2015年9月5日托福阅读真题+题目+答案：Weak Electric Systems in Fish
1. Some blind elephantnose fish produce weak electric signals that are used for detecting objects in their surroundingsa phenomenon called active electrolocation. These fish have specialized electric organs that discharge either in pulses or in a wave-like fashion, depending on the species. Although discharges follow one another almost continuously throughout the life of the fish, their power level is much too low to be detected by human handlers but potent enough to create a stable electric field around the body of the fish. When an object enters into this electric field, it causes distortions in the current that are detected by electroreceptor organs distributed over the fish's skin.
2. A weak electric system may have several uses, including the exploration of novel environments. For example, blind elephantnose fish can easily find the only opening that allows them to cross through a newly installed partition within their aquarium, even though they cannot see it with their eyes. Their electric sense must be implicated because when these individuals become electrically silent (unable to use their electric system through denervation of their electric organs), they can no longer find the opening.
3. During the 1970s, biologists became interested in the role of the weak electric system not only as a means of electrolocation but also as a means of electrical communication between individual fish. Communication is possible because the rate and waveform of the electric discharges can vary between species, between sexes, between individuals, or even between situations in the same individual. Moreover, some fish can temporarily interrupt their normally continuous train of discharges, and these pauses can be full of meaning. The effective range of communication by electric signals can reach a little over 1 meter depending on water resistance.
4. In terms of functions, electric communication is strikingly similar to acoustical vocalization (vocal sounds). Some of these functions are concerned with reproductive activity. In some species, males switch to new electric calls during courtship, resuming their regular programming only after the mating season is over. In species in which each sex has its own distinctive pattern of discharges, females are attracted to the pattern of males, and males to the pattern of females. Females can even be induced to release their eggs in the vicinity of electrodes that imitate a male signal¨Dthe spark of love. As expected, through natural selection, both males and females prefer the electric pattern of their own species to that of other species.
5. Other functions relate to aggression. Aggressive individuals often precede their attacks with an increase in discharge rate, whereas submissive fish may stop emitting altogether. This submissive behavior seems to work. Researchers have found that individuals rendered electrically silent through denervation of their electric organs are seldom attacked by dominant fish. Finally, individual recognition can also be based on electric signatures. In banded knifefish, territory neighbors recognize each other through individually distinctive discharge waveforms.
6. The fact that weak electric fish can use their electric sense to communicate with one another leads to an interesting question: How can a fish distinguish between its own electric bursts and those from another fish In blind elephantnose fish, the problem is solved by the presence of two types of electroreceptors. One of these two types is automatically and briefly shut down each time the fish discharges. Therefore, any signal picked up by these electroreceptors has to come from another animal. Elephantnose fish also have the habit of echoing the discharges of other individuals. They discharge their own electric organ a fixed time after sensing the electric signal of another fish. This response time is extremely shortapproximately 12 millisecondsprobably the most rapid form of communication in the animal kingdom.
7. Knifefish also display a peculiar behavior called the jamming avoidance response. This response allows knifefish to prevent interference with their electric system when they meet other knifefish. In order to avoid confusion, an electric fish must somehow keep track of the discharge rate of another knifefish while remaining aware of its own. If the two rates are too close, each fish alters its frequency of discharge so as to widen the gap between the two. In a sense, they do not want to get their wires crossed. In the laboratory, it is possible, using artificial signals, to force a knifefish to decrease its frequency of firing just by exposing it to a high but slowly decreasing signal rate or to increase its frequency of firing by switching to a low but slowly rising signal rate.
1..Which of the sentences below best expresses the essential information in the highlighted sentence in the passage Incorrect choices change the meaning in important ways or leave out essential information.
A. Throughout the life of the fish, its electric discharges vary in power from undetectable to fairly potent.
B. Nearly continuous discharges undetectable by humans create a stable, low-level electric field around the body of the fish.
C. Human handlers cannot feel the electric discharges because the discharges are at such low power levels.
D. The discharges are so weak that they would have to be fired almost continuously to create a detectable electric field.
2..The word distortions in the passage is closest in meaning to
3..Paragraph 2 describes an experiment with elephantnose fish that were electrically silent and therefore unable to
A. distinguish between new and familiar objects in their aquarium
B. swim around the partition in their aquarium
C. find their way through a hole in a barrier
D. see with their eyes
4..All of the following are mentioned in paragraph 3 as components of communication in electric fish EXCEPT
A. the rate of electric discharges
B. the direction in which discharges travel
C. the waveform of electric discharges
D. pauses between electric discharges
5..The word temporarily in the passage is closest in meaning to
6..The author provides the information that Females can even be induced to release their eggs in the vicinity of electrodes that imitate a male signal in order to
A. emphasize the importance of electric signals in some fish mating behavior
B. argue that many fish are not very particular when choosing a mate
C. show that electric signals alone do not provide adequate information for mate selection
D. imply that identifying a potential mate by its electric signal is more difficult than it might seem
7..All of the following are mentioned in paragraphs 4 and 5 as ways fish communicate using discharge patterns EXCEPT:
A. Some fish increase their rate of discharge right before they attack.
B. Some fish reproduce the electric pattern of another species to hide from a predator.
C. Some male fish switch to a different electric call during mating season.
D. Some fish temporarily stop discharging as a sign of submission.
8..In saying that Aggressive individuals often precede their attacks with an increase in discharge rate the author means that
A. aggressive individuals often prepare for their attacks by increasing the discharge rate
B. aggressive individuals often intensify their attacks by increasing the discharge rate
C. attacks by aggressive individuals often increase when there is an increase in the discharge rate
D. an increase in the discharge rate often occurs before aggressive individuals attack
9..According to paragraph 5, becoming electrically silent can affect a fish by
A. causing it to become more aggressive
B. making it a more frequent target of attacks
C. damaging its organs for sending electric signals
D. making it less likely to be attacked by dominant fish
10..According to paragraph 6, one of the types of electroreceptors of the elephantnose fish shuts down at times in order to
A. avoid confusing its own signals with those of other fish
B. prevent the receptor from becoming damaged
C. distinguish between signals of short and long duration
D. enable the fish to produce echoes of the discharges of other fish
11..The word peculiar in the passage is closest in meaning to
12..According to paragraph 7, what happens when a knifefish comes into contact with another knifefish that has a similar discharge rate
A. Both fish speed up their discharge rate, perhaps out of a sense of competition.
B. Both fish slow down their discharge rate so that they are better able to hear each other.
C. One fish increases its discharge rate while the other decreases it, making the signals less similar.
D. Each fish slowly changes its discharge rate until they match up perfectly.
13.. Look at the four squares that indicate where the following sentence could be added to the passage.
Each fish in an encounter is discharging electric signals at a specific rate.
Where would the sentence best fit Click on a square to add the sentence to the passage.
14..Drag your choices to the spaces where they belong. To review the passage, click on View Text .
A. Some fish have special organs that produce a weak electric field around their body and have receptor organs that allow them to detect disruptions to this field.
B. Electrolocation is used primarily by fish who are blind, whereas electric communication is used by fish who are not equipped for acoustical vocalization.
C. Research suggests that the effective range of communication by electric signals can reach a little over 1 meter, depending on water resistance.
D. Distinctive discharge patterns allow fish with a weak electric system to identify each other by sex or species, or even recognize particular individuals.
E. Most fish with a weak electric system have two forms of electroreceptors so that they can detect both very fast and very slow discharge rates.
F. Fish can deliberately slow, speed up, or pause their rate of discharge in order to communicate and to avoid confusing their own signals with those of another fish.