You should spend about 20 minutes on Questions 15-26 which are based on Reading Passage 2 below.

An examination of the functioning of the senses in cetaceans, the group of mammals comprising whales, dolphins and porpoises

一项对于鲸目动物（包括鲸、海豚、鼠海豚等哺乳动物）的感官功能测试

Some of the senses that we and other terrestrial mammals take for granted are either reduced or absent in cetaceans or fail to function well in water. For example, it appears from their brain structure that toothed species are unable to smell. Baleen species, on the other hand, appear to have some related brain structures but it is not known whether these are functional. It has been speculated that, as the blowholes evolved and migrated to the top of the head, the neural pathways serving sense of smell may have been nearly all sacrificed. Similarly, although at least some cetaceans have taste buds, the nerves serving these have degenerated or are rudimentary.

对于我们和其他陆地哺乳动物来说，一些感官是理所应当的，但是对于鲸来说，这些感官要么衰退或消失，要么在水中无法正常地发挥作用。例如，齿鲸的大脑结构显示，它们不具备嗅觉。然而，虽然须鲸具备与嗅觉相关的大脑结构，人们现在也不知道它们是否起作用。据推测，当它们的（呼吸）气孔进化并移动到头顶时，服务于嗅觉感官的神经通路就已经几乎不见了。同样，尽管还有一些鲸鱼拥有味蕾，服务于它们的神经也已经退化或者变得迟钝了。

The sense of touch has sometimes been described as weak too, but this view is probably mistaken. Trainers of captive dolphins and small whales often remark on their animals’ responsiveness to being touched or rubbed, and both captive and free ranging cetacean individuals of all species (particularly adults and calves, or members of the same subgroup) appear to make frequent contact. This contact may help to maintain order within a group, and stroking or touching are part of the courtship ritual in most species. The area around the blowhole is also particularly sensitive and captive animals often object strongly to being touched there.

有时，人们会认为它们的触觉也很弱，但是这种观点可能是错误的。圈养海豚和小海豚的训练员常常会谈论到它们对于碰触和摩擦的动物反应。而且无论是圈养海豚还是放养海豚，整个族群中个体之间的接触都更加频繁（尤其是成年海豚和幼年海豚之间，或同一亚群的成员之间）。这种接触有利于维护群体内部的秩序，而且对于大多数种类的鲸来说，轻抚和碰触都是求偶行为的一部分。气孔周围的区域是尤其敏感的，圈养鲸类非常反感被碰触那里。

The sense of vision is developed to different degrees in different species. Baleen species studied at close quarters underwater – specifically a grey whale calf in captivity for a year, and free-ranging right whales and humpback whales studied and filmed off Argentina and Hawaii – have obviously tracked objects with vision under- water, and they can apparently see moderately well both in water and in air. However, the position of the eyes so restricts the field of vision in baleen whales that they probably do not have stereoscopic vision.

不同种类的鲸，视觉感官的发达程度也不尽相同。人们在水下近距离研究表明，须鲸（具体来说就人工饲养一年的灰鲸幼崽和阿根廷、夏威夷沿海拍摄的放养露脊鲸和座头鲸）明显具有可以利用视觉在水中追踪猎物，而且它们不论在水中还是在空中都能看得很清楚。然而，须鲸眼睛的位置限制了它们视线的区域，所以它们可能不具备立体视觉。

On the other hand, the position of the eyes in most dolphins and porpoises suggests that they have stereoscopic vision forward and downward. Eye position in freshwater dolphins, which often swim on their side or upside down while feeding, suggests that what vision they have is stereoscopic forward and upward. By comparison, the bottle-nose dolphin has extremely keen vision in water. Judging from the way it watches and tracks airborne flying fish, it can apparently see fairly well through the air–water interface as well. And although preliminary experimental evidence suggests that their in-air vision is poor, the accuracy with which dolphins leap high to take small fish out of a trainer’s hand provides anecdotal evidence to the contrary.

从另一方面看，大部分海豚和江豚的眼睛位置说明它们拥有向前和向下的立体视觉。淡水海豚经常测游，或在进食时肚皮朝上，这说明它们拥有向前和向后的立体视觉。相反的是，宽吻海豚在水下有很好的视力。从它观察和追踪空中飞鱼的方式看来，它穿过水面向空中也可以看得很清楚。而且，尽管初步试验结果表明海豚的空中视力并不好，但它们能从水中跃起很高，并准确地吃到驯兽师手里的小鱼。这就用事实证据表明上述结论是错误的。

Such variation can no doubt be explained with reference to the habitats in which individual species have developed. For example, vision is obviously more useful to species inhabiting clear open waters than to those living in turbid rivers and flooded plains. The South American boutu and Chinese beiji, for instance, appear to have very limited vision, and the Indian susus are blind, their eyes reduced to slits that probably allow them to sense only the direction and intensity of light.

当然，这些变异可以通过个别物种的生存环境来解释。例如，相比居住于混住的河流和洪水泛滥平原上的鲸，对于居住在清澈宽广水域中的鲸来说，视力无疑更加有用。比方说南美洲的亚马逊河豚和中国的白鳍豚视力都很有限，印度恒河豚根本看不见，它们的视力减弱成了两条缝，只能感知到方向和光的强度。

Although the senses of taste and smell appear to have deteriorated, and vision in water appears to be uncertain, such weaknesses are more than compensated for by cetaceans’ well-developed acoustic sense. Most species are highly vocal, although they vary in the range of sounds they produce, and many forage for food using echo- locationv①. Large baleen whales primarily use the lower frequencies and are often limited in their repertoire. Notable exceptions are the nearly song-like choruses of bow-head whales in summer and the complex, haunting utterances of the humpback whales. Toothed species in general employ more of the frequency spectrum, and produce a wider variety of sounds, than baleen species (though the sperm whale apparently produces a monotonous series of high-energy clicks and little else). Some of the more complicated sounds are clearly communicative, although what role they may play in the social life and ‘culture’ of cetaceans has been more the subject of wild speculation than of solid science.

尽管味觉和嗅觉严重退化，水下视力也不确定，这些缺陷却完全可以被它们非常发达的听觉系统所弥补。尽管音域不尽相同，但大部分种类的鲸鱼都很会“唱歌”而且可以利用回声定位来觅食。大须鲸主要使用低频段的声音，而且声音种类很有限。值得一提的例外是夏天里北极露脊鲸歌曲般的合唱和座头鲸复杂又令人难忘的话语。相比须鲸，齿鲸一般可以使用更多的频谱，并且发出更多种类的声音（尽管抹香鲸只能发出一系列单调尖锐的咔哒声）。尽管声音在鲸鱼的社交生活和“文化”中扮演什么角色更是一种胡乱猜测而不是严谨的科学，但显然，一些更复杂的声音更具备交流能力。

①. Echolocation: the perception of objects by means of sound wave echoes.