The cost of getting around

Moving costs energy, and animals pay in food. To compare very different animals, biologists use a number called the cost of transport: the energy needed to carry one kilogram of body mass over one metre.

COT = energy / (mass × distance)

Its unit is joules per kilogram per metre, J/(kg·m). A smaller COT means a cheaper way to travel. Because COT is per kilogram and per metre, a mouse and a horse can be compared on the same scale.

A feeling for the size. A person running uses very roughly 4 J/(kg·m) in total (a common rule of thumb is about 1 kilocalorie per kilogram per kilometre). A 70 kg runner covering 1 km therefore uses about 4 × 70 × 1000 = 280 kJ, around 67 kcal. Brisk walking costs about half as much per metre. These figures vary between people and with speed, so they are only a guide.

Notice that energy per metre is the right measure for a trip. A bird flapping may burn energy fast every second, but it moves so quickly that each metre is cheap.

Swimming, flying and running compared. In a famous 1972 comparison (Knut Schmidt-Nielsen), animals of the same body mass were ranked by how much energy each used to move one metre:

• Swimming (a fish in water) is the cheapest.
• Flying is in the middle.
• Running is the most expensive.

Why? Water holds the body up, so a swimmer only fights drag. A flier pays to stay up, but the air lets it travel fast and straight. A runner has to repeatedly lift its body and speed up and slow down its legs with every step. Note the limits of the ranking: it compares typical animals of equal mass, and swimmers that paddle at the surface, such as ducks or people, pay a much higher price than a streamlined fish because they make waves.

Small animals pay more per kilogram. Across mammals, the running COT per kilogram falls as the animal gets bigger. A mouse uses many times more energy per kilogram per metre than a horse does. Part of the reason is that a small animal takes more strides to cover the same distance, and each stride has a fixed cost.

Why migration works. Travel in the cheapest mode that fits your body. A bar-tailed godwit flies non-stop from Alaska to New Zealand, roughly 11,000 km in around eight days, living on fat it stored beforehand (roughly half its take-off weight). A salmon returning to its birth river swims for hundreds of kilometres against the current without feeding, using up body reserves. Neither trip would be possible by running.