How Dolphins Perceive Their World
The marine world is a dynamic environment filled with a myriad
of life forms, noises, topographical features, differing substrates,
varying water qualities and movements, temperature variations, pressure
variations, chemical variations, and objects of various sorts. Many
or all of these things may be important for the dolphin to sense,
interpret, and act on to protect or enhance its well-being. To achieve
that favorable outcome, dolphins have evolved or adapted sensory
specializations that enable it to monitor its environment and take
advantage of opportunities. Oceanic dolphins have retained the basic
senses of vision, hearing, taste and touch, but smell has been abandoned.
The olfactory receptors and the olfactory lobes of the brain were
apparently lost during the migration of the nostrils to the top
of the head, and because olfaction was no longer useful to an air-breathing
mammal that spent most of its time underwater and no longer sniffed
the air. Other, more useful, senses evolved—echolocation and,
apparently, magnetic-field detection.
Hearing and echolocation
The underwater world is filled with sound, providing information
to the listener on such diverse things as vocalizing schoolmates,
shoaling water, and prey locations. In response to the advantages
of perceiving and interpreting underwater sounds, the dolphin’s
hearing and sound production systems have undergone extensive modifications.
The external ears have disappeared, resulting in a more streamlined
body shape better suited to rapid swimming. New sound pathways to
the inner ear have evolved, including broad area around the sides
of the dolphin’s head and the fat-filled spaces of the lower
jaw. Each of the two inner ears is isolated acoustically from the
other, enabling the dolphin to precisely locate the sources of underwater
sound, something that is very difficult for humans immersed in water.
Hearing is remarkably acute throughout a broad range of frequencies,
and the dolphin is capable of distinguishing small differences in
the frequency (pitch) of sounds. One of the types of sounds produced
by the bottlenosed dolphins and some other dolphin species, is the
whistle, a narrow-band continuous sound that varies in its frequency.
Individual dolphins tend to have unique whistle sounds, called “signatures,”
whose changing frequency characteristics can be easily detected
by the listening dolphin and often identify who among the schoolmates
is whistling. Additionally, the inner ear has been modified to allow
for the perception of high-frequency sounds, reaching some ten times
or more above the upper limit of adult human hearing. The ability
to sense these high-frequency sounds is vital for the dolphin’s
echolocation sense, sometimes called its active sonar system, because
it allow the dolphin to detect very small objects. Series of very
short duration, high-intensity, broad-band clicks containing frequencies
as high as 120-kHz are projected in a narrow beam from the region
of the dolphin’s melon and broadcast in front of the dolphin
into the adjoining waters. When the clicks strike an object, echoes
are returned and sensed by the dolphin through its special pathways
for hearing. Recent research suggests that these echoes may preserve
the spatial structure or shape of the reflecting object and be interpreted
by higher center of the dolphin’s brain as an image of the
object. This echolocation sense seems to be closely integrated with
the dolphin’s visual sense, allowing it to easily relate things
heard to things seen. The echolocation signals of dolphins can penetrate
through many objects, revealing their inner structures. Spotted
and bottlenosed dolphins, for example, have been observed echolocating
into the sand bottom of the ocean, searching through sound for hidden
fish and rooting them out with their snout.
Vision
Because of their exceptional hearing capabilities, their echolocation
sense, and the importance of sound in the underwater world, all
dolphins were thought of as primarily “auditory animals.”
Vision was believed to be not particularly well developed, even
in oceanic dolphins, and of secondary importance at best. We now
know, however, that oceanic dolphins, such as the bottlenosed dolphin,
have excellent vision, and that vision plays an important part in
their natural world. Research has shown that dolphins can see equally
well both underwater and in air. In contrast, we humans are visually
handicapped underwater unless we wear a facemask or goggles. The
dolphin instead seems to rely on adaptations of the optical qualities
of its cornea and lens to allow it to see well in air or underwater.
