Modes of Locomotion and Efficiency

Why swimming, flying, and running each have their own physics and energy cost, and what makes one way of getting around cheaper than another.

Definition

Modes of locomotion are the distinct ways animals propel themselves — chiefly swimming, flying, and terrestrial locomotion — and locomotor efficiency is how economically each mode moves an animal, commonly expressed as the cost of transport, the energy required to carry unit body mass a unit distance.

Scope

This topic covers the major modes of animal locomotion and their energetic efficiency: swimming and the drag of moving through water, flying and the generation of lift, terrestrial locomotion and the use of gaits, and the comparison of these modes through the cost of transport. It treats how the medium, body size, and speed determine the energy needed to travel a given distance and why each mode is efficient under its own conditions. Coverage is comparative and mechanistic.

Core questions

• What physical forces dominate swimming, flying, and running?
• How is the energetic efficiency of locomotion measured and compared?
• Why do animals use different gaits at different speeds?
• How do medium and body size determine which mode of movement is cheapest?

Key theories

Cost of transport across locomotor modes

The energy to move a unit of body mass a unit distance differs systematically among swimming, flying, and running, with swimming generally cheapest and running most expensive per distance, and decreases with body size within each mode.

Gait selection by economy

Terrestrial animals switch between gaits such as walk, trot, and gallop at speeds where each becomes the most economical, reflecting the changing mechanics and energy use of locomotion with speed.

Mechanisms

Each mode of locomotion contends with different forces. Swimmers must overcome the drag and density of water and generate thrust by undulation or appendages, achieving low cost of transport because water supports their weight. Fliers must generate lift to stay aloft as well as thrust to move forward, making flight powerful but, per distance, moderately economical for animals that travel far. Terrestrial animals support their weight against gravity and accelerate and decelerate their limbs and bodies with each stride, which makes running relatively costly, though elastic energy storage and the choice of gait reduce the expense. Comparing modes with the cost of transport shows that, for a given body size, swimming is generally cheapest and running dearest, and that larger animals move more economically per unit mass within each mode. These patterns explain why animals are adapted to particular media and travel distances.

Clinical relevance

The comparative study of locomotor efficiency informs the understanding of the energetic cost of movement, gait, and exercise and guides the design of efficient vehicles and robots. This entry is educational reference material and does not provide medical guidance.

History

Schmidt-Nielsen and Taylor's measurements of the energy cost of locomotion across animals and modes established the cost-of-transport framework, Tucker's studies of flight energetics quantified the cost of flying, and Alexander's analyses of gait and dynamic similarity explained why animals move as they do.

Key figures

• Knut Schmidt-Nielsen
• Robert McNeill Alexander
• Charles Richard Taylor
• Vance Tucker

Seminal works

• schmidtnielsen1997
• alexander2003
• hill2016

Frequently asked questions

Which way of getting around is most energy-efficient?

For a given body size, swimming generally has the lowest cost of transport because the water supports the animal's weight, while running tends to be the most expensive per distance travelled.

Why do animals change gait as they speed up?

Each gait is most economical over a certain range of speeds, so animals switch from walking to trotting to galloping at the points where the new gait uses less energy.