Principles of Animal Locomotion

  • R. McNeill Alexander
Princeton University Press: 2003. 376 pp. $49.50, £35

How did the chicken cross the road? A neurobiologist might answer that commands from the central nervous system, modulated by sensory feedback, generated cyclical walking movements. Alternatively, a physiologist could say that walking came about through myosin's interaction with actin, which created muscular tension to produce the necessary joint torques. On the other hand, a biomechanist might view the whole bird as a point mass rising and falling with each step, transforming gravitational potential energy into forward kinetic energy and back again, like an inverted pendulum. Such disparate responses highlight the wonderfully intricate and dynamic interplay that transpires when animals move through their environment.

In Principles of Animal Locomotion, R. McNeill Alexander approaches movement from a physical point of view. He seeks to explore the mechanical principles at work in organisms as different as burrowing earthworms and hovering hummingbirds. More over, he tries to account for the metabolic cost, and thus the relative energetic merit, of alternative forms of locomotion. These might seem like lofty goals, but Alexander is up to the task. His publications over the past six decades span such topics as swimming, scaling, gaits, elastic storage, and optimization (to name just a few), giving him the breadth and depth needed to summarize the field.

The book begins with a lucid summary of performance objectives — speed, acceleration, manoeuvrability, endurance, economy and stability — and their potential relationship to fitness. This is followed by a brief discussion of compromises (not all of the objectives are compatible), constraints (not all changes are possible) and optimization (which solution is best?). Alexander then covers muscle mechanics, energetics, scaling and recording techniques, before delving into specific forms of locomotion. Terrestrial progression is addressed first through simple theoretical models and then by examining walking, running, hopping, climbing, jumping, crawling and burrowing in real animals. Aerial and aquatic locomotion are then tackled, although cilia-driven propulsion is not included. There is also a chapter on the effectiveness of devices for augmenting human locomotion.

Although this book is aimed at an audience ranging from advanced undergraduates to researchers and university teachers, I think that most undergraduates will be overwhelmed and intimidated if it is used as a textbook. Alexander packs in so much information that clarity is often sacrificed for coverage. His curt style is so direct and quantitative (equations are ubiquitous, starting on page 2) that every paragraph is extremely dense. But despite its failings as a textbook, it is an excellent reference tome. Citations (up to 2001) and 33 pages of references (of which 77 are authored by Alexander) will quickly direct readers to the relevant literature. I will use Principles of Animal Locomotion and recommend it to my graduate students.

Alexander's facility with simple, quantitative models is both a strength and a weakness. Regrettably, the translation from a living, breathing animal to a mathematical representation of its underlying physics is never explained. In creating such models, which details can be omitted, and which need to be retained? If Alexander wants to appeal to a broad range of scientists, he must take readers step-by-step through the transition to a mechanical perspective, rather than just diving in. Most biologists, even those with training in mathematics and physics, don't think like engineers.

Finally, I lament the book's lack of phylogenetic foundation. Despite the first chapter's evolutionary tone, Alexander never returns to ancestry or history; instead, each organism is treated as a separate case study of adaptation. In a disappointingly short epilogue (just five-and-a-half pages), he attempts “some generalizations about locomotion”, but comes to few conclusions. His search for generalizations fails because he treats each organism as an independent point on a graph, rather than as a member of a hierarchical tree of life. From an evolutionary perspective, physical mechanisms that are shared among taxa are either homologous or convergent, not just common.

If most walking animals, whether vertebrate or arthropod, use an inverted pendulum mechanism to save energy, how many times has this evolved? How did organisms that were optimized for swimming evolve into organisms optimized for running? And then how did runners evolve into organisms that are optimized for flying? Such transitions are completely ignored in this book.

Principles of Animal Locomotion is a valuable reference book written by a leader in the field. But it also serves as proof that enough studies have accumulated to warrant an evolutionary analysis of locomotor mechanics — a synthesis that I await eagerly.