Swimming muscle helps warm the brain of lamnid sharks

Summary

Lamnid sharks are known to have warm red muscle and warm brains. We describe a large vein in lamnid sharks that provides a route for transfer of warm blood from the red muscle to the central nervous system. This ‘red muscle vein’ runs longitudinally in the red muscle and is valved to direct blood flow anteriorly. It joins the myelonal vein in the neural canal, thus providing a route for blood flow from the red muscle to the brain. Temperature profiles along the neural canal of freshly caught mako sharks show that warm blood enters the myelonal vein from the red muscle vein. Experiments with heat generation by model brains indicate that the metabolic heat produced by the brain is probably not sufficient to cause the temperature elevations observed. Metabolic heat imported from the red swimming muscle may be a valuable addition to the heat budget of the head.