This realization has raised basic questions about the metabolic functions of deep-diving mammals. To appreciate the physiological feat of marine mammals, consider the contrasting diving ability between elephant seals and humans. A northern elephant seal can dive, after a short surface interval, for over an hour, reaching depths below 1,500 meters. The free diving record for humans, assisted by weights, was to a depth of 133 meters and lasted just over two minutes. But, when diving, particularly on deep dives, the availability of oxygen is insufficient for aerobic metabolism, and anaerobic metabolism may take over.
Energy is generated by breaking down a common cellular component, which produces lactic acid.
The acid accumulates in tissue and is ultimately removed with normal aerobic respiration, after the mammals resurface. Relying primarily on laboratory studies, scientists have generated numerous equations that show the necessary balance between oxygen concentration, metabolic rate, production of lactic acid, and time necessary for anaerobic recovery. A deep-diving mammal, it's believed, could finesse but not transgress these limits, using strategies like maximizing oxygen loading while at the surface and slowing its metabolic rate when submerged. The dives of elephant seals, deeper and longer than anyone thought, challenged the expert's equations. Every other diving mammal follows a long dive with a surface recovery period of an hour or more.