One why of the warmth of warm-bodied fish.

Tunas are unusual among fish in that they are warm bodied. In the present essay we argue that one adaptive advantage to being warm is that the warmth increases the rate of delivery of oxygen from the cell boundary to the mitochondria by myoglobin. This argument is supported by the following. 1) Tuna have extremely high rates of oxygen uptake, much higher than other fish and close to the rates achieved by mammals. 2) Tuna have an extraordinary capacity to maintain high cruising speeds for a long time. 3) Tuna have much red muscle that contains a high concentration of myoglobin. 4) The effect of temperature on simple diffusion of oxygen is very small whereas the potential affect on facilitated diffusion by myoglobin is large.