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  3. SOCIALITY IN ANIMALS (1) Social insects represent the high point of invertebrate evolution. Some species live in communities

SOCIALITY IN ANIMALS (1) Social insects represent the high point of invertebrate evolution. Some species live in communities

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问题                                             SOCIALITY IN ANIMALS
    (1) Social insects represent the high point of invertebrate evolution. Some species live in communities of millions, coordinating their building and foraging, their reproduction, and their offspring care. Yet sociality is found in only a few species of insects, and is rare among vertebrates as well: wildebeest (large antelope) and lions are the exception rather than the rule. Nearly all fish, amphibians, reptiles, birds, and mammals are solitary, except when courting and mating. Birds and mammals usually rear their young, but year-round family groups are almost unknown, though they are intensely studied where they do exist. The same is true for insects.
    (2) We know, or think we know, that social groups are good. [A] Wolves are better predators when they hunt in packs, and pigeons escape from falcons far more often when feeding in flocks. [B] Group building projects—the dams beavers build to block a body of water provide them with relative safety from predators and the lodges they build for shelter, for instance—can provide a high level of protection and comfort. [C] Why, then, are social species so very rare? [D] In fact, living socially presents inevitable problems that transcend habitat needs so that only when these costs are offset by corresponding benefits is group living a plus.
    (3) The most obvious cost is competition. All the members of a species share the same habitat; when they live together, they are trying to eat the same food and occupy the same nesting sites. In general, there is far less competition away from a group, and selection should favor any individual who (all things being equal) sets off on its own, leaving the members of its group behind to compete among themselves for limited resources. Another difficulty is that concentrations of individuals facilitate disease and parasite transmission. On the whole, social animals carry more parasites and species-specific diseases than do solitary animals. Parasites and diseases diminish the strength and limit the growth of animals, and among highly social creatures, epidemics can devastate whole populations. Distemper (a viral disease) has been known to wipe out entire colonies of seals, for instance. So the penalty of social life is potentially huge.
    (4) But in some instances, the payoffs can be even greater. Two have already been mentioned: cooperative hunting and defensive groups. Social hunting is likely to evolve where prey is too large to be taken by individuals operating alone. To capture wildebeest some members of a group of lions follow their prey and herd them toward others lying in ambush. In other species, individuals forage or hunt simultaneously and share the food. Vampire bats that have had a bad day, for instance, are fed by more successful members of the community, but they are expected to return the favor in the future. Cooperation can even involve sharing information about the location of food. Some colonial birds, such as bank swallows, use the departure direction of a successful forager (food hunter) to locate concentrations of prey. Information transfer can be unintentional though some species make use of special assembly calls or behavior.
    (5) Cooperation in group defense, such as we see in circles of musk oxen or elephants, is quite rare among vertebrates but is prevalent among the social insects. The strategy of employing many eyes to watch for danger, on the other hand, is widespread in birds and mammals. A herd of gazelles (small antelope) is far more likely to spot a lurking lion or a concealed cheetah than is a lone individual, and at a greater distance. In fact, a group enters into a kind of time-sharing arrangement in which individual antelopes alternate biting off a mouthful of grass with a period of erect and watchful chewing. A larger group can afford more bites per individual per minute, there being more eyes to scan for danger. For a small antelope living in a forest where visibility is limited, however, remaining hidden is probably a better bet than assembling into noisy herds.
    (6) Among the millions of species of insects, only a few thousand are social. Those rarities are generally confined to termites and Hymenoptera. All termites are social: their diet (cellulose) requires that each generation feeds a special kind of bacteria or fungi to the next generation to aid in its digestion. Of the numerous hymenopterans, some are social—including all ants and a few bees and wasps—but many are solitary.
What does paragraph 3 say about the relationship between natural selection and animals that live apart from other members of their species?
选项 A、Natural selection does not favor living apart because individuals are unable to obtain resources equal to those of group members.
B、Natural selection does not favor living apart because of the intense competition for resources among individuals apart, from a group.
C、Natural selection favors living apart because individuals are unlikely to attract the attention of predators.
D、Natural selection favors living apart because group living increases competition for limited resources of food and nesting sites.
答案D
解析 本题属于事实信息题，询问自然选择和离群索居的动物之间的关系，第3段第3句提到，自然选择会更加有利于独自启程、留下其他团体成员相互竞争有限资源的个体，故选D项“自然选择更有利于独居的动物，因为群居使争夺食物和巢穴等有限资源的竞争更加激烈”。A项和B项的观点都是“自然选择不会更加有利于独居动物”，和原文相悖。C项“自然选择更有利于独居动物，因为个体不太可能吸引捕食者的注意”没有依据。
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