American Scientist

Cats' Paws and Catapults: Mechanical Worlds of Nature and People. Steven Vogel. 382 pp. Norton, 1998. $27.50.

Any comparison needs a benchmark. A mountain is not a mountain without the foothills or the sea for reference. Unique items or categories resist evaluation and understanding because they are unique. The human penchant for creating technologies ranks alongside language as a distinctive characteristic of our species. Although a few other species such as Galapágos finches and chimpanzees use tools, few shape and manipulate environmental objects to improve their utility the way our species does. Chimps strip twigs off sticks to make better termite catchers, beavers trim and arrange logs and sticks to make dams, various worms and insect larvae select sand grains from their environment and glue them together to make their domiciles, but each of these species is a one-trick pony. Where might one look for an appropriate foil for human technology? Assuming that the U.S. government really doesn't have a flying saucer tucked away in a warehouse outside Roswell, New Mexico, no appropriate comparison comes readily to mind.

Why bother? What's at stake here is a real understanding of what appears to be a basic characteristic of our species, one that may even have predated the origin of Homo sapiens during our evolutionary history. How much of our technology is the natural and inevitable development of a reasoning (if not always rational) species, dictated by the rules of the physical universe? How much is a consequence of our own biological characteristics—the fact that we have an endoskeleton based on levers, are very large animals when viewed in the context of all the metazoans and are terrestrial rather than aquatic? If biology looms large, comparisons with chimps, for example, may tell us very little. A search for extraterrestrial intelligence based on signatures in the electromagnetic spectrum must ask whether a technology that exploits that spectrum, necessarily based on metals and electricity, is inevitable, likely or merely possible for an intelligent species.Cats' Paws and Catapults is aimed at a broad general audience. There isn't an equation to be found in the text, but this doesn't mean that Vogel eschews either rigor or quantification; the relevant mechanics is thoroughly explained and its implications explored, with sufficient quantification that the similarities and differences between biological and human technology are explicit. The book is accessible to a bright and curious high school student and any undergraduate. The prose is often as elegant as D'Arcy Thompson's On Growth and Form but without the mysticism of the latter; this is hard science made easy, not watered down. (However, if you find puns distasteful, beware—they pervade the book.) Unlike many popularizations, the text includes an excellent set of footnotes and a wonderfully eclectic and thorough set of references; those whose curiosity is piqued will not be frustrated by a lack of guidance for where to dig deeper.

The thesis of Steven Vogel's Cats' Paws and Catapults is that an appropriate comparison for human technology does exist: the evolved technology of living organisms. Can biology qualify as a technology? Vogel argues compellingly that it does, and I have to agree. The fact that biological technology has been shaped by natural selection rather than "designed" in the colloquial sense only adds to the power of the comparison; commonalities imply external constraints, whereas differences may arise either from history or subtle differences in context. Such differences may be fodder for our own technology, especially as we move increasingly toward a size scale comparable to biological systems and begin to directly manipulate their genetic underpinnings. One might argue that the biological world includes a host of technologies—mechanical, chemical and computational, to name the most obvious. The subtitle of the book, Mechanical Worlds of Nature and People, establishes the limits to Vogel's purview; the technology of biological chemistry must await another author, whereas the technology of biological computation still remains largely unexplored and poorly understood. Still, the comparison of engineering and biomechanics is rich enough to supply extraordinary insights into both, and indirectly into one of the major evolutionary innovations in the history of this planet. Prokaryotes are biochemical specialists, manipulating chemistry to make their living and to distinguish their niches, but they do remarkably little manipulation of external mechanical forces. Eukaryotes, on the other hand, are mechanical tinkerers, first with their evolution of motor protein systems (actin and myosin; dyenin, kinesins and microtubules), later with the repeated evolution of multicellularity, implying adhesive interactions and the attainment of sizes large enough that external mechanical forces (as opposed to the internal forces that arise from osmotic effects) become significant.

Cats' Paws and Catapults is aimed at a broad general audience. There isn't an equation to be found in the text, but this doesn't mean that Vogel eschews either rigor or quantification; the relevant mechanics is throughly explained and its implications explored, with sufficient quanification that the similarities and differences between biological and human technology are explicit. The book is accessible to a bright and curious high school student and any undergraduate. The prose is often as elegant as D'Arcy Thompson's On Growth and Form but without the mysticism of the latter; this is hard science made easy, not watered down. (However, if you find puns distasteful, beware—they pervade the book.) Unlike many popularizations, the text includes an excellent set of footnotes and a wonderfully eclectic and thorough set of references; those whose curiosity is piqued will not be frustrated by a lack of guidance for where to dig deeper.

But Vogel's book is not just for a popular audience. The professional engineer will find here an introduction to the basics of natural selection, to the remarkable information paucity of biological systems (despite the information density of DNA), to the ubiquity of negative feedback systems at all levels of biological organization, to biological diversity and to biology's intriguingly different solutions to problems the engineer will find familiar. The professional biologist will find a painless introduction to the basic mechanics relevant to plants and animals, with examples of the phenomena that are comfortingly simple and familiar—bookshelves for beams, bundles of spaghetti for columns and as models for crack resistance in composite materials, balloons for hydrostats and a recipe for an edible composite whose toughness can be tailored to taste. There is real insight here, even for those who consider themselves professional biomechanics (as I do). On the consequences of size per se: "Being big means having lots of inside relative to your outside; being small means having lots of outside relative to your inside." And, "The technology of people must be different from that of nature simply because the two span different size scales." On the ubiquity of composite materials in biology: "Composites are what a mindless, blundering, information-starved, and minimally coordinated system might be expected to make.… Their properties are highly sensitive to tinkering with the amounts and arrangements of their constituents on a microscopic level." On the way biological systems produce structures: "What matters here is the strangeness of a manufacturing system [cells] whose products are larger than its factories."

Given the scope of this book (ranging from engineering to biology to archaeology to the history of technology), I found precious little to quibble with. At one point, Vogel seems to imply that bacterial flagella are driven by an electrical motor. On a different subject, not all biological jets involve a positive-displacement pump (like squid); the flatworm Convoluta and salps both use a fluid-dynamic pump (cilia). Nor do biological systems completely ignore the possibility of inertial energy storage; baleen whales use the energy stored in their body's inertia to inflate their throats while suspension feeding, trading the inertial energy for elastic energy, then using the elastic energy to filter the captured fluid.

In a lengthy section on the absence of structural metals in biology, Vogel seems convinced that animals would be better off if they had them. I am not. The ductility of metals underlies their high toughness, but organisms have found other (arguably better) routes to that end. Ductility also underlies the utility of metals in human technology, but I can conceive of no mechanism that would allow an animal to hammer a bent tooth or bone back into shape; controlled wear with continual growth (as in horse, cow, rodent and sea urchin teeth) or continual replacement (as in sharks) is probably a more useful strategy. The argument that the metals we use in our technology (mainly iron and aluminum) were overlooked by evolution because of their low abundance in seawater loses force when viewed in the appropriate geological perspective. Three billion years ago, when free oxygen was a scarce commodity, iron (and uranium) was exquisitely soluble; the great iron deposits of Michigan are the result of the precipitation of that iron when oxidative photosynthesis really took off.

Such minor issues weigh little against the pervasive differences between human and biological technology and the insights into each that the comparison of the two brings. As we approach true nanotechnology and biotechnology, Vogel's book provides a literate guide for discriminating the hype from the potential of each. Cats' Paws and Catapults is not a paean to either human or biological technology, nor is it an indictment of either. However, this elegant comparison of the two will forever change the way you look at each.