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  3. COST AND BENEFITS OF SOCIAL LIFE (1) Many think that the reason why so many animals live with others of their species is tha

COST AND BENEFITS OF SOCIAL LIFE (1) Many think that the reason why so many animals live with others of their species is tha

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问题                                         COST AND BENEFITS OF SOCIAL LIFE
    (1) Many think that the reason why so many animals live with others of their species is that social creatures are higher up the evolutionary scale and so are better adapted and leave more offspring than do animals that live solitary lives. However, in each and every species, generation after generation, relatively social and relatively solitary types compete unconsciously with one another in ways that determine who leaves more offspring on average. In some species, the more social individuals have won out, but in a large majority, it is the solitary types that have consistently left more surviving descendants on average.
    (2) But how can living alone ever be superior to living together? Under some conditions, a cost-benefit comparison favors solitary life over a more social existence. For example, among most social species, animals have to expend time and energy competing for social status. Those that do not occupy the top positions regularly have to signal their submissive state to their superiors if they are to be permitted to remain in the group. This can take up a major share of a social subordinate’s life. In fact, even in small social groups there are both subtle competition and not-so-subtle competition.
    (3) Social groups also offer opportunities for reproductive interference. Breeding males that live in close association with more attractive rivals may lose their mates to these individuals. In addition, sociality has two other potential disadvantages. The first is heightened competition for food, which occurs in animals as different as colonial fieldfares (a kind of songbird) and groups of lions, whose females are often pushed from their food by hungry males. [A] The second is increased vulnerability to parasites and diseases, which plague social species of all sorts. [B] While it is true that some social animals have evolved special responses designed to combat parasites and diseases, those responses can only reduce, but cannot totally eliminate, the damage caused by those threats, and the responses may even carry their own costs. [C] Thus, honeybees warm their hives in response to an infestation by a fungal pathogen, which apparently helps kill the heat-sensitive fungus, but at the price of time and energy expended by the heat-producing workers. [D]
    (4) If social living carries a heightened risk of infection, then the larger the group, the greater the risk. This prediction holds for cliff swallows, which pack their nests side by side in colonies composed of anywhere from a handful of birds to several thousand pairs. The more swallows nesting together, the greater the chance that at least one bird will be infested with swallow bugs, which can then readily spread from one nest to another.
    (5) The parasites and fungi that make life miserable for swallows and other social creatures demonstrate that if sociality is to evolve, the assorted costs of living together must be outweighed by compensatory benefits. Cliff swallows may join others to take advantage of the improved foraging that comes from following companions to good feeding sites, while other animals, such as male imperial penguins, save thermal energy by huddling shoulder to shoulder during the brutal Antarctica winter. Still others, such as lionesses, join forces to fend off enemies of their own species.
    (6) The most widespread fitness benefit for social animals, however, probably is improved protection against predators. Many studies have shown that animals in groups gain by reducing the individual risk of being captured, or by spotting danger sooner, or by attacking their enemies in groups. Males in nesting colonies of bluegills cooperate in driving egg-eating bullhead catfish away from their nests at the bottom of a freshwater lake. While bluegills have adopted social behavior to avoid predation, closely related species that nest alone have evolved means to protect themselves while nesting alone. Thus, the solitary pumpkinseed sunfish, a member of the same genus as the bluegill, has a powerful biting jaw and so can repel egg-eating enemies on its own, whereas bluegills have small, delicate mouths good only for inhaling small, soft-bodied insect larvae. Pumpkinseed sunfish are in no way inferior to or less well adapted than bluegills because they are solitary; they simply gain less through social living, which makes solitary nesting the adaptive tactic for them.
An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
There are costs and benefits associated with living in groups.
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Answer Choices
(A) Solitary animals are sometimes thought to be higher up the evolutionary scale because they leave more surviving descendants on average than social animals do.
(B) Social animals have evolved fewer mechanisms for reducing the costs of competing for social status than they have for reducing the risks of exposure to diseases and parasites.
(C) The need for protection from predators makes animals adopt social living, but animals that have the ability to individually fight off predators benefit more from solitary living.
(D) Social living has advantages such as making it possible for animals to become more successful at locating food, but it also has risks such as increased competition for mates.
(E) The numerous disadvantages of living together lead animals to adopt social living only when the benefits are greater than the costs.
(F) Animals that live alone such as pumpkinseed sunfish need to evolve a special mouth or some other special organs to help them combat predators.
选项
答案C,D,E
解析 本题属于文章总结题。C项“防御捕食者的需求使动物们适应群居生活，而那些有能力单独击退捕食者的动物则从独居生活中获益更多”是第5段最后一句和第6段的整合，指出群居生活的一个好处：防御敌人或捕食者，是文章的一个主要观点。D项“群居生活有很多好处，比如可以让动物更容易找到食物；但也有风险，比如争夺配偶的竞争加剧”是第3段第2句和第5段第2句的整合，分别阐述了群居的好处和风险，可知D项也是文章的一个主要观点。E项“群居的诸多弊端导致动物们只有在益处大于代价的情况下才会选择群居生活”是对第3到5段的概括，这几个关键段落分别阐述了群居的好处和代价，可知E项也是文章的主要观点。A项“人们有时认为独居动物在进化上处于较高的地位，因为它们通常能比群居动物留下更多的后代”，但文章首句提到，许多人认为群居动物的进化级别更高，它们能比独居动物留下更多的后代。B项“群居动物进化出的减少群体地位竞争代价的机制，比它们减少接触疾病和寄生虫风险的机制要少”缺乏依据，F项“独居动物，如瓜仁太阳鱼，需要进化出一张特殊的嘴或其他一些特殊的器官来帮助它们抵御捕食者”是第6段的细节信息，且该项主要阐述与独居生活有关的内容，不符合题意。
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