Buoyancy regulation through modulation of onboard gas volume by resting sperm whales (Physeter macrocephalus)

Sperm whales rest while drifting under the surface in a unique stereotypical vertical position. This position is thought to enable them to sleep effectively by buffering them from surface wave action, while avoiding the costs of transiting to deeper depths. While resting, sperm whales release bubbles, hypothesised to help them regulate their buoyancy. As sperm whales are breath-hold divers, if they slowly drift up while resting, the gas in their lung will slowly expand following Boyle-Mariotte's gas law. Hence, releasing bubbles would enable them to counteract the expanding diving gas and achieve neutral buoyancy. To test this hypothesis and examine how buoyancy influences resting sperm whales, we created a simulation based on tissue density, drag coefficient and onboard gas volume, and matched it to depth profiles from 10 of 42 tagged individuals in Norway that rested primarily by diving head-down. All individuals resting during such dives released bubbles, and the results from the simulation confirmed that release of bubbles functioned to regulate their buoyancy to remain submerged while resting. Additionally, the results indicate that sperm whales start resting dives with less diving gas volume than for deep foraging dives. These results demonstrate that sperm whales actively regulate their buoyancy while resting by releasing bubbles and adjusting their initial diving gas volume. We cannot rule out the possibility that releasing bubbles is related to the offgassing of excess CO2 or N2 from tissues into the lungs, so this behaviour could also have implications for metabolic gas exchange.