concept

Cost of transport

The energy an animal spends per kilogram of body mass per metre travelled — biology's fuel-economy figure, in joules per kilogram per metre. Because it divides out both mass and distance, it compares bodies that differ wildly in size and gait. Respirometry puts pelagic squid at roughly 1.5–4.5 while fishes and rays of similar mass sit at 0.2–0.6: jetting costs three to five times what flapping does, which is the difference between an animal that can cross an ocean and one that can cross a room.