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.