Celebrating blurry boundaries

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The Extended Organism: The Physiology of Animal-Built Structures

Harvard University Press: 2000. 235 pp. $47.50, £32.95

The snail's shell is an integral part of the snail, but the shell of the hermit crab is just clothing. There is no ambiguity here — the snail grows its shell, whereas the crab must shop repeatedly for empty shells that fit. Our hairs make up part of us, but the spider's web is just a structure that it builds. In this example, the distinction works less well. We may remain attached to our hairs, but the spider can (and does) reingest and recycle its fibroin, something we cannot do with our keratin.

Scott Turner begins his book by arguing that what we consider to be the outer boundaries of organisms are mainly habits derived from perceptual accidents. He goes on to assert that such habits have led to an excessively restrictive view of what constitutes an organism, and that such a view has had unfortunate consequences for physiology. He bases his argument not on history, philosophy, cognition or our predilection for sharp dichotomizations, but rather on the weight of examples. With case upon case he shows how the sharp, traditional line between organism and external world often proves at least a nuisance and how, almost as often, we tacitly ignore it. And he concludes that our outlook on how organisms function would be empowered by drawing a more encompassing line.

If applied to a less eclectic field, or if it were the account of a less eclectic scientist, Turner's case-by-case approach might be tedious. But few readers of this book will fail to be fascinated by the examples. Turner's tales of the subtle ways organisms capitalize on the opportunities afforded them by their physical and chemical surroundings provide more than ample reason to read the book. Although no area of biology brings into its purview more diverse elements of the physical sciences than physiology, few physiological accounts consider both the present range of non-biological factors and the full range of plants, animals and microorganisms in the way that Turner does.

On the physical (or mathematical) side, one encounters diffusion, fractals, redox potentials, acoustics, thermodynamics and chemical kinetics, hydrostatics and dynamics, convective and radiative heat transfer, soil mechanics, surface tension, climatology and control systems — each with clear explanations adequate for their concomitant biological stories.

Community life: termite colonies operate communally built ventilation systems. Credit: WOLFGANG KAEHLER/CORBIS

The biological side is no less diverse. One learns about bioconvection and the way biological and physical factors interact to produce large-scale order in populations of swimming microorganisms; about plastrons and the other external bubble-lungs used by aquatic but air-breathing insects and spiders; about the way gall midges manipulate the surface temperatures of solar-heated leaves; and about the thermoregulatory tricks of the aerial bee colonies and subterranean termite colonies that operate communally built ventilation systems.

Turner gives particular attention to two general topics, each providing a set of elaborate cases where physiology must take account of “external” structures. Burrowing appears in multidimensional splendour, from its palaeontology to such things as the mechanics of digging, electron acceptors at different depths and in different substrata, the necessary sensitivity of earthworms to soil-water potential, and the use of burrows as feeding devices. The latter focuses on how lugworms — as cleverly adapted as they are consummately ugly — cultivate microorganisms in their burrows, and provides a splendid mix of zoology, microbiology, physical chemistry and fluid mechanics.

The other general topic is animal acoustics and the way external structures help small animals produce loud sounds whose characteristics are attuned to their functional roles, such as the cricket that improves the emission of its song by cutting a hole in a leaf that then serves as a baffle. The acoustic account then returns to burrows, specifically those of mole crickets and the way in which the shape of these residences allows them to work like different musical instruments — and to the feedback schemes by which mole crickets, like musicians, listen and retune.

Turner's views are less a radical revision than a reminder or rejoinder. Indeed, the argument might be pressed still farther. One can equally well take a narrower view of the organism–surroundings boundary, limiting the organism to intracellular material that participates in the continuous processes of breakdown, excretion, resynthesis and resupply. Hair, horn, exocuticle, even wood then join the domain of burrow, nest and cocoon. The zone of context-dependent definition of an organism extends more certainly inwards than further outwards towards 'Gaia' and superorganisms, with their metaphorical and metaphysical baggage; although, to be fair, these latter get only brief mention in the book.

The attempt to wrap a larger picture around the rich complexity of organisms takes a bit of shoehorning and discrete tucking in of loose shirt-tails — but one could make the same comment about Darwin's Origin. As Mark Twain put it in Huckleberry Finn: “There was things which he stretched, but mainly he told the truth.”

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