the ants don’t feed regularly on the liquid from the caterpillar’s honey gland, the gland overloads and gets infected. [04:17.62]The infection will kill the caterpillar and it'll never reach its final stage of development—becoming a butterfly. John?

[04:25.25]MALE STUDENT: OK. I just wanna make sure I'm following here. [04:28.64]The caterpillar needs the ants, or it won't make it to the stage where it can become a butterfly. [04:34.13]And, the ants do this because they get an easy meal out of it, right? [04:38.62]But the ants don't absolutely need the caterpillar for survival 'cuz they can get food from other places, right? [04:45.23]So it's still called mutualism even though it seems like the caterpillar's getting way more out of it? [sudden realization] [04:51.60]Oh. Wait. You said they don't have to equally benefit.[04:55.45]Never mind. Sorry.
[04:57.00]FEMALE PROFESSOR: Yes, but there is a type of mutualism where the relationship is necessary for both organisms to survive.[05:03.49]It's called obligatory mutualism. [05:05.99]We'll talk about that in next class.