Whales and other marine mammals that dive to obtain food underwater often dive in bouts. The bouts are periods when the whale dives repeatedly, with only a short amount of time at the surface before the next dive. We think that marine mammals dive in bouts to get as much food as possible while they are diving. For example, if they find a large school of fish, they do not want to spend so much time at the surface that the school escapes, so they dive again to quickly get down to catch more fish. Therefore, they only stay at the surface long enough to replenish their oxygen. However, the time at the surface may not be long enough to remove all the nitrogen from their blood. Because of this, the amount of nitrogen increases slowly from dive to dive, and sometimes it increases enough that bubbles may form in the whales when they return to the surface.

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