A

River

Niagara Falls, Ontario

Niagara Falls, NY

I American Falls

•'Goat Island , Horseshoe Falls

Niagara River

The Identity of Permanent Eddies

Just downstream from Niagara Falls, there is a permanent eddy in the Niagara River, called simply The Whirlpool (Fig. 1.1). The Whirlpool is located at a dogleg turn of the river, at an old plunge pool formed when the Falls were located downstream from where they now are. As in all eddies, it is hard to see where The Whirlpool begins or where it ends. Nevertheless, The Whirlpool does seem to have an identity. It has a proper name. Its existence is obvious to anyone who looks over the cliff above the river. Its location can be found on maps. There is even a plaque on the walk along the riverbank that describes it. So, despite there being no obvious boundary separating The Whirlpool from the rest of the Niagara River, it does seem to have an identity. From where, then, does The Whirlpool's identity come?

The Whirlpool seems to derive its identity not from its distinctiveness—that is, in a clear demarcation between Whirlpool and not-Whirlpool—but from its

Figure 1.1 The Whirlpool is located downstream from Niagara Falls. Inset: Detail of The Whirlpool, showing general trajectory of flow.

persistence. Like Jupiter's Great Red Spot, The Whirlpool has persisted long enough for cartographers to put it on their maps and for landscape architects to incorporate it into their park designs. In this sense, The Whirlpool is like an organism: both are persistent in the way an eddy in the wake of a boat is not. So, perhaps we need to explore the analogy a bit further and ask: what is it about organisms and permanent eddies like The Whirlpool that confers on them persistence?

The Whirlpool is a persistent feature of the Niagara River for two reasons. First, the flow of the Niagara River provides a steady source of energy and matter to keep it swirling. Second, the flowing water interacts with specific structural features of the riverbed to channel the flow in a particular way: the dogleg turn of the river forces the water to change direction as it flows past—water, having mass and inertia, will resist this change—and the old plunge pool provides a venue for the dissipation of the water's inertia before the water is forced into the sharp turn. Both act together to modify the field of potential energy driving water down the river. The result is The Whirlpool.

For as long as it persists, an organism also modifies energy flowing through it, albeit in a very different way. An organism's persistence comes from the tangible boundary separating it from its environment. Even though it seems quite solid, an organism's outermost boundary is actually very permeable, allowing a steady stream of matter and energy to pass continually through it. But, the boundary is not passively permeable, as a sieve would be. Rather, it exerts adaptive control2 over the flows of matter and energy across it. Here is the real breakdown in the analogy between a permanent eddy like The Whirlpool and an organism. Turn down the source of potential energy driving The Whirlpool (which the engineers of the New York Power Authority can do by diverting water away from the Falls), and The Whirlpool disappears. Turn down the potential energy driving matter and energy through an organism, and the organism will alter the nature of the boundary separating it from its environment so that it can maintain that flow. It is not the boundary itself that makes an organism distinctive, but what that boundary does. In other words, the boundary is not a thing, it is a process, conferring upon the organism a persistence that endures as long as its boundary can adaptively modify the flows of energy and matter through it.

A curious and paradoxical consequence arises from following the analogy between eddies and organisms

2. I am using adaptive in the engineering sense of maintaining a state in the face of changing conditions, like thermostatic control of room temperature. In biology, adaptation is a loaded word, in part because it has been used carelessly and recklessly (what animal is not "wonderfully adapted to its environment"?) and in part because it implies a purposefulness that is anathema to many evolutionary biologists.

as far as we have: the obvious and seemingly clearly demarcated boundary separating the organism from its environment disappears. To see why, take the analogy just one step further. An eddy is a consumer of energy, taking in kinetic energy in flowing water and dissipating it as heat. An eddy like The Whirlpool has an indistinct boundary because this inward flow of energy and matter also influences flows elsewhere in the Niagara River. The strength of this influence diminishes with distance: its effects are easy to see close to The Whirlpool's center but become less distinct the further upstream or downstream you look. Nevertheless, the presence of The Whirlpool leaves an imprint on the flows of matter and energy that extends rather far from the obvious center of its activity. In the jargon of thermodynamics, The Whirlpool is at the center of a field of potential energy that both drives energy or matter through it and that, in turn, is influenced by The Whirlpool's presence.

