Scientists now have direct evidence that the north Pacific salmon shark maintains its red muscle (RM) at 68-86 degrees Fahrenheit (F), much warmer than the 47 F water in which it lives. The elevated muscle temperature presumably helps the salmon shark survive the cold waters of the north Pacific and take advantage of the abundant food supply there. The heat also appears to factor into the fish's impressive swimming ability.
Very strange. How do humans stay warm? I never thought much about it, but I assume the chemical reactions in a human stomach are exothermic and give off heat, and that this heat is transmitted to the blood by conduction and from the blood to the rest of the body through convection. Then again, our other cells must also generate heat when they process adenosine triphosphate for energy.
I'll have to do some more research, but The Straight Dope confirms most of my hunches about being warm-blooded.
First off, let's get a few terms straight. "Warm-blooded" and "cold-blooded" are misleading labels. Modern biologists generally use two other contrasting pairs of terms to describe the thermal physiology of animals: homeotherm/poikilotherm and endotherm/ectotherm. (Don't things seem clearer already?) Basically, a homeotherm is an organism that maintains its body temperature at a nearly constant level, while a poikilotherm experiences much larger fluctuations. The latter terms refer to the source of the body's heat. In endotherms, most of the heat is generated internally, through metabolism, while in ectotherms, most of the heat comes from external sources, such as the sun.
By this categorization the fish above are clear poikilotherms, since their internal temperature can vary across an 18-degree range. It's interesting though that they generate some heat from their metabolism indirectly through muscle movement.