Rorqual Lunge-Feeding Energetics Near and Away from the Kinematic Threshold of Optimal Efficiency

Synopsis Humpback and blue whales are large baleen-bearing cetaceans, which use a unique prey-acquisition strategylunge feeding-to engulf entire patches of large plankton or schools of forage fish and the water in which they are embedded. Dynamically, and while foraging on krill, lunge-feeding incurs metabolic expenditures estimated at up to 20.0 MJ. Because of prey abundance and its capture in bulk, lunge feeding is carried out at high acquired-to-expended energy ratios of up to 30 at the largest body sizes (similar to 27 m). We use bio-logging tag data and the work-energy theorem to show that when krill-feeding at depth while using a wide range of prey approach swimming speeds (2-5 m/s), rorquals generate significant and widely varying metabolic power output during engulfment, typically ranging from 10 to 50 times the basal metabolic rate of land mammals. At equal prey field density, such output variations lower their feeding efficiency two- to three-fold at high foraging speeds, thereby allowing slow and smaller rorquals to feed more efficiently than fast and larger rorquals. The analysis also shows how the slowest speeds of harvest so far measured may be connected to the biomechanics of the buccal cavity and the prey's ability to collectively avoid engulfment. Such minimal speeds are important as they generate the most efficient lunges.

Automated summary

Humpback and blue whales use lunge feeding to engulf large plankton or forage fish, incurring high metabolic expenditures while foraging on krill. The study shows that different prey approach swimming speeds can significantly affect the metabolic power output of rorquals during engulfment, impacting their feeding efficiency. Slow and smaller rorquals are found to feed more efficiently than fast and larger rorquals due to variations in power output.