Anna C. Gonzalez

Natsci - Article I

May 11, 1998

Apparently, an enduring question in Biology concerning fish is "why be warm blooded." Of the thirty-thousand species that are cold blooded, at least 30 have formed some type of body heat control.

Barbara A. Block, an animal physiologist at the University of Chicago, says these are the selective force behind the evolution of endothermy in fish. Warm-blooded fish are all large-bodied and like birds and animals, have central heating: their entire bodies are kept warm by recycling the heat generated by an abundance of aerobic-energy-producing red muscle. Researchers have strapped sensors and transmitters to wild sword fish, which have especially large brain heaters, in order to trace their movements and temperature throughout the course of the day. The heater cells in the thermogenic organs are actually skeletal muscle cells tat have lost their contractile filaments and, as such, are probably derived from the steak-sized eye muscles of these predatory fish, says Block. The calcium ions that normally signal a muscle filament to contract by suddenly pouring into the cell cytoplasm flow steadily between the cytoplasm and a dense network of membranes.

What does the fish achieve by heating only its head? The most important advantage of endothermy in any animal is protection of the temperature-sensitive brain. The advantage, of course, of being able to cruise through wide temperature ranges is access to squid and other vertically migrating prey organisms that other fish cannot track.

Thus, niche expansion was probably the selective force for endothermy in physiological performance.