by
Reviewed by
MIT Press, 1999
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Review
Observers of the animal world have long explained mysterious behavior in terms of instinct, a catchall word for "we really don't know." That is beginning to change.
| Sensory Exotica explores uncommon sense. |
Sensory Exotica, by Howard C. Hughes of Dartmouth College, explores the previously unseen, unheard realm in which bats stalk insect prey in the night sky and dolphins navigate through the depths of the ocean. These and other familiar creatures, such as trout and salmon, migratory birds, bees, and ants, are guided by incredibly sophisticated sensory organs that have largely remained beyond the intellectual grasp of humans until recent times.
Hughes' book is an entertaining survey of current understanding of these parallel states of existence. He covers tentative conclusions and educated theories, as well as established facts concerning the amazing varieties and capabilities of animal sensory systems that remain unfamiliar to those of us limited to five classic senses.
| "Exotic" senses evolved. |
Hughes groups the "exotic" senses into four categories: biosonar, biological compasses, electroreception, and scents of attraction. Each is the product of evolutionary adaptation to the natural world. The book's title, Sensory Exotica, is a thoughtful, eloquent expression of the cleverness and beauty of creation as expressed through natural selection.
The book begins by analyzing the biosonar systems of bats and dolphins. These are among the earth's most superbly adapted mammals. Both species have mastered sound waves for navigation and for hunting. So acutely sensitive is the auditory detection skill of Pallid bats, for example, that they have been observed stalking centipedes, apparently attracted by the sound of their footsteps!
| In navigational ability, bats rival night-flying planes. |
The anatomical mechanisms that have evolved rival and are often superior to the complex technologies created by human engineers. The echolocation detection of the mustache bat, for instance, compares favorably - it is better, in some respects - to the navigation equipment of night-flying aircraft. That is both a sobering and inspiring reflection when considering the size of bats that use biosonar. These microchiropteran bats, unlike the larger fruit-eating megachiropteran species, are quite small, sometimes no bigger than the human hand.
Hughes examines the auditory and neurological systems of bats to show how these flying mammals process the echo frequencies that enable them to detect prey. Dolphin biosonar receives a similar analysis, with particular attention given to the way these marine animals are able to emit signals underwater.
| Fat in dolphin foreheads is an acoustic tool. |
Dolphins, unlike bats, must deal with an air-water interface. Although water conducts sound, it also creates an acoustic impedance factor that must be overcome if the dolphin is to communicate or navigate by emitting sonar signals. A dolphin deals with this problem by means of a melon-shaped deposit of fat tissue under its forehead. It is this unique structure that enables them to transmit an acoustic signal from their nasal cavities into the water, thus initiating the biosonar process.
These examples of adaptation by bats and dolphins to their respective environments through biosonar are matched by other feats of evolutionary engineering, involving no less extraordinary systems for sensing the environment. Among the more impressive examples is that provided by marine bacteria that live without oxygen in the sediment of lake bottoms and ocean floors. Each of these anaerobes contains a particle of magnetite that responds to the Earth's magnetic field. These magnetosomes enable the bacteria to orient themselves to the lake or sea floor when exposed to harmful oxygen that is stirred into the sediment by water currents.
| The magnetic oddballs may save the species. |
This amazing, if entirely passive, escape maneuver is not the whole story. Hughes notes that in the Northern Hemisphere, most of these bacteria have north-seeking magnetosomes, which guide them in the right direction for safety. The opposite is true in the Southern Hemisphere. Significant numbers in both parts of the globe, however, have their magnetosomes oriented in the inverse direction. The reason? Hughes says that the magnetic poles of the earth have shifted over the ages. In the event of a reoccurrence, the populations of marine bacteria would be saved from extinction by these exceptional members of their population.
