Constraints on lunge feeding: krill can clog the baleen of filter-feeding whales.

Rorqual whales (Balaenopteridae) grow to exceptional size by lunging to engulf entire swarms of fish or krill, then rapidly concentrating prey using a baleen filter. A 20-m-long fin whale can engulf 60 m3 of water and 144 kg of krill in a single gulp and expel the water in 31 s, allowing the animal to make six lunges in a single 8 min dive. But there is a problem hidden in this apparently efficient scheme. Spread uniformly across the 2.9 m2 of the baleen window, 144 kg of krill forms a layer 6.3 cm thick. If the accumulated krill clog the filter, it could increase the time needed to expel water, decreasing the rate at which lunges could be made, and negatively impacting foraging efficiency. We devised a model that incorporates measurements of the hydraulic resistance of accumulating krill and the pressure applied to water in the buccal pouch by contraction of muscles and elastic rebound of blubber in the pouch's walls. Given the stress typical of mammalian muscle, the model suggests that approximately 16 min would be required to empty the buccal pouch of an average gulp of krill - twice the duration of typical foraging dives - severely reducing feeding efficiency. Filter time could be reduced to the observed 31 s if a fraction of the baleen window is maintained clear of krill, but it is unclear how that might be achieved. Important questions remain as to how baleen whales feed.