Principles of Animal Locomotion

1. What they did

This book by zoologist R. McNeill Alexander explains how animals move on land, in water and through the air, and what that movement costs them in energy. It focuses on understanding and principles rather than detailed description, and uses rough calculations and simple mathematics to test explanations. It covers only muscle-powered movement, and was written mainly for advanced students, researchers and university teachers of biology.

The book is in 19 chapters:

• Foundations (Chapters 1 to 4). What natural selection might favour in movement (speed, acceleration, maneuverability, endurance, energy economy, stability), how muscles work, where the energy goes, and how body size changes things.
• Methods (Chapter 5). Filming, treadmills and wind or water tunnels, measuring oxygen use, force plates, recording muscle activity, and transmitters and data loggers on free-ranging animals.
• Land (Chapters 6 to 9). Simple models of crawling, hopping, walking and running, then walking, running and hopping in real animals, climbing and jumping, and crawling and burrowing.
• Air (Chapters 10 to 12). Gliding and soaring, hovering, and flapping flight.
• Water (Chapters 13 to 17). Moving on the water’s surface, swimming with oars and hydrofoils, swimming by undulation and by jet propulsion, and buoyancy.
• People and summary (Chapters 18 and 19). Shoes, bicycles, scuba, boats and gliders, then a short epilogue comparing running, swimming and flight.

2. What they found

Speed and chases

• Acceleration can matter more than top speed in a short chase. In films of lions hunting Thomson’s gazelles, the lions’ initial acceleration averaged 9.5 metres per second squared and the gazelles’ only 4.5.
• Prey that turn more tightly than their predator can escape even if they are slower, but only if they swerve at the last moment.
• Over long distances, a predator with better endurance, such as African hunting dogs chasing antelopes over several kilometres, can catch faster prey.
• Most published speeds of large wild mammals are unreliable estimates. A cheetah timed at 29 metres per second is the most reliable record for the fastest runner. Mammal sprint speeds roughly increase with body size, but the very largest mammals are relatively slow.

Gaits

• A gait is a pattern of movement used over a limited range of speeds, which changes abruptly at the switch to another gait. People walk to go slowly and run to go fast; horses walk, trot and gallop.
• Animals of different sizes tend to change gait at the same Froude number (speed squared divided by hip height times gravity). Quadrupeds change from trotting to galloping at a Froude number of about 2 to 3, and from walking to trotting (or people from walking to running) at about 0.3 to 0.8. So bigger animals change gait at higher speeds: dogs at about half the speeds of horses with legs four times as long.
• Animals speed up by taking longer strides, faster strides, or both. Galloping mammals mostly lengthen their strides. Feet stay on the ground for a smaller part of each stride as speed rises: in four-legged mammals, from about 70% in slow walking to 20% to 30% in fast galloping.
• In cursorial (running-adapted) mammals and birds, the feet pass beneath the hips and shoulders, so the lines of left and right footprints are close together. In noncursorial mammals, such as weasels and rodents, the lines are farther apart for their stride length. Fossil footprints show that dinosaurs walked more like birds and mammals than like modern reptiles.
• Each gait seems to be the most energy-saving one over the speeds at which it is used, at least in horses and people.

Energy

• Tendons act as springs. In hopping wallabies, elastic recoil of the ankle tendons did about 20 times as much work as the muscle fibres, and cut the work needed from the leg muscles by 45% at 6 metres per second.
• Per kilogram and per metre, larger animals move more cheaply. Running is the most costly way to travel, flapping flight moderate, and swimming by fish the cheapest.
• Walking on soft sand cost people up to 2.5 times as much as walking on concrete, and deep snow up to five times as much as a treadmill. To save energy, a walker may detour around soft ground, a zigzag path saves energy on steep hills, and a path around the shoulder of a hill can save energy over climbing it. Old mountain footpaths seem to fit these predictions.

Other modes of travel

The book shows how the same principles (gaits, size effects, elastic energy saving and energy costs) apply to jumping, climbing, crawling, burrowing, gliding, soaring, hovering and flapping flight, and to many styles of swimming. Flying is generally much faster than running or swimming; most birds fly at 9 to 19 metres per second over distances of a few hundred metres or more.

3. What they concluded

• The cost of transport scales in a similar way with body mass for running, swimming and flight, though at very different levels. Why the muscles of small runners and fliers seem to work less efficiently than those of large ones is not yet explained.
• Gait changes are found in runners, swimmers and fliers. For legged animals, the prediction that gaits change at equal Froude numbers works reasonably well, but gait changes in flying and swimming animals are still poorly understood.
• The role of elastic energy saving in running is well established; its role in flight and swimming is much less clear.
• Most research has studied animals moving at steady speed in laboratories. Much less is known about accelerating, slowing down, turning and staying stable, and about how animals move, how fast, and at what energy cost in their natural environments.
• The author expects that some of the explanations in the book will be shown to be wrong, but cannot say which.

4. How tracking was or was not used

Tracking was not used. The book touches on footprints in its discussion of gaits: the spacing between the lines of left and right footprints differs between cursorial and noncursorial mammals (citing fox, deer, weasel and rodent tracks in Lawrence and Brown 1974), and fossil footprints show how dinosaurs walked.

6. Other technologies, skills or innovations

• Film and video recording, treadmills, and wind and water tunnels.
• Force plates that record the forces feet exert on the ground.
• Measurement of oxygen consumption to estimate energy use.
• Electromyography (recording muscle activity), tendon buckles to measure tendon forces, and sonomicrometry crystals to measure muscle length changes.
• Radio and ultrasonic transmitters and data loggers carried by free-ranging animals, such as gray seals and black-browed albatrosses.