As a large animal swims, water is accelerated around the body and produces drag as a resistive force to locomotion. The physics of fluid flow also generate a boundary layer (BL) around the animal that consists of zero-velocity flow directly at body surface and a gradient of flows with increasing velocity away from body. The BL is dynamic and complex. Its thickness generally increases posteriorly and the flows within it transition from laminar to turbulent at some point along the body; fluid interactions at the whale surface also change as the whales body changes shape during swimming. For example, a blue whale’s BL is estimated to be only several millimeters thick at the rostrum and about 25 cm at the fluke. Therefore, suction-cup attached tags may reside in the BL and allow a unique opportunity to study its physical and biological properties. Here we report video observations and kinematic data from 101 multi-sensor -suction-cup tag deployments on several large baleen whale species. To calculate BL hydrodynamics, we used first-order modeling and computational fluid dynamics parameterized with idealized whale morphology, kinematic data, and location within the BL. Our recordings documented behaviors of vertebrate and invertebrate species within the BL. On blue whales, remoras (Echeneidae) exhibited rapid attachment-detachment behaviors, agile maneuvers, and foraging on zooplankton. On humpback whales, we revealed movements and social interactions among whale lice (Cyamus). In both gray whales and humpback whales, we observed active filter feeding and mating attempts in barnacles (Coronula). Organisms within the BL may not only benefit from lower drag, but also have access to plankton associated with the stratified flows next to the whale’s surface. Our analyses provide the first characterization of a specialized ecological niche that is formed by the hydrodynamic environment around large marine vertebrates.
Goldbogen, J., D. Cade, J. Calambokidis, A. Stimpert, B. Flammang, A. Friedlaender, J. Potvin. 2017. Living in a whale’s boundary layer: Swimming hydrodynamics of large vertebrates generate a low-drag ecological niche for commensal marine organisms. Abstract (Proceedings) 22nd Biennial on the Biology of Marine Mammals, Halifax, Nova Scotia, October 22-27, 2017.