Passage 56

     Although numbers of animals in a given region may

   fluctuate from year to year, the fluctuations are often

   temporary and, over long periods, trivial. Scientists

   have advanced three theories of population control to

(5) account for this relative constancy.

   The first theory attributes a relatively constant popu-

   lation to periodic climatic catastrophes that decimate

   populations with such frequency as to prevent them

   from exceeding some particular limit. In the case of

(10) small organisms with short life cycles, climatic changes

   need not be catastrophic: normal seasonal changes in

   photoperiod (daily amount of sunlight), for example,

   can govern population growth. This theory---the

   density-independent view---asserts that climatic factors

(15) exert the same regulatory effect on population regard-

   less of the number of individuals in a region.

      A second theory argues that population growth is

   primarily density-dependent---that is, the rate of

   growth of a population in a region decreases as the

(20) number of animals increases. The mechanisms that

   manage regulation may vary. For example, as numbers

   increase, the food supply would probably diminish,

   which would increase mortality. In addition, as Lotka

   and Volterra have shown, predators can find prey more

(25) easily in high-density populations. Other regulators

   include physiological control mechanisms: for example.

   Christian and Davis have demonstrated how the

   crowding that results from a rise in numbers may bring

about hormonal changes in the pituitary and adrenal

(30) glands that in turn may regulate population by lowering

   sexual activity and inhibiting sexual maturation. There

   is evidence that these effects may persist for three

   generations in the absence of the original provocation.

   One challenge for density-dependent theorists is to

(35) develop models that would allow the precise prediction

   of the effects of crowding.

      A third theory, proposed by Wynne-Edwards and

   termed “epideictic,” argues that organisms have evolved

   a “code”in the form of social or epideictic behavior

(40) displays, such as winter-roosting aggregations or group

   vocalizing; such codes provide organisms with infor-

   mation on population size in a region so that they can,

   if necessary, exercise reproductive restraint. However,

   wynne-Edwards’ theory, linking animal social behavior

(45) and population control, has been challenged, with some

   justification, by several studies.

4.According to the Wynne-Edwards theory as it is described in the passage, epideictic behavior displays serve the function of

(A) determining roosting aggregations

(B) locating food

(C) attracting predators

(D) regulating sexual activity

(E) triggering hormonal changes

答案是D,我认为选A。D项不是第二个theory里提到的吗？Wynne-Edwards theory根本没有提到regulating sexual activity呀。

   termed “epideictic,” argues that organisms have evolved

   a “code”in the form of social or epideictic behavior

(40) displays, such as winter-roosting aggregations or group

   vocalizing; such codes provide organisms with infor-

   mation on population size in a region so that they can,

   if necessary, exercise reproductive restraint. However,

   wynne-Edwards’ theory, linking animal social behavior

(45) and population control, has been challenged, with some

   justification, by several studies.

供参考。倾向D为正确选项.