Now consider a bizarre question. What would have to happen to make The Whirlpool behave more like a living thing? We have already ruled out adaptive control of the flow of matter and energy across a tangible boundary, like that which occurs in an organism, because turbulent eddies have no tangible boundaries. Suppose, however, that one night, when the New York Power Authority engineers divert water away from the Falls, The Whirlpool effects a change in the shape of the riverbed surrounding it, perhaps by forcing the riverbed downstream to sink in response to the diminished potential energy upstream. In this fanciful scenario, The Whirlpool might persist even in the face of the changing field of potential energy. In other words, if The Whirlpool could persist by adaptively modifying structural features of the environment surrounding it, the distinction between The Whirlpool and an organism—the adaptive control of the flows of energy and mass—would disappear. Could The Whirlpool then fairly be said to be "alive"? Well, that would be stretching the analogy further than even I am comfortable with, but I hope you would agree that we are now in that gray area Mark Twain referred to in his famous wisecrack about the identity of the author of The Iliad and The Odyssey: it was either Homer or another blind Greek poet with the same name.

However, it is precisely this "fuzzy" boundary between living and nonliving that is at the crux of the physiology of the extended phenotype. If The Whirlpool can be nudged closer to the realm of the living by conferring upon it the ability to adaptively modify its environment, then what should we think about organisms that do the same? If an organism modifies its environment for adaptive purposes, is it fair to say that in so doing it confers a degree of livingness to its apparently inanimate surroundings? If we agree, just for the sake of argument, that it does, then the boundary between organism and not-organism, the boundary that seems so tangible—so obvious—to our senses of vision and touch, dissipates into an indistinct blur, much as a turbulent eddy merges imperceptibly into the water surrounding it.

The Physiology of the Environment

The idea that organisms are integral with the world outside them, like the notion of the extended pheno-type, is not new but neither is it an idea that sits comfortably with modern biology, especially neo-Darwin-ian biology. Consider the simple example of adaptation to some aspect of the environment, say temperature. Generally, organisms seem to have evolved to function well at the prevailing temperatures they normally experience. So, for example, a desert pupfish and an Antarctic ice fish live in very different temperature regimes, yet they each seem to function well in their own environments. Take a pupfish and an icefish and move them to each other's environments, however, and you will soon have two dead fish. In short, the two species have adapted to function well in their respective, albeit very different environments.

We have a pretty good idea how this process of adaptation works. The conventional story goes something like this: A cohort of individuals exists in an en vironment with a certain temperature. Because there will be variation in how well the individuals function at the prevailing temperature, there will be variations in the ability of the members of the cohort to reproduce. To the extent that these functional variations are genetic, the genetic attributes that confer "good" function will translate into high fitness and will be passed on to the next generation. Those that confer "poor" function will not be. The result over many generations will be adaptation, in the evolutionary sense.

What happens to this pretty picture, though, if you suggest that there is no real division between an organism and its environment? The notion of adaptation to the environment thus becomes problematic, because how can an organism adapt to itself? Even more strange, in this view the environment and not just the organism, never mind the genes in the organism, can be subject to selection and adaptation. In other words, the environment, and not just the organism, can have fitness. This kind of thinking gives many biologists fits, as is clear in dogmatic statements like the following: "Adaptation is always asymmetrical-organisms adapt to their environment, never vice versa"(emphasis added).3 Nevertheless, the problem of just what the organism is and its proper relationship with the environment is too big (dare I say too obvious?) to be confined by dogma, and biology, fortunately, is returning to this problem in a serious way.

