Sensory Exotica: A World Beyond Human Experience by Howard C. Hughes

MIT Press, 1999. $26.95 (345 pages)ISBN 0 262 08279 9Back in 1974 Thomas Nagel posed the philosophical question, ‘What is it like to be a bat?’1xWhat is it like to be a bat?. Nagel, T. Philos. Rev. 1974; 83: 435–450CrossrefSee all References. Although we can never precisely know the answer to this question, thanks to the seminal work of Donald Griffin and colleagues2xListening in the Dark: The Acoustic Orientation of Bats and Men. Griffin, D.R. See all References a considerable amount of information is known about the mechanisms of how a bat perceives its world. As a result of modern technology, we now have techniques that allow us to detect and analyze the sensations of other animals that we would not normally be able to detect with our own sensory apparatus. Indeed, considerable progress has been made in discovering how a range of different animal species perceive their worlds and process sensory information essential for their survival. Examples include the echolocation of dolphins, the ultraviolet vision of birds and bees, and the electric communication found in some species of fish.So why study the sensory abilities of other animals? It is easy to see why the study of animals that are either evolutionarily related to humans, or possess sensory capabilities similar to our own, is important to understanding the basis of our own ‘sensory toolkit’, in the form of neural mechanisms or sensory apparatus. What is less clear, however, are the reasons for studying animals that have sensory systems with no perceivable relationship to our own. In this book, Howard Hughes delivers a strong case for making these forays into the worldly perceptions of other animals, aside from interesting curiosities.On the one hand this book is unusual in that it is written so that a layperson can follow the arguments clearly, and so will appeal to a wider audience. On the other hand, it is a useful handbook for any biologist or psychologist not acquainted with the field but interested in the mechanisms of animal sensation. Some chapters achieve this goal more effectively than others, such as the first section of the book, Biosonar: Echoes in the Night. By contrast, the final section, The Scents of Attraction, is more populist in its approach and is therefore more speculative. Given that the goal of the book is to describe the sensations of animals, it is not entirely clear why there is a chapter on human pheromones.One major focus of the author’s thesis is the link between a sensation and an appropriate sensory receptor; for example, the detection of light and photoreceptors in the eye. Hughes discusses many sensations that humans don’t appear to possess, such as electric communication. However, he does not describe in any detail the undetectable sensations that we do possess. For example, although we have photoreceptors that can detect light, we cannot perceive ultraviolet light. It seems surprising that the author did not elaborate on the ability, or otherwise, to see ultraviolet wavelengths, especially given the importance of this signaling system in both natural and sexual selection to insects, fish and birds. Another sense that we make use of is that of touch. Although this sensation is important for maternal stimulation of the infant, social bonding and sexual behavior in humans, it is a primary method of interacting with the environment in some non-human animals. The only example of this described in the book is that of the star-nosed mole, and this is only briefly mentioned in the epilogue. Other examples abound. Some species of ants rely on tactile communication for foraging. Cats and rats use tactile information perceived via their whiskers for navigating, which is processed by specialized touch receptors attached to the whiskers that project topographically to specific areas within the barrel cortex.Technological advances have shaped the study of comparative sensory mechanisms and, reflecting this, the author takes a historical perspective. This is a major strength of the book: there are far too few books that adopt a historical approach to understanding the mechanisms of behavior generally and sensory systems in particular. Sensory Exotica is refreshing because the author describes not only what has been discovered, but also how. In so doing, the chapters introducing each of the four sections conjure up the feeling of a detective story. Particularly interesting was the first section describing biosonar; what better way to start than with the title The Discovery, which begins with a detailed description of how 20 million Mexican free-tailed bats in Texas leave the depths of the Bracken Cave around dusk each day to catch insects. He then goes on to describe the bats’ ability to navigate, from the pioneering studies of Lazzaro Spallanzani to the current neurobiological research that was largely stimulated by Donald Griffin’s findings in the 1930s. This pattern is repeated throughout the book, in sections discussing biological compasses, electroreception and pheromones. The last section, however, does not live up to the promise of the previous sections, lacking detailed mechanistic explanations and with less emphasis on how and why these discoveries were made.In summary, Howard Hughes’ book provides an intriguing glimpse of animal minds through their sensory experiences and successfully argues that this research provides us with an opportunity to learn more about our place in the natural order, our evolution and some of our inadequacies. To our knowledge, this book is unique in that the discussion of sensory systems is not concerned only with communicating with conspecifics, but with other functions, such as predation, foraging and navigation.