Book review – How to Walk on Water and Climb up Walls: Animal Movement and the Robots of the Future

Animals move in many different ways – hopping, gliding, flying, slithering, walking, swimming, etc. their way through our world. Studying how they do this brings together biologists, engineers, and physicists in disciplines such as biomechanics, bioengineering and robotics. Author David L. Hu, for example, is a professor of mechanical engineering and biology. How to Walk on Water and Climb up Walls is a light and amusing romp through the many remarkable forms of animal locomotion, and the equally remarkable experiments that are informing the robots of the future, although it leaves out some notable examples.

“How to Walk on Water and Climb up Walls: Animal Movement and the Robots of the Future“, written by David L. Hu, published by Princeton University Press in December 2018 (hardback, 236 pages)

Hu’s entry into this world was water striders and the question of how they move on the water surface. But first, how do they even stand on water? One recurrent theme in a book like this is scaling. The physical rules of the world operate differently at different scales, and, as Hu mentions, small animals are sensitive to forces we would find negligible (see also my review of Scale: The Universal Laws of Life and Death in Organisms, Cities and Companies, and pop-science introductions such as Why Size Matters: From Bacteria to Blue Whales and Nature’s Giants: The Biology and Evolution of the World’s Largest Lifeforms). At their size, water striders benefit from surface tension and the microscopic hairs that cover their legs making it even easier to stand on the water surface. But the question of how they gain the traction to start moving on such a surface was a paradox. Hu describes his observations on live water striders, and his attempts to mimic their movement with a mechanical water strider.

This introduction sets the tone for the book. In eight chapters, Hu looks at different physical principles and modes of locomotion. Using a rather formulaic approach, each chapter discusses two or three case studies. A third-person story introduces the researcher, often catching them in the process of conducting an experiment, outlines the problem or question at hand, gives an overview of what has been done in the past, and explains the latest findings of the protagonist.

“Flying snakes are dropped off rickety towers […] Dead fish are shown to be capable of passive swimming”

What follows is a miscellany of some truly remarkable organisms and wacky experiments. Undulatory motion is used by snakes, burrowing worms, but also the sand fish, with high-speed X-ray imagery revealing that this lizard effectively swims through sand (see the image on the right). Flying snakes are dropped off rickety towers to study how they deform their body into an airfoil during their gliding fall. Dead fish are shown to be capable of passive swimming. Insects employ crumple zones in their wings to survive frequent impacts with raindrops and vegetation. And cockroaches can flatten themselves to an extreme degree and still run at full tilt (!), inspiring robots that can continue to function while being compressed. And do not get me started on ant colonies. Do they behave as a fluid or as a solid?

Some projects are so well known that multiple authors will write about them, such as the Kilobot robot swarm described by Lisa Margonelli (see my review of Underbug: An Obsessive Tale of Termites and Technology). Others are very obscure, so I was not terribly surprised to see no mention of John Long’s evolving robots (see Darwin’s Devices: What Evolving Robots Can Teach Us About the History of Life and the Future of Technology). But, given that Hu has worked at MIT, I was surprised to read nothing about the walking robots from well-known engineering schools such as the MIT Biomimetic Robotics Lab, Boston Dynamics, or Caltech. Their designs for mobile all-terrain robots are (literally) advancing by leaps and bounds, and the footage coming out of these labs of walking, running, jumping, backflipping, door-opening robots is as fascinating as it is terrifying. They regularly make the news, prompting both clickbaity headlines playing at people’s fears, but also serious concerns about how the military will be using these (a lot of this research is sponsored by DARPA, the Defense Advanced Research Projects Agency of the US Department of Defense). Instead, Hu features Steve Collins’s passive-dynamic walking robot that only uses gravity as a driving force. And where are the titular wall-climbers in this book? Finally, the book lacks much in the way of a discussion of evolutionary and physical constraints that impose limits on locomotion (see e.g. Evolutionary Biomechanics: Selection, Phylogeny, and Constraint, Feats of Strength: How Evolution Shapes Animal Athletic Abilities, and The Equations of Life: The Hidden Rules Shaping Evolution).

“I was surprised to read nothing about […] engineering schools such as Boston Dynamics […] Their designs for mobile all-terrain robots are (literally) advancing by leaps and bounds […]”

Those omissions notwithstanding, the topics that are featured are given a nice treatment. I was especially impressed with the explanatory diagrams, many of them redrawn for this book, the useful black-and-white stills and photos, and the colour plate section. Finally, an author who actually refers to his plates in the text! Without using any formulas, Hu gives concise explanations of biological and physical principles such as allometry, pendulums, propulsion by crack propagation, or conservation of angular momentum. How to Walk on Water and Climb up Walls is not intended to be a complete overview of the field of biomechanics and biorobotics and Hu does not go into great detail, just enough to cover the basics. Interested readers may want to look up Life’s Devices: The Physical World of Animals and Plants, The Biomechanics of Insect Flight: Form, Function, Evolution, Nature’s Flyers: Birds, Insects, and the Biomechanics of Flight, Collective Animal Behaviour, or the forthcoming The Rules of the Flock: Self-Organization and Swarm Structure in Animal Societies to name but a few. A clever conclusion shows the value of this kind of research, which is often lampooned for being a waste of taxpayer’s money.

Where a competing pop-science title such as Furry Logic: The Physics of Animal Life runs the whole gamut of physics (covering some of Hu’s examples), Hu has carved himself a nice little niche by focusing on the intersection of animal locomotion and robotics. Not intended to be comprehensive, How to Walk on Water and Climb up Walls is an amusing and easy read, and Hu’s choice of fascinating examples is likely to suck readers deeper into the topic of biomechanics.

Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.

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