The Physics of Whale Movement: Drag and Thrust Production to Measure Whale Propulsive Efficiency

The group of whales known as the rorquals contains some of the largest animals that have ever lived. As a result, this group presents an interesting case study for examining how morphology at the extremes of body size can affect aspects of behavior such as locomotion as well as bioenergetics. Here, we explore how body size effects hydrodynamic factors such as thrust power and propulsive efficiency. The current study uses data from inertial whale-borne sensors to calculate kinematic parameters during normal and maximum effort swimming. By combining these data with morphometric measurements (i.e., total length, fluke area) taken from aerial drone footage, we have produced some of the first estimates of thrust power derived from direct kinematics measures. Absolute thrust power (Watts) predictable increases with body size while our results for mass-specific thrust power per flukebeat during normal effort swimming showed no significant change between the smallest species in our dataset (minke; ~8m) and the largest (blue; ~25m). Maximum effort swimming, on the other hand, resulted in higher mass-specific thrust power for humpback whales than for blue whales. We also found that propulsive efficiency decreases slightly with increasing body size from humpback whales to blue whales. These results suggest that the unique morphology of the humpback whale (larger tail and flippers, a less streamlined body shape) impacts the thrust generation and overall performance of the species and could explain their more generalist foraging habits and highly maneuverable lifestyle.