MOST OF us know fish to be cold-blooded creatures that alter their temperature to match that of the surrounding water. But some fish, the tuna for example, are warm-blooded, a fact that had long puzzled biologists.

Barbara A Block, biologist at the University of Chicago and three of her students have now traced the warm-bloodedness found in three groups of fishes -- the tuna, butterfly mackerel and billfishes (such as swordfish and marlin) -- to the wanderlust of their ancestors, which took them to colder waters. In order to be able to survive the cold, these piscean adventurers slowly entered into the warm-blooded fold. Most evolutionary biologists endorse Block's conclusion.

The billfishes and the butterfly mackerel warm only their brain and eyes through special heat-producing eye muscles. Tunas, on the other hand, warm their brains by capturing the heat produced by their muscles before it is lost through their gills. They have similar heat exchange mechanisms for other parts of their bodies as well.

These selective heating mechanisms protect the fish's temperature-sensitive brain and retina so that they can spot and pursue prey even in colder climates, says Block. She explains that warming the brain allows the swordfish to pursue squids from the warm surface waters into chilly waters hundreds of metres deep. Similarly, the heat generation and retention capacity allows the bluefin tuna to migrate between the tropics and subpolar seas.

Block and her team charted the evolutionary histories of the three groups of warm fishes, their cold-blooded cousins and other distant relatives. The team found that the three groups of warm-blooded fish evolved independently of each other, even though all of them were driven by their experience of a wider range of temperatures than their cold-blooded relatives.