In Norwegian waters off the Lofoten Islands, sperm whales display a remarkable behaviour: they rest vertically, barely touching the surface, and at the same time release bubbles of air. A new study has shown that it is precisely this mechanism that helps them maintain neutral buoyancy and avoid floating up involuntarily.
Sperm whales are the only whales that rest in precisely this posture. Their head contains a huge spermaceti organ filled with a waxy fat that is lighter than water. The lungs and respiratory tract are also located closer to the head, which helps maintain the vertical position. When the animal rises slightly, the pressure drops, the air in the lungs expands and pushes the whale upward. Releasing bubbles compensates for this ascent.
Scientists analysed data from 42 tagged sperm whales collected from 2005 to 2019. Seventeen individuals spent anywhere from a few minutes to three hours in vertical rest. The volume of air they inhaled before such rest differed from that of ordinary foraging dives. This suggests that the animals consciously regulate their breathing depending on their state.
For now it cannot be asserted that sperm whales are actually sleeping: that would require electroencephalographic studies, which are not yet possible on the open sea. That is why scientists speak specifically of resting. A similar use of bubbles to control buoyancy has been observed in other whales as well — grey whales and bowhead whales — but in sperm whales it has been studied in the most detail.
Understanding these subtle mechanisms is important for the conservation of the species. Knowing when and how the animals are in a vulnerable state near the surface makes it possible to better assess the risks of collisions with ships and to develop measures to reduce the threats.
