Book Review: Animal Movement

Reviewed by Ephraim Nissan (London, England)

Hu, D. L. 2018. How to Walk on Water and Climb Up Walls: Animal Movement and the Robots of the Future. Princeton University Press, Princeton, NJ, ix+228 pp. ($19.96 cloth, $11.96 paper, $11.96 e-book with 20% PS discount.)

A gentle and engaging introduction to biomechanics is something that not only neontologists need, but also students who want to become palaeontologists. This book is a wonderful propaedeutic survey, without formulae, of how biomechanics works and inspires bioengineering. This is a book that lights the sparkle of motivation that would give students the fortitude to endure the dreariness that the field inevitably entails.

“One of the first to ask these questions was Leonardo da Vinci” (6). Then came D’Arcy Thompson, a pioneer of mathematical biology (6–7). “Cambridge zoologist Sir James Gray is arguably the father of modern biomechanics, the study of the physics of animal movement and shape. Today, biomechanics is not as clear-cut an area as it was then, as it now intersects with studies of microscopic things such as the biophysics of the cell and with human-centered topics such as exercise physiology. […] In the 1930s, Gray conducted the first studies of the swimming of fish and dolphins” (7).

Sandwiched between the introduction and conclusion chapters are eight numbered chapters: “Walking on Water”, “Swimming under Sand”, “The Shape of a Flying Snake”, “Of Eyelashes and Sharkskin”, “Dead Fish Swimming” (this is Ch. 4, which also discusses 3D printing), “Flying in the Rain”, “The Brain behind the Brawn”, and “Are Ants a Fluid or a Solid?” A dozen colour plates follow p. 138.

The author writes in the first person singular, sometimes autobiographically. The introduction, “The World of Animal Motion”, begins by relating about how a poodle his future wife brought along to their first meeting became the subject of an experiment of filming with a high-speed camera as the dog was removing sticky notes from its fur, and then finding the percentage of water removed by a wet-dog shake, and next using a simulator, and also filming other animal species shaking off water. “Why do smaller animals shake more times per second? A smaller animals [sic] has a smaller radius, which causes it to generate less centripetal force” (2) (recte centrifugal, like in a washing machine), hence the need to compensate.

Apart from “moving from A to B” (3), “[a]nimals transport matter in and out of themselves” (3). And to combat parasites, animals move (4). Walking on water evolved once “terrestrial insects and spiders began to colonize the water surface. […] The most primitive surviving water-walkers, the Velia or water treaders, look quite similar to the bugs from which they evolved” (4), but their walking like an ant “is not very effective on water, and they move frantically and with little progress, as if perpetually slipping on ice. Moreover, their slow gait restricts them to being close to shore” (5). More effective are such water-walkers whose “legs became so long that they could act like oars. This new species, called the water strider, can row like a rowboat, balancing on its remaining legs as pontoons for support” (5). Their movement on the water surface is so effective that “such insects are nearly impossible to catch by hand, awkwardly dragging their canoe-like legs behind them” (5). The basilisk lizard “can run on water in short bursts if frightened. It has long fringed toes that it uses to slap the water surface and support its weight. Similarly, the western grebe, a black and white bird with red eyes, also runs on water, despite weighing up to ten times as much as a basilisk lizard” (5), “distances up to 50 meters in order to attract females” (5), and then couples “will run across water together to reinforce pair bonding” (5).

“Chapter 1 sets the stage for the rest of the book in that it begins with a simple question about animal movement, and finishes with a proof-of-concept, here the construction of a device that can walk on water” (12). The book “pay[s] particular attention to key concepts that scientists use to understand the diversity of animal movement, showing that just a few physical concepts can give an intuitive understanding of a number of animal shapes and movements” (8), and how this can inspire engineering solutions (this is what is called biomimetic robotics). “[T]his book is heavily focused on fluid mechanics” (9), but without formulae. Hu “structured this book such that each chapter focuses on the stories of several key scientists” (9), “convey[ing] the scientific process as if it were a mystery story” (9). Hu has “chosen studies that were done early in a scientist’s career, so you can see what it’s like to embark on new territory” (18). Those are studies from the 21st century. The bibliography at the end of the book is organised by chapter.

“Studying animals can also teach us about the importance of body size, which can in turn influence how we design machines. In physics, bigger is different. As animals grow, certain forces that were previously negligible become important. For example, large animals simply cannot afford to fall down because they will be easily hurt. But as animals get smaller, their bones [or exoskeletons] appear to be effectively stronger due to the physics of scaling. This is why fleas can jump 120 times their body length without being injured. Such invincibility at smaller scales gives small animals a wider range of behaviors. Small animals are naturally more robust so much so that they regularly crash into objects without damage” (11–12). “As said by British biologist J.B.S. Haldane, ‘you can drop a mouse down a thousand-yard mine shaft; and, in arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes’” (79).

