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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
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.
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