? Studies of the Weddell seal in the laboratory have
?described the physiological mechanisms that allow the
?seal to cope with the extreme oxygen deprivation that
?occurs during its longest dives, which can extend 500
(5) meters below the ocean’s surface and last for over 70
?minutes. Recent field studies, however, suggest that
?during more typical dives in the wild, this seal’s physio-
?logical behavior is different.
??In the laboratory, when the seal dives below the
(10) surface of the water and stops breathing, its heart beats
? more slowly, requiring less oxygen, and its arteries
? become constricted, ensuring that the seal’s blood
? remains concentrated near those organs most crucial to
? its ability to navigate underwater. The seal essentially
(15) shuts off the flow of blood to other organs, which either
? stop functioning until the seal surfaces or switch to an
? anaerobic (oxygen-independent) metabolism. The latter
? results in the production of large amounts of lactic acid
which can adversely affect the pH of the seal’s blood
(20) but since the anaerobic metabolism occurs only in those
? tissues which have been isolated from the seal’s blood
? supply, the lactic acid is released into the seal’s blood
? only after the seal surfaces, when the lungs, liver, and
? other organs quickly clear the acid from the seal’s blood-
(25) stream.
??Recent field studies, however, reveal that on dives in
?the wild, the seal usually heads directly for its prey and
?returns to the surface in less than twenty minutes. The
?absence of high levels of lactic acid in the seal’s blood
(30) after such dives suggests that during them, the seal’s
?organs do not resort to the anaerobic metabolism
?observed in the laboratory, but are supplied with oxygen
?from the blood. The seal’s longer excursions underwater,
during which it appears to be