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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.
In paragraph 6, why does the author compare the bluegill to the pumpkinseed sunfish?
选项 A、To show that nesting has different costs and benefits for solitary organisms than it has for organisms living with other members of the same species
B、To provide evidence that social nesting is usually superior to solitary nesting
C、To argue that, in closely related species, the feeding organs of solitary organisms usually differ from those of social organisms
D、To show that closely related species can be social or solitary depending on which behavior better enables them to fight off predators
答案D
解析 本题属于修辞目的题，问在第6段中，作者为什么把蓝鳃太阳鱼和瓜仁太阳鱼作比较。蓝鳃太阳鱼的例子是为了说明群居对一些物种来说有利于抵御捕食者的攻击；而作为蓝鳃太阳鱼同属物种的瓜仁太阳鱼的例子，则是为了说明独居对于这个物种来说更利于抵御捕食者的攻击。另外，结合该段末句“瓜仁太阳鱼并没有因为独居而比蓝鳃太阳鱼更低等或适应能力更差，它们只是从群居生活中获益较少，这使得单独筑巢成为它们适应生存的策略”也可知，作者比较两者是为了“证明近亲物种群居或独居取决于哪种能更好地帮助它们击退捕食者”(D项)。A项“表明筑巢对群居物种和独居物种有着不同的代价与益处”、B项“提证据证明群居筑巢通常优于独居筑巢”和C项“认为在近亲物种中，独居生物的进食器官通常与群居生物的不同”均不是作者的目的。
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