This book is undertaken very much in that spirit, and it is built around the simple idea that structures built by animals are akin to The Whirlpool's "adaptive modification" of the bed of the Niagara River. By structurally modifying the environment, I will suggest, organisms manipulate and adaptively modify the ways energy and matter flow through the environment. In so doing, they modify the ways energy and matter flow through them. Thus, an animal's physio

3. G. C. Williams, "Gaia, nature worship, and biocentric fallacies," Quarterly Review of Biology 67 (1992): 479-486.

logical function is comprised really of 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.

I have organized my argument for this view into roughly three parts. The first section, comprising Chapters 2 through 4, will build the notion of a physiology that extends outside the conventionally defined boundaries of the organism. Chapter 2 delves into a basic discussion of what physiology is and of the thermodynamic principles governing all physiological function, whether it be internal or external. My overt agenda in that chapter is to convince you that external physiology can exist, that the environment can have physiology. Chapter 3 continues the line of thinking begun in Chapter 2 but focuses more specifically on how external physiology can work. I conclude Chapter 3 with a brief and very general discussion of how, practically, animal-built structures can modify the flows of energy and matter in the environment. Chapter 4, the end of the beginning, explores the apparently spontaneous emergence of orderliness in living systems and outlines a specific example. The interesting feature of this example will be the emergence of physiological function operating in the environment at a scale many times larger than the organisms that generate it. This imposition of orderliness at a large scale, I shall argue, is at the heart of the ability of organisms to be architects and engineers of their environment.

Chapters 5 through 11 represent the biological heart of the book. Each chapter explores how particular animal-built structures function as external organs of physiology. Chapter 5, for example, examines the indivisible link between permanent structures like coral reefs, or the "bodies" of sponges, and the flows of energy and matter in the environment. Chapter 6, on the tunnels dug by invertebrates in marine muds, argues that these structures are devices for tapping one of the largest potential energy gradients on the planet, the oxidation-reduction potential between our oxygen-rich atmosphere and the reducing muds that are a remnant of the early anoxic Earth. Carrying this discussion to the terrestrial sphere, Chapter 7 considers how earthworms manipulate the physical properties of the soil environment: in so doing, they make the soil an "accessory kidney" that enables them to survive an otherwise forbidding environment. Chapter 8 looks at woven structures, like silken webs of diving spiders and certain types of aquatic cocoons, that serve as accessory lungs and gills. Again, the theme in this chapter is the functioning of an external physiology to create an environment in which the organism's internal physiology may be maintained. Chapter 9 takes an unusual turn, proposing that leaf galls are animal-built structures that serve to modify leaf microclimates. As part of a rather speculative discussion, I suggest that galls change for the energy budgets of leaves in favor of the parasites infesting them. Chapter 10 presents animal-built structures as communications tools, focusing on the "singing burrows" of mole crickets. Finally, Chapter 11 explores the interaction between structure and physiological function in the nests of social insects, culminating in a discussion of what I regard as the most spectacular animal-built structures on the planet, the large mound nests constructed by certain species of African termites. These mounds, I assert, are not simply houses for the colony but are accessory gas-exchange systems that confer on the termites the power of adaptation to a wide range of environmental conditions. The interesting twist here is that the mounds function at a scale many times larger than the creatures building them. How they create such a system is a fascinating problem in biology, one that cannot be fully understood, I think, without understanding the external physiology that underlies the phenomenon.

For the final section, Chapter 12 returns to the theme of the extended phenotype and the many ways in which animal-built structures illustrate it. I frame the chapter around a discussion of the Gaia hypothesis, which asserts that the Earth is a singular living thing, an entity whose biota are engaged in a massive global physiology. Gaia is, I shall argue, simply the extended phenotype taken to its logical conclusion. I must note, however, that a physiological approach practiced on a global scale leads one to conclusions about evolution, natural selection, and adaptation that, I think it fair to say, sit uneasily with mainstream evolutionary biology.

The only interesting questions left in biology are molecular. —apocrypnaL statement sometimes attributed to james watson chapter two

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