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 evolvedecholocation 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 dolphins 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 dolphins 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 dolphins 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 dolphins 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 dolphins brain as an image of the object. This echolocation sense seems to be closely integrated with the dolphins 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.
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 dolphins 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 dolphins 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 dolphins 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.
It has been suggested that dolphins may possess a magnetic sense that enables them to use the Earths 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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