Essentials of the Living World 5th Edition by George Johnson

Exercise 4

Which Mode of Locomotion Is the Most Efficient?
Running, flying, and swimming require more energy than sitting still, but how do they compare? The greatest differences between moving on land, in the air, and in water result from the differences in support and resistance to movement provided by water and air. The weight of a swimming animal is fully supported by the surrounding water, and no effort goes into supporting the body, while running and flying animals must support the full weight of their bodies. On the other hand, water presents considerable resistance to movement, air much less, so that flying and running require less energy to push the medium out of the way.
A simple way to compare the costs of moving for different animals is to determine how much energy it takes to move. The energy cost to run, fly, or swim is in each case the energy required to move one unit of body mass over one unit of distance with that mode of locomotion. (Energy is measured in the metric system as a kilocalorie [ kcal ] or technically 4.184 kilojoules [note that the Calorie measured in food diets and written with a capital C is equivalent to 1 kcal]; body mass is measured in kilograms, where 1 kilogram [ kg ] is 2.2 pounds; distance is measured in kilometers, where 1 kilometer [ km ] is 0.62 miles). The graph to the right displays three such "cost-of-motion" studies. The blue squares are running, the red circles are flying, and the green triangles are swimming. In each study, the line is drawn as the statistical "best-fit" for the points. Some animals such as humans have data in two lines, as they both run (well) and swim (poorly). Ducks have data in all three lines, as they not only fly (very well), but also run and swim (poorly).
Applying Concepts Do the three modes of locomotion have the same or different costs?