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Over the years, biolists have suggested two main pathways by which sexual selection may have shaped the evolution of male birdsong. In the first, male competition and intrasexual selection produce relatively short, simple songs used mainly in territorial behavior. In the second, female choice and intersexual selection produce longer, more complicated songs used mainly in mate attraction; like such visual ornamentation as the peacock’s tail, elaborate vocal characteristics increase the male’s chances of being chosen as a mate, and he thus enjoys more reproductive success than his less ostentatious rivals. The two pathways are not mutually exclusive, and we can expect to find examples that reflect their interaction. Teasing them apart has been an important challenge to evolutionary biolists.

Early research confirmed the role of intrasexual selection. In a variety of experiments in the field, males responded aggressively to recorded songs by exhibiting territorial behavior near the speakers. The breakthrough for research into intersexual selection came in the development of a new technique for investigating female response in the laboratory. When female cowbirds raised in isolation in sound-proof chambers were exposed to recordings of male song, they responded by exhibiting mating behavior. By quantifying the responses, researchers were able to determine what particular features of the song were most important. In further experiments on song sparrows, researchers found that when exposed to a single song type repeated several times or to a repertoire of different song types, females responded more to the latter. The beauty of the experimental design is that it effectively rules out confounding variables; acoustic isolation assures that the female can respond only to the song structure itself.

If intersexual selection operates as theorized, males with more complicated songs should not only attract females more readily but should also enjoy greater reproductive success. At first, however, researchers doing fieldwork with song sparrows found no correlation between larger repertoires and early mating, which has been shown to be one indicator of reproductive success; further, common measures of male quality used to predict reproductive success, such as weight, size, age, and territory, also failed to correlate with song complexity.

The confirmation researchers had been seeking was finally achieved in studies involving two varieties of warblers. Unlike the song sparrow, which repeats one of its several song types in bouts before switching to another, the warbler continuously composes much longer and more variable songs without repetition. For the first time, researchers found a significant correlation between repertoire size and early mating, and they discovered further that repertoire size had a more significant effect than any other measure of male quality on the number of young produced. The evidence suggests that warblers use their extremely elaborate songs primarily to attract females, clearly confirming the effect of intersexual selection on the evolution of birdsong.

17.1. The passage is primarily concerned with

(A) showing that intrasexual selection has a greater effect on birdsong than does intersexual selection

(B) contrasting the role of song complexity in several species of birds

(C) describing research confirming the suspected relationship between intersexual selection and the complexity of birdsong

(D) demonstrating the superiority of laboratory work over field studies in evolutionary bioly

(E) illustrating the effectiveness of a particular approach to experimental design in evolutionary bioly

17.2. The author mentions the peacock’s tail in line 8 most probably in order to

(A) cite an exception to the theory of the relationship between intrasexual selection and male competition

(B) illustrate the importance of both of the pathways that shaped the evolution of birdsong

(C) draw a distinction between competing theories of intersexual selection

(D) give an example of a feature that may have evolved through intersexual selection by female choice

(E) refute a commonly held assumption about the role of song in mate attraction

17.3. According to the passage, which of the following is specifically related to intrasexual selection?

(A) Female choice

(B) Territorial behavior

(C) Complex song types

(D) Large song repertoires

(E) Visual ornamentation

17.4. Which of the following, if true, would most clearly demonstrate the interaction mentioned in lines 11-13?

(A) Female larks respond similarly both to short, simple songs and to longer, more complicated songs.

(B) Male canaries use visual ornamentation as well as elaborate song repertoires for mate attraction.

(C) Both male and female blackbirds develop elaborate visual and vocal characteristics.

(D) Male jays use songs to compete among themselves and to attract females.

(E) Male robins with elaborate visual ornamentation have as much reproductive success as rivals with elaborate vocal characteristics.

17.5. The passage indicates that researchers raised female cowbirds in acoustic isolation in order to

(A) eliminate confounding variables

(B) approximate field conditions

(C) measure reproductive success

(D) quantify repertoire complexity

(E) prevent early mating

17.6. According to the passage, the song sparrow is unlike the warbler in that the song sparrow

(A) uses songs mainly in territorial behavior

(B) continuously composes long and complex songs

(C) has a much larger song repertoire

(D) repeats one song type before switching to another

(E) responds aggressively to recorded songs

17.7. The passage suggests that the song sparrow experiments mentioned in lines 37-43 failed to confirm the role of intersexual selection because

(A) females were allowed to respond only to the song structure

(B) song sparrows are unlike other species of birds

(C) the experiments provided no evidence that elaborate songs increased male reproductive success

(D) the experiments included the songs of only a small number of different song sparrows

(E) the experiments duplicated some of the limitations of previous field studies

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