These sensory attributes appear to be "exotic" to us because we have an egocentric perspective. This limiting, if understandable, view of nature has obscured insights into animal behavior even when accurate and conclusive evidence has been available. Experiments on bats during the late 1700s, for example, determined that these night-flyers navigated by hearing rather than sight. Yet these findings were misinterpreted in favor of theories invoking a "sixth sense" based on touch to guide bats. Not until twentieth-century technology in the 1930s enabled scientists to record bat calls and to play them at a level audible to humans were the discoveries first made in the Age of Reason confirmed.
| Similarity: the ears have it, the eyes don't. |
Hughes, who teaches psychology at Dartmouth, delves in considerable detail into human sensory and perceptual systems. As a result, the reader is able to grasp both the similarities and the differences between human beings and their evolutionary kindred. The cochlea in the inner ear, for example, is essentially the same in shape and function for all mammals, while the structure of a bee's eye is radically different from a human's.
The "sensory exotica" of bats, dolphins, and other creatures are remarkable mechanisms for performing specific tasks essential to their lives. By contrast, the nighttime navigational skills of humans or our ability to orient ourselves by the position of the sun seems poor in comparison to the innate abilities of "lower" animals. The human body, however, is a much more complex system and more difficult to understand. It is, to borrow Hughes' description of human vision, a "remarkably acute, general-purpose system."
| Specialized sensory systems make great models. |
The study of the sensory abilities of animals that are restricted to a "few essential functions" is a natural place to begin comprehending human perception. Locating receptor cells, for instance, is proving to be a particularly elusive task in humans, but the successes recorded among less complex organisms such as the marine bacteria are a good start.
Sensory Exotica is itself a "remarkably acute, general-purpose" book, of value to specialist and general reader alike. The abundance of clear, well-designed charts and diagrams complements Hughes' engaging, often witty text. Occasionally, some extra effort is required to follow the details of scientific theory, as in the analysis of electrical frequencies in the biosonar chapter. This is due more to the multidisciplinary nature of the book than to any fault of the author. In Hughes' capable hands, there are few points in his wonder-filled book that remain obscure. He succeeds in making the "exotic" understandable.
Ed Voves is a news researcher for Philadelphia Newspapers Inc., publishers of the Philadelphia Inquirer and Daily News. For the past twelve years, he has written book reviews, author interviews, and other news articles for both papers.
Biosonar is a wonderful example of convergent evolution: unrelated species evolving a similar ability through independent selection pressures. As such, there is no compelling reason that these systems as elaborated in bats and marine mammals should be the same. Just as the differences we've noted relate to differences in their respective environments, so the similarities probably relate to the most remarkable thing about the evolution of species: that they so often find optimal solutions to problems of adapting to the constraints imposed by the physical world. So the similarities are likely the result of a form of coercion by the physical environment itself.
You may purchase this book (344 pp., hardcover) directly from:
Learning and Navigation by Honeybees - a succinct article dealing with the way honeybees judge distance and use landmarks, cognitive maps, and celestial maps to find their way to food sources and then return to the hive.
Mastering the Mysteries of Bird Migration - an interesting interview with Ken Able, a biologist at the University of Albany and a leading authority on bird migration. He discusses the orientation and navigation of Savannah sparrows, which migrate individually using a magnetic compass and polarized light patterns.
Magnetic Cues - a discussion of the classic 1971 experiment by William Keeton that determined that homing pigeons use magnetic compasses to navigate on cloudy days when their sun compasses are obscured.
Interdisciplinary Center for Bioacoustics and Environmental Research - provides an overview of acoustic communication in mammals, abstracts from bioacoustic workshops, a sound library of Mediterranean cetaceans, and more. From the Universitá di Pavia, Italy.
Bat Ecology and Bioacoustics Laboratory - offers an extensive bibliography and collection of Web links. From the University of Bristol.
Chiroptera - provides a general introduction to the life history, ecology, morphology, systematics, and fossil record of bats. From the University of California Museum of Paleontology.
Dolphin Study Group - a rich resource with links to research, news, publications, and databases.
Magnetotactic Bacteria - a great introduction with an extensive bibliography. From California Polytechnic State University.
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