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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.
Which of the following is NOT identified in paragraph 5 as a need that some animals can better satisfy when they live with other members of their species?
选项 A、The need to find food
B、The need to locate breeding sites
C、The need to preserve body heat
D、The need to protect themselves from attackers
答案B
解析 本题属于否定事实信息题，问在第5段中，没有提到哪项需求是群居生活才能更好地实现的。B项“它们需要找到繁殖地”在第5段中找不到事实依据，且该段第2句提到的是寻找优越的觅食地(feeding sites)，而不是繁殖地(breeding sites)。B项不符合原文说法，符合题意，故选。A项“它们需要觅食”、C项“它们需要保存身体热量”和D项“它们需要防御攻击”均可以在原文第5段中找到事实依据，不符合题意。
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