托福阅读真题+题目+答案：Sexual Dimorphism in Lamprologus Callipterus

托福阅读真题+题目+答案：Sexual Dimorphism in Lamprologus Callipterus

Lamprologus callipterus is one of over 300 species of cichlid (a freshwater fish) found only in Lake Tanganyika in Africa. Most African cichlids, including L.callipterus , exhibit sexual size dimorphism-the males are larger than the females. What is unusual about L. callipterus is the magnitude of the size difference. The key to understanding the sexual dimorphism of L.callipterus lies in the reproductive behavior of the species, particularly spawning (release of eggs by females and release of sperm by males) and brooding (protection of eggs). Female L.callipterus spawn and brood their eggs in empty snail shells on the lake bottom. In all other shell-brooding cichlids, both sexes enter the shell to spawn and thus both must be small enough to fit into the shells available on the lake bottom. In L. callipterus , however, only the females enter the shell to spawn and brood their eggs. The males remain outside, releasing their sperm only at the entrance to the shell. Consequently, female L. callipterus are small enough to fit into the shells, but the males, unconstrained by the need to enter shells, are much larger.

1.According to paragraph 1, all of the following are true about callipterus EXCEPT:

A They are found in Lake Tanganyika.

B The males are larger than the females.

C They carry out reproductive activities in empty snail shells.

D Both males and females are small enough to fit available shells.

Spawning and brooding inside shells puts females in an evolutionary dilemma with respect to their body size. In cichlids, as in most fishes, the number of eggs spawned per batch (the fecundity) increases with female body size, and this favors females that grow large. In L. callipterus the correlation between female length and number of eggs spawned is as high as 0.95, which means that more than 90 percent of the variation in fecundity among females can be explained by differences in body length. Further, because fecundity is more a function of body volume than length, it increases disproportionally as body length increases. In one study, for example, an increase in female length from 3 centimeters to 4 centimeters was associated with an increase in fecundity from 59 to 134 eggs. In addition to this huge fecundity boost, larger females are able to exclude smaller females from access to the larger, preferred shells and so gain in the competition for spawning sites. All this suggests that female L. callipterus should be large, but this is where the dilemma comes in. No matter how beneficial large size might be, the scarcity of large shells ultimately limits the size that females attain in a given population. In laboratory experiments females actually adjust their growth rates so that their body size at maturity matches that of the available shells, and in the wild the correlation between shell size and female size explains more than 98 percent of the variation in female size among populations. The ultimate size of female L. callipterus is thus a compromise between growing as large as possible to maximize fecundity and remaining small enough to be able to find suitable shells for spawning and brooding their young.

2.The word ”it” in the passage refers to

A fecundity

B function

C body volume

D body length

3.In paragraph 2, why does the author discuss the relationship between the number of eggs spawned and the body size of female cichlids?

A To emphasize how beneficial large size can be for the fecundity of female cichlids

B To explain the evolutionary dilemma faced by female L.callipterus

C To report the results of studies on fecundity and body length in female cichlids

D To compare the growth rates of female L.Callipterus in the laboratory and in the wild

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