When northern elephant seals (Mirounga angustirostris) migrate between their breeding and foraging grounds, they spend as long as eight months at a time at sea. They’re almost always underwater, devoting only a few minutes to breathing at the surface between dives—hardly long enough for a nap. After a sip of air, they often sink quickly to 500 feet, then drift farther down in a shallow descent. Some experts have suggested that the drift is when the seals catch their Zs.
To find out, a team led by Yoko Mitani of Hokkaido University in Japan fitted six juvenile elephant seals with satellite transmitters and newfangled data loggers capable of recording such information as body position, flipper strokes, and the 3-D path of movement. They tracked the seals for up to eight days off the California coast. The resulting data revealed that drifting seals usually rolled over on their backs, stopped stroking, and spiraled peacefully down for a dozen minutes or so. (The team dubbed it the “falling-leaf phase” of the descent.)
The belly-up position is consistent with slumber: ventral blubber tends to flip an unresponsive seal’s body. What’s more, a few animals that drifted in shallow areas hit the seafloor without reacting.
The initial rapid descent is important, Mitani’s team points out. It takes the seals below the usual cruising depths of their main predators, killer whales and [white sharks. And their slow sinking thereafter makes for a relatively short ascent for air once they awake. (Biology Letters)