The dolphin eye, though lacking color vision, is highly sensitive
to light in the blue region of the visible spectrum, which is in
keeping with its off-shore blue-green underwater world. Dolphins,
like dogs and cats, have “eye shine,” produced when light
reflects back out of the eye from “mirror cells” located
behind the receptor cells of the retina. Mirror cells act to amplify
the light entering the eye, making it possible for the dolphin to
see well at night or in the dim light of its underwater world. Dolphin
eyes are laterally placed, providing a wide field of view enabling
the dolphin to see forward, laterally, and even rearward. Also,
unlike our eyes, the dolphin’s eyes act independently. For
example, as the dolphin swims on its side, the pupil of the downward
looking eye, gazing into the dark, may be fully open, while the
pupil of the upward looking eye, gazing toward the bright surface,
may be tightly constricted. In the wild, underwater vision helps
the dolphin to capture swift-moving prey and to keep in contact
with schoolmates swimming nearby. It also helps the dolphin to identify
its own species, recognize familiar individuals from its social
group, and to interpret behavior in the context of age, size, and
sex differences of individuals. In-air vision helps the dolphin
to keep in contact with more distant leaping members of its group,
to detect circling birds that may indicate the presence of prey
fish, and to recognize land features in its coastal habitat. Laboratory
studies have confirmed that vision is an important information source
for the dolphin. Dolphins can attend to and interpret human gestures,
follow the direction in which a human is pointing, monitor rapidly
occurring visual symbols appearing on a television screen and report
the occurrence of certain key symbols, and easily recognize the
same objects across the senses of vision and echolocation. It is
clear, therefore, that we should regard dolphins as both auditory
and visual specialists, making their way though their world through
both sound and vision, just like us.
Taste and touch
The areas of the tongues of adult dolphins that lie closer to its
base appear to have pits and bud-like structures that may represent
taste receptors. . Studies have shown that dolphins can in fact
detect the four primary taste qualities of salt, bitter, sweet,
and sour, although sensitivity to these qualities seems to be about
an order of magnitude less than human sensitivity. Nonetheless,
there is value in being able to detect the various chemicals in
the water that are produced by biological organisms or by natural
chemical processes. For example, large aggregations of prey species
will leave or disperse chemical traces of their presence (such as
excretory products) that may guide the dolphin toward that location.
Likewise, schoolmates may also be detected though their chemical
traces produced by frequent urination. Different “tastes’
of water may offer cues to prey presence or may possibly assist
in recognition of certain regions of their habitat that may have
distinctive chemical composition. More importantly, perhaps, taste
may help in communication about the sexual or receptive state of
other members of their species. It has been suggested that excreted
urine may contain pheromones (chemical messengers) that may be sensed
though taste, and that may signal the sexual state of the urinating
animal or even function as an alarm signal.
The skin of dolphins is richly provided in nerve endings. Studies
examining the dolphin’s sensitivity to touch have found highly
sensitive areas around the blowhole, eyes, snout, lower jaw, and
melon, areas where nerve bundles are especially prominent. . The
measured sensitivity in some of these areas was comparable to the
levels of sensitivity of humans in the most sensitive skin areas,
such as the fingers and lips. Sensitive touch receptors may help
the dolphin in feeling and snapping at prey near its mouth, an areas
that may not be visually available to the dolphin. Touch also seems
to be part of the normal social interactions among dolphins as they
brush against one another, deliberately or otherwise. Pressure,
as well as touch, may be sensed by the skin receptors, and may provide
useful information during diving or high-speed swimming. The dolphin’s
body, except for pectoral fins, dorsal fin, and tail flukes is well
insulated by a thick blubber layer. Detection of temperature, therefore,
if used by dolphins, would likely be restricted t these appendages
without blubber, although there appear to be no studies of skin
sensitivity to temperature stimuli.
Magnetic sense
It has been suggested that dolphins may possess a magnetic sense
that enables them to use the Earth’s magnetic field for orientation
and navigation. Evidence for a magnetic sense is still tentative
and consists in part of some correlations between regions where
many stranding of dolphin species have occurred and the presence
of geomagnetic anomalies or disturbances in these region s. Other
suggestive evidence comes from the presence of iron oxide crystals
(magnetite) in various locations in the head region of some dolphin
or whale species. Recent satellite tagging of several humpback whales
off the coast of the island of Kauai in the Hawaiian chain have
shown that they seemed to deviate very little form a magnetic north
heading during their long journey back to the feeding grounds in
the higher latitude near Alaska.
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