Behavioral data suggest adaptive buoyancy control during shallow dives in humpback whales

Marine mammals have been proposed to have a passively changing buoyancy that influences their swimming efforts depending on the depth, as a result of pulmonary alveolar compression/expansion as ambient pressure changes. Mysticetes have been described to have the same passive mechanism without considering their unique respiratory system, which may provide them with the ability to change their buoyancy actively to enhance vertical movements. Here, we present behavioral evidence supporting that adult mysticetes actively change buoyancy during shallow dives by analyzing diving data from multi-sensor tags placed on humpback whale mother-calf pairs. We show that adult female humpback whales display low effort to swim downward and upward regardless of the depth and can stay perfectly static (stationary) at different depths. In contrast, calves display depth-dependent swimming effort and only remain stationary with external help or at a specific depth. Our study adds to the body of evidence regarding the unique features of mysticetes' dives.