Section D In this section, there is one passage
followed by five questions. Read the passage carefully, and then answer the
questions in a maximum of 10 words. Remember to write the answers on the answer
sheet.
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. 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. 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 underwater, 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. 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
bottlenose 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. 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 sinus are blind, their eyes reduced to slits that
probably allow them to sense only the direction and intensity of light.
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 echolocation. Large baleen whales
primarily use the lower frequencies and are often limited in their repertoire.
Notable exceptions are the nearly song-like choruses of bowhead 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. Questions: What can bottlenose dolphins follow from under the water