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  3. Peahens, though not as showy as cocks, are by no means plain. Their heads have attractive crests and their necks are bright blue

Peahens, though not as showy as cocks, are by no means plain. Their heads have attractive crests and their necks are bright blue

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问题    Peahens, though not as showy as cocks, are by no means plain. Their heads have attractive crests and their necks are bright blue. Such decoration is costly to grow and likely to attract predators. If you do not have to show off your stuff to get a mate, why do you need it? Even more confusingly, in many species both sexes are equally showy. To understand things better James Dale of Massey University, in Auckland, New Zealand, and his colleagues have therefore examined the feathers of both sexes of all 5,983 species of passerine bird (雀形目鸟类) and compared them in exquisite detail. 【F1】The researchers started from the fact that, despite the exceptions, showiness is still more a male than a female phenomenon.
   【F2】Their first observation, as they report this week in Nature, was that in species where a few males exclusively control all the females, with a consequent lack of male involvement in parental care, males were more colourful than females. This is what the theory of sexual selection would predict. What it would not predict in its simple form, though, was a second finding—that females in co-operatively breeding species (those in which, for lack of other opportunities, several females collaborate to raise the young of only one of them) are more ornamented than those in which all adult females have a chance of breeding. 【F3】In this case it is females who are competing for the right to reproduce, thus putting themselves in a more male-like position.
   Another widespread belief Dr Dale and his colleagues confirmed is that tropical species are more colourful than those from temperate climes. But again, there was a twist—the effect was much more marked in females than in males. 【F4】It may be that tropical birds, which face more intense competition for food and nesting sites than temperate ones do because the tropics have more species, form more stable and collaborative pair-bonds than do temperate birds. In these circumstances males also need to be choosy, and females competitive.
   The final effect the researchers found was that big species are more colourful than small ones. That is true of both sexes, and probably reflects the fact that bigger birds are more difficult prey and thus have less need to hide. When released from the threat of predation, then, females tend to be showy. 【F5】That suggests showiness is always good when it can be got away with and probably explains the decorated necks and heads of peahens, which are among the biggest of birds.
【F4】
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答案这可能是因为热带地区鸟类更多，热带鸟类比温带鸟类面临更激烈的食物和筑巢竞争，由此导致热带鸟类形成了比温带鸟类更为稳定的、具有协作性的一雄一雌对偶匹配。
解析 ①本句为多重复合句，主句It may be that…为固定句型，It指代上文提到的the effect…than in males。②that引导主句的表语从句包含more stable…than do temperate birds的倒装结构，其正常语序为more stable…than temperate birds do。③which引导定语从句修饰先行词tropical birds“热带鸟类”，说明这些热带鸟类面临更激烈的竞争，for food and nesting sites作后置定语修饰competition；该定语从句中还包含because引导的原因状语从句，说明热带鸟类面临更激烈竞争的原因；tropics为复数名词，指the tropical zones“热带地区”。
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