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The Extended Organism: The Physiology of Animal-Built Structures (review)
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Perspectives in Biology and Medicine 44.2 (2001) 297-300

Book Review

The Extended Organism: The Physiology of Animal-Built Structures

The Extended Organism: The Physiology of Animal-Built Structures. By J. Scott Turner. Cambridge: Harvard Univ. Press, 2000. Pp. x + 235. $47.50.

When one divides the world into biotic entities, such as plants and animals, and abiotic substances, such as water or stone, where do animal-built structures fit in? The conventional wisdom is that they not alive and therefore abiotic, yet this categorization is unsatisfactory in many respects. Artifacts such as bird's nests or termite mounds have a degree of order that characterizes the living. Moreover, the edifices constructed by animals are alive in the sense that they do work, altering flows of matter and energy between the organism and its environment in a manner that benefits the organism. In this respect, animal structures are external organs of physiology, no different from the more conventionally defined organs such as hearts or lungs.

This is the premise of Scott Turner's stimulating book. By structurally modifying the environment, Turner demonstrates that organisms manipulate and adaptively modify the ways energy and matter flow through the environment and through themselves. In fact, nests, burrows, and other structures that organisms manufacture are much more than dwellings. They are supplementary physiological appendages that organisms construct to act as accessory kidneys, lungs, and foraging tools. Turner argues that animals have two physiologies: "the conventionally defined 'internal physiology,' governed by structures and devices inside the integumentary boundary of the organism, and an 'external physiology,' which results from adaptive modification of the environment" (p. 7).

If at first this appears a little eccentric, ask yourself why? As Turner points out, physiology is essentially the means by which animals use energy to do order-producing work. If animals use energy to do work on the "external" environment, their activity is as much physiology as when they use energy to do work on the "internal" environment. In both cases, the principles of thermodynamics govern how organisms channel energy through their bodies and create orderliness in the process.

In entitling his work The Extended Organism, Turner doffs his cap to Richard Dawkins, the British evolutionary biologist who developed the concept of the "extended phenotype." Dawkins (1982) pointed out that genes frequently express themselves outside the bodies of the organisms that carry them. For instance, the beaver's dam is just as much an expression of the beaver's genes as its teeth or tail, and can be subject to evolution is essentially the same way. For Turner, the extended phenotype is incomplete without an understanding of the underlying physiology. Genes can act outside of the bodies of the organisms containing them only if they can manipulate flows of matter and energy between organisms and their environment. One of the real strengths of this book is the extraordinary yet fascinating detail in which Turner unravels the physics, chemistry, and physiology that underlie how countless organisms construct aspects of their niches. However, while Dawkins' extended phenotype rests comfortably within the mainstream of evolutionary thinking, Turner's extended physiology "is not an idea that sits comfortably with neo-Darwinian biology" (p. 6), and he regards his ideas as more in tune with James Lovelock's Gaia hypothesis, the controversial and somewhat fantastic notion that the planet is a superorganism. In fact, Turner's discomfort stems from a genuine problem in evolutionary biology: there is something wrong with our concept of adaptation.

The Extended Organism is far from a dry physiological textbook. It is a multidisciplinary romp through the wonderfully eclectic and engaging mind of a scientist, one with a real talent for explaining complex ideas in an absorbing manner. The book should be read not just for its provocative theoretical ideas, which are expressed with marvellous clarity, but for the richness of the natural history. Turner bases his argument on a series of compelling examples of organisms that impose orderliness on the environment, frequently at a massive scale.

Take, for example, the humble earthworm. Put a worm through one of those high-school classificatory exercises and it will rapidly become apparent that it has no business living where it does...

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