题目内容
The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, (31) this is largely because, (32) animals, we stand upright. This means that our noses are (33) to perceiving those smells which float through the air, (34) the majority of smells which stick to surfaces. In fact, (35) , we are extremely sensitive to smells, (36) we do not generally realize it. Our noses are capable of (37) human smells even when these are (38) to far below one part in one million. Strangely, some people find that they can smell one type of flower but not another, (39) others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate (40) smell receptors in the nose. These receptors are the cells which sense smells and send (41) to the brain. However, it has been found that even people insensitive to a certain smell (42) can suddenly become sensitive to it when (43) to it often enough. The explanation for insensitivity to smell seems to be that the brain finds it (44) to keep all smell receptors working all the time but can (45) new receptors if necessary. This may (46) explain why we are not usually sensitive to our own smells—we simply do not need to be. We are not (47) of the usual smell of our own house, but we (48) new smells when we visit someone else’s. The brain finds it best to keep smell receptors (49) for unfamiliar and emergency signals (50) the smell of smoke, which might indicate the danger of fire.
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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 account for this relative constancy. The first theory attributes a relatively constant population to periodic climatic catastrophes that decimate populations with such frequency as to prevent them from exceeding some particular limit. In the case of 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 exert the same regulatory effect on population regardless 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 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 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 (垂体) and adrenal glands (肾上腺) 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 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 displays, such as winter roosting aggregations or group vocalizing; such codes provide organisms with information on population size in a region so that they can, if necessary, exercise reproductive restraint. However, Wynne-Edwards’ theory, linking animal social behavior and population control, has been challenged, with some justification, by several studies. Which of the following may be a density-independent factor that affects animal population
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