Mighty mouths: how whales keep the heat.

Anyone who has eaten ice cream on a cold winter day can relate to the way gray whales feed. Swimming in near-freezing arctic waters, the behe-mouths slurp up sediments by the tubful to strain out enough tiny crustaceans to sustain their 35-ton forms.

"These animals have big mouths, they're feeding in cold waters--they could be losing a tremendous amount of heat," says John E. Heyning, curator of mammals at the Natural History Museum in Los Angeles.

Yet even with mouths agape, the marine mammals manage to stay about as warm as a person eating ice cream. The whale's secret lies in its massive tongue. A clever arrangement of veins and arteries keeps the animal's heat from being squandered through the uninsulated tongue--fully 5 percent of the body's surface area.

Heyning found the array of vessels by happenstance, while dissecting a whale's tongue for a study of its muscles. He and James G. Mead of the National Museum of Natural History in Washington, D.C., report the anatomical discovery in the Nov. 7 Science.

The vessels follow a well-known design in engineering--and in other animal parts (SN: 5/14/94, p. 312). In human limbs, blood vessels lie side by side, modestly conserving heat as the blood flowing out warms the blood coming in. Marine mammals excel in the use of these countercurrent heat exchangers, especially in their flukes and fins.

"Classically," says Heyning, the structure is "a central artery with a sheath of veins around it so it looks like a little rosette." In the gray whale, a large bundle of 50 such structures carries warm blood into the muscular, 5-foot-long tongue. By the time the blood reaches the surface of the tongue, the heat has been dumped into the cooled, incoming blood of the adjacent veins. Says Heyning, "Blood coming back into the body core can be just about the same temperature as blood leaving the body core."

It's "a wonderful bit of anatomy," says Ann Pabst of the University of North Carolina at Wilmington, who studies the biomechanics of marine mammals. Equally impressive, she says, is the researchers' report of a whale's tongue in action. Using a thermal sensor on a captive gray whale calf as it fed, they found little difference between water and tongue temperature.

Despite people's ancient interest in the animals--gray whales were cleaned out of the Atlantic Ocean by the 1700s--their size makes them difficult to study and good specimens are rare. The result, says Pabst, is that "a lot of very interesting functional aspects of their anatomy are being described today."