Hu spent his “postdoctoral years studying the motion of snakes” (12), and this is the subject of Ch. 2, as is optimality of body shape for moving through a particular medium, be it sand or mud. The sandfish, i.e., the lizard Scincus scincus, swims under the sand (48–55). Optimality of body shape is further pursued in Ch. 3, but it begins discussing bladders, however, before turning to the propulsion, by jetting, of jellyfish and squids, and then (76) to the motion of the flying snake family and other gliding animals. “Strangely, the flying snake has no obvious gliding surfaces. Its body cross section is round. The flying snake would seem as good a glider as a stick” (78). “With no wings and no clear landing gear, the flying snake seems like the worst possible glider” (79).

“[F]lying snakes use gliding as a last resort if they need to escape” (80), and it is they who initiate the gliding, unlike being dropped from a tower (as then they do poorly). “The snake’s ribs are hinged, and when it expands them, it assumes a triangular cross section that reduces the effects of aerodynamic stall” (83). “The snake finally took the leap (Fig. 34A). Like a rubber band shot from a finger, it accelerated up and away from the branch. It first straightened its body, transforming into a spear. Then the snake flattened like a cobra’s hood. Its ribs, initially pointing toward the ground, swung outward laterally like wings. Its width doubled, giving the snake a gentle concave shape. The snake had transformed its entire body into a wing. The snake dived with its head angled toward the ground. The world, initially still, began accelerating at a terrifying 10 meters per second every second. It’s so fast that one second of acceleration results in an increase of speed from zero to 18 miles per hour. If we were in free fall, we would likely feel disoriented as the water in our vestibular canal began floating. This free-fall acceleration may be frightening to the snake the way it would be to us, but it’s necessary. The snake’s ribs could only help generate lift if the snake ramped up its body speed. The free fall lasted a distance of more than two meters as the snake gained speed. As it sped up, it readjusted the orientation of its body in space. Originally angled downward, it lifted its head and lowered its tail, to make its body become more horizontal. It configured its body into an S‑shape and began undulating as if swimming through the air. This was a specialised gait for the air only, with a lower frequency and higher amplitude than its motion on the ground. Now with its ribs outstretched, and in its characteristic body motion, the snake began to change from a stick to a true glider, traveling with its body around 30 degrees relative to the horizontal. The snake’s trajectory shallowed as it started generating more lift. If you had been standing below the snake, it would have appeared as if it were going fall on your head, then it would seem to pull up and pass over you. Eventually, the snake reached a nearly steady traveling speed and direction. […] it was still alert to its surroundings” (83 – 84). Before describing the landing, Hu describes how a flying snake would turn, if facing an obstacle. This long quotation gives an idea of why this book is so readable.

Those jellyfish that are shaped like dinner plates “creat[e] a much clearer wake” (73) — “a single vortex ring […] The rest of the fluid [remains] quiescent”. They are “much more economical with their energy, using far less energy for each centimeter traveled”:  they eat “plankton, a low-calorie source of food, making saving energy more important than speed,” unlike with bullet jellyfish, quick and “leaving behind a messy wake” (73).

“Shape is not the only way to influence flow, however. By evolving specialized surface features, such as hairs or scales, animals can also influence the way fluids flow around them” (87), which is the subject of Ch. 4. “Growth is how a shark develops the fine scales on its surface, and how you grow eyelashes to protect your eyes” (13). Chapter 4 discusses “the hydrodynamic properties of each of these fine structures” (13). Chapter 5 is concerned with “animals that can move using very little energy” (14), which they do by energy transfer. “Fish push this idea to its limits by harvesting energy from their surroundings similarly to the way a kite is pushed by the wind to move” (14). Even dead fish.

Chapter 6 is about obstructions when insects are flying: how mosquitoes survive a rainstorm, by an injury-preventing strategy, or how a bee, “surrounded by thousands of plant stems, each of them waving in the wind” (14), does not avoid them, but “simply crashes into the stems over and over on its way to find pollen. Its wings have special crush-zones that store elastic energy like springs, bending without breaking” (14).

Chapter 7 is about the nervous system being “put to the test in particular in insect flight, where one of the more difficult tasks is to stay motionless, or hover. It is difficult for a fruit fly to hover, because the fruit fly’s body is inherently unstable. Like a sheet of paper, the fruit fly does not tend to fall straight when dropped. The fly is affected by the air currents that it itself generates as it falls. The nervous system works with the body to put hovering and other kinds of locomotion under automatic control” (14). Hu turns to cooperation in flocks or swarms, in Ch. 8, e.g., in swarms of fire ants. 

The study of animal motion has the potential of helping to breed domestic animals in a more humane way. Take the analysis of the gait of walking chickens. “The problem with the domesticated chicken is that it is bred to grow its breast so quickly that its leg muscles cannot keep up and eventually the chicken is unable to walk properly. For the last segment of its life, it is too large to stand up and sits, unable to move” (209).

On pp. 212–213, Hu enumerates the kind of employment that students may expect, while looking for careers or jobs in animal motion. Hu points out that “taxonomists […] are […] going extinct themselves” (212), so the next generation is trained to use the tools of genetics. By now, our present readers will have understood the merits of the book under review.

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