Northern elephant seals (Mirounga angustirostris) spend months at sea, surfacing only briefly between 20-minute dives. Yoko Mitani of Hokkaido University in Japan and her colleagues attached data recorders to six animals (see picture) to see if they could be resting on 'drift dives', which involve little or no active swimming.
During such dives, they found, the seals descend rapidly to at least 135 metres, then roll onto their backs and drift downwards, wobbling like a falling leaf. This slows the descent rate significantly.
The authors suggest the seals dive quickly to below where killer whales and great white sharks normally hunt and then go belly-up to avoid sinking too far into the depths while they nap.
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Animal behaviour: Deep sleep. Nature 462, 15 (2009). https://doi.org/10.1038/462015c
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DOI: https://doi.org/10.1038/462015c
