Drift diving in female northern elephant seals: Implications for food processing

We tested predictions from the hypothesis that northern elephant seals, Mirounga angustirostris, drift during the bottom segment of some dives (called C dives) using oxygen saved from reduced locomotion to process food. Sixteen free-ranging dive records were obtained with microcomputer dive recorders attached to 13 adult females from Ano Nuevo, California, during biannual foraging trips when they were in early or late stages of gestation; swim speed was recorded throughout one dive record. Body composition was measured before and after trips to sea. C dives with a bout length of 2-10 dives and a mean duration similar to those of other dive types made up 6.3 +/- 1.9% of the dives recorded. Swim speed was near or below the recorder stall speed (0.22 m/s) during the second, drift segment of these dives. The rate of vertical depth change while drifting varied little within bouts, was initially significantly correlated with the ratio of fat to lean body mass at departure, and changed systematically as the seals fed while at sea. Females in early gestation, with initial mean body fat of 24%, drifted down at a mean rate of 0.31 +/- 0.04 m/s; females in late gestation, with mean body fat approaching 36%, drifted up at an average rate of 0.17 +/- 0.05 m/s. The frequency, duration, and temporal pattern of drift dives were correlated with foraging behavior, supporting the hypothesis that drifting while diving is associated with the metabolic cost of processing food. This study provides indirect support for the hypothesis that elephant seals suspend active swimming on certain dives, during which a greater proportion of oxygen stores is allocated to the processing of food, without interrupting the seals' normal pattern of continuous diving and allowing them to remain within their aerobic dive limit.