How Sharks Stay Warm

North Pacific salmon sharks spend their entire lives swimming. If they didn't, they'd sink. Now new research shows that swimming actually warms up these cold-blooded creatures, allowing them to survive chilly 47 degree Fahrenheit waters.

As the sharks are swimming, the red muscle used to propel them through the water generates heat. As this muscle heats up, the warmth spreads from their body's core to the surrounding white muscle and warms the rest of the body. To keep the rest of their body warm, these sharks keep their red muscle temperature between 68 and 86 degrees Fahrenheit.

Scientist had believed in this mechanism for years, but didn't have evidence until Robert Shadwick of the Scripps Institution of Oceanography caught several sharks in the Gulf of Alaska and measured temperatures throughout their bodies and tested the mechanical power of red muscle.

Shadwick and his colleagues showed that the red muscle is much more efficient at warmer temperatures. At 50 degrees Fahrenheit, red muscle produces only 25 to 50 percent of the power it produces at 79 degrees. The researchers concluded that if red muscle temperatures fall below 68 degrees Fahrenheit for an extended period, the shark's muscle function could be permanently impaired.

Salmon sharks are members of the lamnid group of sharks, which also include the mako and great whites. Sharks in this group share many physiological and anatomical characteristics with bony tuna and probably shared a common ancestor more than 400 million years ago.

These results are published in the Oct. 27 issue of the journal Nature.