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 freeranging
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 susus 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
echolocation1. 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.
1. echolocation:
the perception of objects by means of sound wave echoes.
Answer the
questions below using no more than three words from the passage for each
answer.
1 Which ofthe
senses is described here as being involved in mating?
2 Which species
swims upside down while eating?
3 What can
bottlenose dolphins follow from under the water?
4 Which type
of habitat is related to good visual ability?
5 Which of the senses
is best developed in cetaceans?
